Pseudo-backcrossing design for rapidly pyramiding multiple traits into a preferential rice variety
Ruengphayak et al. Rice
Pseudo-backcrossing design for rapidly pyramiding multiple traits into a preferential rice variety
Siriphat Ruengphayak 1 2
Ekawat Chaichumpoo 1
Supaporn Phromphan 1
Wintai Kamolsukyunyong 0
Wissarut Sukhaket 1
Ekapol Phuvanartnarubal 1
Siripar Korinsak 0
Siriporn Korinsak 0
Apichart Vanavichit 0 1 3
0 Rice Gene Discovery, National Center for Genetic Engineering and Biotechnology (BIOTEC) National Science and Technology Development Agency (NSTDA), Kasetsart University , Kamphaengsaen, Nakhon Pathom 73140 , Thailand
1 Rice Science Center, Kasetsart University , Kamphaeng Saen, Nakhon Pathom 73140 , Thailand
2 Interdisciplinary Graduate Program in Genetic Engineering, Kasetsart University , Chatuchak, Bangkok 10900 , Thailand
3 Agronomy Department, Faculty of Agriculture at Kamphaeng Saen, Kasetsart University , Kamphaeng Saen, Nakhon Pathom 73140 , Thailand
Background: Pyramiding multiple genes into a desirable genetic background can take years to accomplish. In this paper, a pseudo-backcrossing scheme was designed to shorten the backcrossing cycle needed. PinK3, an aromatic and potentially high-yielding rice variety-although one that is intolerant to flash flooding (Sub) and susceptible to bacterial leaf blight (BB), leaf-neck blast (BL) and the brown planthopper (BPH)-was used as a genetic basis for significant improvements through gene pyramiding. Results: Four resistance donors with five target genes (Sub1A-C, xa5, Xa21, TPS and SSIIa) and three QTLs (qBph3, qBL1 and qBL11) were backcrossed individually using markers into the pseudo-recurrent parent 'PinK3' via one cycle of backcrossing followed by two cycles of pseudo-backcrossing and three selfings with rigorous foreground marker-assisted selection. In total, 29 pseudo-backcross inbred lines (BILs) were developed. Genome composition was surveyed using 61 simple sequence repeats (SSRs), 35 of which were located on six carrier chromosomes, with the remainder located on six non-carrier chromosomes. The recurrent genome content (%RGC) and donor genome content (%DGC), which were based on the physical positions of BC1F2, ranged from 69.99 to 88.98% and 11.02 to 30.01%, respectively. For the pseudo-BC3F3BILs, the %RGC and %DGC ranged from 74.50 to 81.30% and 18.70 to 25.50%, respectively. These results indicated that without direct background selection, no further increases in %RGC were obtained during pseudo-backcrossing, whereas rigorous foreground marker-assisted selection tended to reduce linkage drag during pseudo-backcrossing. The evaluation of new traits in selected pseudo-BC3F3BILs indicated significant improvements in resistance to BB, BL, BPH and Sub compared with PinK3, as well as significant improvements in grain yield (21-68%) over the donors, although yield was 7-26% lower than in 'PinK3'. All pyramided lines were aromatic and exhibited improved starch profiles, rendering them suitable for industrial food applications. Conclusions: Results show that our new pyramiding platform, which is based on marker-assisted pseudo-backcrossing, can fix five target genes and three QTLs into a high-yielding pseudo-recurrent background within seven breeding cycles in four years. This multiple pseudo-backcrossing platform decreases the time required to generate new rice varieties exhibiting complex, durable resistance to biotic and abiotic stresses in backgrounds with desirable qualities.
Gene pyramiding; Multiple backcrossing; Submergence tolerance; Bacterial leaf blight resistance; Blast resistance; Brown planthopper resistance; Linkage drag; Pseudo-backcrossing; Recurrent genome content; Donor genome content
Rice (Oryza sativa L.) is an important staple food crop
and a major part of the diet of more than half of the
worlds population. Approximately 90% of rice is grown
in Asia. Worldwide, approximately 79 million ha of
irrigated lowland rice provides 75% of the worlds rice
production (Maclean et al. 2002; Bouman et al. 2007).
Therefore, irrigated rice remains the most important
production system for maintaining food security,
particularly in Asian countries. Rice production in irrigated
areas of Thailand has been frequently and strongly
affected by abiotic stresses resulting from unfavorable
climatic changes, such as flooding and drought, as well as
by biotic stresses caused by bacterial leaf blight (BB),
leaf/neck blast (BL) and brown planthopper (BPH).
Therefore, new successful breeding lines must possess
multiple types of resistance to both biotic and abiotic
stresses, as well as demonstrating specific grain qualities
and high yield.
Three popular breeding methods used for gene
pyramiding are pedigree, backcrossing and recurrent
selection. In cross-pollinating crops, gene pyramiding is
accomplished through recurrent selection. Successful
quantitative trait locus (QTL) introgression depends on
the optimized expression of newly introgressed QTLs in
the recipient genome background, with the aim of
maximizing productivity. A general framework for addressing
these considerations through the pyramiding of multiple
QTLs into a single favorable genetic background has been
proposed, although such techniques are time-consuming
(Servin et al. 2004).
To pyramid several new QTLs, stepwise crossing
schemes can be designed, although such schemes can
require many generations of breeding. If the ultimate goal
is to improve a specific desired variety, parallel
backcrossing of single donors can most effectively be carried
out using both foreground and background selection.
Recently, successful gene/QTL pyramiding programs
were reported in Thai Jasmine (Win et al. 2012; Luo and
Yin, 2013), Basmati (Singh et al. 2012; Singh et al. 2013),
Koshihikari (Ashikari and Matsuoka, 2006; Tomita,
2009), Zhenshan 97 (Wang et al. 2012) and 9311 (Zong
et al. 2012) rice varieties. However, when the target traits
are quantitatively controlled, combining several QTLs can
take years to accomplish. For multiple QTL pyramiding,
three phases are necessary: the creation of near-isogenic
lines (NILs), genotype assembly, and the extraction of
pure lines. Specific backcrossed recombinant inbred
lines (RILs) carrying four main-effect QTLs and four
epistatic-effect QTLs were pyramided into the elite
cultivar Zhenshan97 (Wang et al. 2012). To combine greater
numbers of QTLs, marker-assisted phenotypic selection
(MAS) has been developed, which is a novel approach
for QTL pyramiding of up to 24 QTLs from a single
crossing (Zong et al. 2012). QTL pyramiding via NILs
was successfully used to improve disease and lodging
resistance, as well as to increase the harvest index (Luo
and Yin, 2013). However, all of these approaches require
many years to complete.
To shorten the backcross breeding cycle, we propose a
modified form of pseudo-backcrossing. The original
design of pseudo-backcrossing originated from tree
breeding methods, where F1 plants resulting from a single
cross are backcrossed to alternate recurrent parents to
avoid inbreeding depression (Bouquet 1986).
Pseudobackcrossing is commonly used in perennial plants,
including grape (Molnr et al. 2007), eucalyptus (Kullan
et al. 2012), poplar (Novaes et al. 2009) and oil palm
(Montoya et al. 2014). For multiple gene/QTL
pyramiding, several donors are used to create newly improved
genotypes; however, the need to maintain the preferred
genetic background is equally important. Therefore, the
introduction of pseudo-backcrossing could benefit
multiplex gene pyramiding.
Most rice varieties are not tolerant to flash flooding.
Submergence tolerance is determined by the major
Sub1QTL on chromosome 9 with relatively high
heritability. The new Thai jasmine rice variety Khao Dawk
Mali 105 (KDML105) was developed for submergence
(Sub) tolerance using marker-assisted backcross
breeding (MAB) and has been released to farmers (Siangliw
et al. 2003). Growing rice plants also suffer from
epidemic diseases such as rice blast (BL) and bacterial leaf
blight (BB). Rice BL, which is caused by the fungal
pathogen Pyricularia oryzae (teleomorph: Magnaporthe
oryzae), is a major rice disease in irrigated rice-growing
areas worldwide (Ou 1985). QTLs for broad-spectrum
resistance to rice BL have been reported on
chromosomes 1 and 11 from JHN and on chromosomes 2 and
12 from IR64 (Sirithunya et al. 2002; Noenplab et al.
2006; Sreewongchai et al. 2010). Major effective QTLs
for Thai blast isolates were located on chromosomes 1
and 11, which are flanked by RM212 and RM319 on
qBL1 and by RM224-RM144 on qBL11 (Noenplab et al.
2006; Wongsaprom et al. 2010). The presence of qBL1
and qBL11 have strong effects on blast resistance in
Thailand. BB, which is caused by the bacterium
Xanthomonas oryzae pv oryzae can be effectively controlled
using resistant varieties. Several resistance genes, including
Xa4, xa5 and Xa21, are effective sources of resistance for
marker-assisted gene pyramiding in rice (Korinsak et al.
2009b; Suh et al. 2013).
Among insect rice pests, the brown planthopper (BPH,
Nilaparvata lugens Sta l), is considered one of the most
serious pests of irrigated rice. The damage caused by
BPH feeding has a major effect on crop growth and yield
(Watanabe and Kitagawa, 2000; Yuan et al. 2005). BPH
not only feeds on the rice plant directly but also transmits
viruses that cause severe diseases (Heinrichs 1979). The
use of BPH resistance genes has been recognized as the
most economic, effective and environmentally friendly
solution to this problem. The stability of BPH resistance in
Rathu Heenati (RH), a traditional Sri Lankan rice cultivar
containing qBph3, has made this strain one of the most
popular hopper-resistance donors in the Mekong
subregion, where rice production is highly intensive. QTL
mapping located qBph3 to the short arm of chromosome
6 between RM589 and RM588 based on KDML105
Rathu Heenati (Jairin et al. 2009) and PTB33 RD6 (Jairin
et al. 2007) crossings. Other BPH resistance QTLs have
also been reported, including Bph17 on chromosome 4
(Sun et al. 2005), Bph4 on chromosome 6 (Kawaguchi et al.
2001; Sun et al. 2006) and Bph18 on chromosome 12 (Jena
et al. 2006). In addition to resistance QTLs, a putative
sesquiterpene synthase (TPS) gene (Os04g27430) was
identified using SFP mapping with isogenic lines derived from
the backcross of RH and KDML105 (Kamolsukyunyong
et al. 2013). The TPS gene is induced after 5 days of BPH
feeding. Functional markers were identified in exon 5 of
the TPS gene that resulted in the deletion of seven amino
acids in the susceptible rice line, as well as three additional
SNPs associated with a transcriptional binding site,
accounting for the differential response of TPS during the
anti-feeding test (Kamolsukyunyong et al. 2013).
Alkali disintegration has been used as a biomarker for
gelatinization temperature (GT) in rice (Waters et al.
2006; Kate-ngam et al. 2008; Masouleh et al. 2012). The
alkali disintegration locus (ALK) was identified as the
starch synthase IIa (SSIIa) gene on chromosome 6 that
determines amylopectin structure (Bao et al. 2004; Bao
et al. 2006a; Umemoto et al. 2002; Umemoto et al. 2004;
Umemoto and Aoki, 2005; Waters et al. 2006; Lu et al.
2010). Two functional SNPs were identified: GC/TT and
G/A at positions 4329/4330 bp and 4198 bp, respectively
(accession AY423717) (Bao et al. 2006b; Umemoto et al.
2002; Umemoto et al. 2004; Umemoto and Aoki, 2005;
Waters et al. 2006; Lu et al. 2010). Successful marker
assisted selection (MAS) programs utilizing the GT
haplotypes for improving starch profiles have been reported
(Tian et al. 2009; Lu et al. 2010).
PinK3 is an aromatic, high-yielding,
non-photoperiodsensitive, high-amylose rice variety, but it is susceptible to
BPH, BB, BL and Sub stresses. Here, we report the
successful gene/QTL pyramiding of five functional genes
(xa5, Xa21, Sub1A-C, SSIIa, TPS) and three QTLs (qBph3,
qBL1, qBL11) into the PinK3 genome background using a
multiplex pseudo-backcrossing approach based on MAS.
The new, improved lines have a high-yield phenotype that
confers submergence tolerance and resistance to BPH, BB
and BL. This is the first report describing the application of
pseudo-backcrossing to significantly shorten the time
required for gene/QTL pyramiding in an annual crop (rice).
Streamline gene pyramiding in rice
Four sets of donors (Table 1) containing a set of target
genes/QTLsCholsub (Sub1A-C and SSIIa), Xa497 (xa5
and Xa21), Bph162 (qBph3 and TPS), and RBPiQ (qBL1
and qBL11)were backcrossed in parallel once to PinK3
by targeting MAB to generate four sets of BC1F1 progeny.
The resulting BC1F1 progeny were stepwise crossed to
recombine four sets of the target genes/QTLs into a single
set of pseudo-backcrossed progenies. By pair-wise crossing
two pairs of BC1F1 lines, two sets of pseudo-backcrossed
progenyBC2F1 (Sub1A-C, SSIIa, xa5 and Xa21) and
BC2F1 (qBph3, TPS, qBL1 and qBL11)were generated
(Figure 1). In cycle 4, all target genes/QTLs (Sub1A-C,
SSIIa, xa5, Xa21, TPS, qBph3, qBL1 and qBL11) were
recombined by crossing the two pseudo-backcrossed
BC2F1 lines to generate 2,630 pseudo-backcrossed BC3F1
progeny. Using target MAS and plant-type selection, 158
fully heterozygous pseudo-backcrossed BC3F1 lines were
selected and selfed (cycle 5) to generate 11,405 F2 progeny
for large-scale, full-target MAS to generate 29 families for
the target MAS purification. The numbers of positive
plants (pseudo BC3F2) for all target genes/QTLs were
segregated with Mendelian pattern (homozygous preference
genotype = 1/4n). In cycle 6, selfing and full-target MAS
yielded 29 best-selected, fully homozygous
pseudobackcrossed inbred lines (pseudo-BILs) carrying positive
homozygous alleles of all of the donor genes, including
Sub1A-C, SSIIa, xa5, Xa21, and TPS as well as qBph3 and
Of these lines, five pseudo-BC3F3 BILs were chosen
for field evaluation. The five pseudo BC3F3 BILs (PinK + 4)
were selected based on completion of the target genomic
regions with interesting starch profiles suitable for
glycemic index research in the future (data not shown). In
total, four donors and one recipient were intensively
crossed and selected for seven cycles: two cycles to
generate BC1F1, another two cycles to generate
pseudobackcrossed BC3F1, and three cycles of selfing to fix the
final best-selected pseudo-BC3F3BIL for field evaluation.
Graphical genotyping of pseudo-BIL
To determine the effects of pseudo-backcrossing on
genomic background composition, nine elite pseudo-BIL
(BC3F2) families were selected for genome scanning
using 61 SSRs, 35 and 26 of which were located on six
carrier and six non-carrier chromosomes, respectively
(Additional file 1). The whole genome composition of
the selected pseudo-BILs was characterized as the
percentages of recurrent genome content (%RGC) and
donor genome content (%DGC) based on the physical
intervals of the SSR-based genome scanning using five
BC1F2 (Table 2 and Additional file 2), two BC2F2 (data
not shown) and nine pseudo-BC3F3BILs (Table 3 and
+ = desirable allele.
- = undesirable allele.
Rice Science Center (unpublished)
Jantaboon et al. 2011
Rice Science Center (unpublished) Jairin et al. 2009
Figure 1 The gene/QTL pyramiding scheme used to generate the high-yield pseudo-BC BILPinK + 4 line exhibiting submergence
tolerance (Sub1A-C), bacterial leaf blight resistance (Xa21, xa5), rice blast resistance (qBL1 and qBL11), brown planthopper resistance
(qBph3 and TPS) and desired cooking qualities (Wx, SSIIa, Os2AP).
Heterozygous segment unlink1/
Table 3 The percentage genome compositions (average per target locus) in nine selected pseudo-BC3F3BILs
Donor segment link
Heterozygous segment link
Donor segment unlink
Heterozygous segment unlink
Sum of donor segments on target carrier chromosome
Donor segments on non-carrier chromosome (6ch)
Heterozygous segments on non-carrier chromosome (6ch)
Sum of donor segments on non-carrier chromosome
1/Xa21 located within qBL11 on chromosome 11.
2/SSIIa located within qBph3 on chromosome 6.
Additional file 3). The %RGC and %DGC of BC1F2
ranged from 69.99 to 88.98% and from 11.02 to 30.01%,
respectively (Additional file 2), whereas those variables
for the pseudo-BC3F3BILs ranged from 74.50 to 81.30%
and 18.70 to 25.50%, respectively (Additional file 3). The
mean %RGC and %DGC values for BC1F2 were 80.04%
and 19.96%, respectively (Table 2), whereas these values
for the pseudo-BC3F3BILs were 77.48% and 22.52%,
respectively (Table 3). These results indicated that at the
BC1F1 step, there were no significant gains or losses of %
RGC and %DGC from the two cycles of
pseudobackcrossing. For this reason, the theoretical RGC in
pseudo-BC3F3 was not met.
We then looked into the distribution of donor genome
segments across the six carrier and non-carrier
chromosomes. For the carrier chromosomes, linkage drags were
identified upstream and/or downstream of the donated
target gene/QTL following transmission to the recipient
genome. Even considering the stringent MAB on all
target genes/QTLs during the BC1 cycle, linkage drags were
still detected for almost every donated locus, both
homozygous and heterozygous, on one or both sides of
the target gene/QTL, constituting more than half of the
total donor segments on the carrier chromosomes
(Table 2). The largest linkage drags were detected on
both sides of the Xa21 locus (Table 2). However, only a
small heterozygous linkage drag was detected at the xa5
locus. The contrast between the degree of linkage drag
for the two functional genes Xa21 and xa5 was
unexpected, as the pair of loci was inherited from the same
donor, Xa497. As both xa5 and Xa21 were MAB using
their functional markers, it is interesting to speculate on
the differences in linkage drag between the two
functional genes. The size of the linkage of Xa21 is much
greater than for the two functional genes Sub1 and
TPS, and the other QTLs, qBL1, qBL11 and qBph3.
Heterozygous linkage drags were identified for five of
the eight target genes/QTLs, including the Xa21 locus
(Table 2), Donor-unlinksthe additional donor
segments co-transmitted on the opposite (unlinked)
chromosome arms of the target genes/QTLswere identified on
the carrier chromosomes containing Xa21, qBL1, qBL11
and TPS (Table 2). Therefore, the total donor segments
transmitted via BC1 along with Xa21 were obviously the
largest among the target loci under MAB (Table 2).
Multiple target loci from donors are combined in the
recipient genome, which include both linkage and
nonlinkage drags on the carrier chromosomes. Following two
successive cycles of marker-assisted pseudo-backcrossing,
BC3F3BILs linkage drags were detected innearly every
case, with the exception of the qBph3-SSIIa locus. The
total linkage drag (combined homozygous and
heterozygous types) was 8.53%, which is more than half of the total
donor component on carrier chromosomes (Table 3). The
unlinkage drag values of the donated segments on the
carrier chromosomes were between 5.91 and 40% of the total
donor component on the carrier chromosomes (Table 3).
To trace the potential origins of these large donor
components, the sum of all donor compositions in the selected
BC1F2 lines was compared with those components in the
selected BC3F3BILs. On average, the donor genome
content in BC3F3BILs was not significantly different from that
of BC1F2 (Table 2 and Table 3). The same was true for the
homozygous components of linkage drag and donor
unlinked, whereas the heterozygous components were
significantly decreased during pseudo-backcrossing. These
results could indicate that high frequency of
recombination between donated segments and the recipient
genome contents of the carrier chromosomes in the BC1
cycle was primarily due to the high donor genome
content in successive pseudo-backcrossings. Furthermore,
the significant reduction in heterozygous donor
components for the BC3F3BILs highlights an advantage of
pseudo-backcrossing in gene pyramiding.
For the non-carrier chromosomes, the total donor
components on all six non-carrier chromosomes were
not significantly affected (Table 2 and Table 3). However,
the heterozygous donor components on both carrier and
non-carrier chromosomes persisted, even in the selected
pseudo-backcrossed BC3F3BILs. These results show that
pseudo-backcrossing has similar effects as conventional
backcrossing when considering the recovery of the
genome content on non-carrier chromosomes.
Evaluation of the PinK + 4 phenotype
Field evaluations of agronomic and grain quality were
performed based on complete target genes/QTLs, plant
types, days to harvest and grain quality. The foreground
selection successfully fixed homozygosity of the five
target genes (Sub1A-C, SSIIa, xa5, Xa21 and TPS) and
three QTLs for BL and BPH into approximately 77% of
the genetic background of the pseudo-recurrent parent
PinK3 (Table 3 and Additional file 3). However, the
majority of the advanced progeny exhibited significant
phenotypic variation from their pseudo-recurrent parent
PinK3 with respect to nearly all evaluated traits, with
the exception of amylose content, grain length per width
ratio and polished grain length (Tables 4 and 5).
The uniformity of the pseudo-BC3F3BILs was the
result of MAS. Even with respect to complex traits, such
as grain yield, some of these progeny performed as well
as the pseudo-recurrent parent PinK3 and significantly
outperformed their donors (Table 4 and Figure 2).
However, some progeny inherited inferior characteristics from
the resistance donors, which affected maturity, grain
numbers per panicle (NGP), % seed fertility (PSF) and grain
yield (GY). In all cases, early-maturing progeny produced
a lower grain number per panicle and lower grain yield
(Table 4). Therefore, without rigorous background
selection, pseudo-backcrossed progeny may not possess the
desirable characteristics of their pseudo-recurrent parent.
For the Sub1 selection, all progeny exhibited a
significantly improved ability to withstand flash flooding during
seedling stages compared with the susceptible PinK3
(Table 4). However, significant phenotypic variation (31
56%) was observed among the Sub1-Pink3 families
compared with the control phenotype. Two reasons for this
variation; First, the Sub1 gene was not directly inherited
from the original FR13A but from CholSub1 donor one of
the RILs from the mapping pop KD FR13A (IR57514)
for submergence tolerance QTL by Siangliw et al. (2003).
The second reason is the quantitative nature of multiple
gene/QTL may partially regulated by recipient genetic
background of CholSub1 donor and the PinK3, the highly
susceptible recipient. Furthermore, the fact that significant
variations in traits related to submergence tolerance were
observed among the individual progeny of IR57514/Kao
Dawk Mali 105 (Jantaboon et al. 2011) is also consistent
Table 4 Trait evaluation and agronomic characteristics of four selected pseudo-BC3F3BILs (PinK + 4) and parental lines
with respect to submergence, bacterial blight, brown planthopper and blast resistance
No Name Family Trait evaluation1/ Agronomic characteristics2/
PH (cm) NGP PSF (%) TGW (g) GY (kg/ha)
70.1bc 276.7e 82.5ef 42.9f 7777ef
122.0b 10.7bc 62.7ab
Sub = Average % plant survival (%PS) after 15 days of flash flooding.
BLB = average lesion length in centimeters of the damage caused by the BB isolate TXO156.
Bph = Severity scores with UBN biotype at 9 DAI when TN1, the susceptible control died.
Blast = Average blast injury score when attacked by a Mixed#2 blast isolate from Thailand.
DM (days to maturity), NTP (number of tillers per plant), PH (plant height from the soil surface to the neck of the panicle), NGP (number of grains per panicle), PSF
(percent spikelet fertility), TGW (1,000 grain weight) and GY (grain yield).
3/Average values marked with different letters in the same column are significantly different at the 95% confidence level using LSD.
with our findings, indicating the quantitative nature of
For BB, selection based on xa5 and Xa21 resulted in
significant improvements in bacterial leaf blight resistance,
specifically to the TXO156 virulent isolate (Table 4) and
to several BB isolates identified in Thailand (Additional
file 4). For leaf/neck blast resistance, QTL pyramiding of
the two QTLs on chromosomes 1 and 11 onto the PinK3
background successfully improved resistance to a wide
range of blast isolates collected in Thailand (Table 4).
For BPH resistance, based on the UBN biotype (a BPH
biotype that has been well characterized in Thailand),
the progeny exhibited significant improvement over their
pseudo-recurrent parent PinK3 but were not as
resistant as the resistance donor. The Bph3 QTL and the TPS
gene from the Bph162 donor were co-inherited from the
broad spectrum BPH resistance cultivar Rathu Heenati
in crosses with KDML105, as reported by Jairin et al.
(2009). Some selected introgression lines from that
report were moderately resistant (MR) to various BPH
Figure 2 Plant and grain types of pseudo-BC BIL PinK + 4 compared with PinK3 (pseudo-recurrent parent). A) PinK + 4#1E06, B) PinK +
4#20A09, C) PinK + 4#66B09 and D) PinK3.
biotypes. It appears that introgression of only qBph3 and
TPS from Bph162 in pseudo-backcrossed BILs was not
enough to withstand some of the BPH biotypes used in
All selected pseudo-BC3F3 BILs (PinK + 4) contained
the aromatic allele and the Wxa allele known to confer
high amylose content and a starch profile suitable for
further analysis and industrial food applications.
Streamline backcrossing design
In a gene-pyramiding project, several donors, each
providing a target QTL with flanking markers, are used as
parental sources for new traits with the goal of
improving a favorable variety that has a preferred genetic
background. To best facilitate the efficient integration of
multiple QTLs into a single optimal variety, an improved
breeding platform was developed based on
pseudobackcrossing. In conventional backcross breeding
experiments, nearly isogenic lines from each donor were
developed prior to pyramiding (Luo and Yin, 2013; Singh
et al. 2013) to recover the background genome of the
recurrent parent. However, when introducing multiple traits,
this approach can be tedious and time-consuming. This
novel platform based on pseudo-backcross breeding
involves both multiple foreground selection and
background genome recovery in an abridged manner. Within
seven cyclesconsisting of a single backcross, two cycles
of pseudo-backcrossing and three cycles of line fixation
the entire project was accomplished within four years.
Recurrent genome background recovery
Transferring multiple resistance genes using conventional
MAB requires at least three to four backcrosses to
guarantee a high recovery of the recurrent parent phenotype
(Joshi and Nayak, 2010; Suh et al. 2013). In this study,
without background selection, the pseudo-BC3F3BILs
(PinK + 4) contained 74.50 to 81.30%RGC, which is
significantly below the theoretical value of 93.75% possible
following three conventional backcrossings. When
comparing graphical genotyping among BC1F2 progeny and
pseudo-BC3F3BILs, the total %DGC was only slightly
increased. These results indicate that without background
selection, pseudo-backcrossing can only maintain the %
RGC gained during the first backcross generation. A low
background recovery rate was also reported for the
introgression of stripe rust resistance in wheat. Without
marker-assisted background selection, the %RGC was only
82% in BC4F7 progeny (Randhawa et al. 2009). However,
when combined with phenotypic selection, %RGC was
improved to 8592% in BC3 (Sundaram et al. 2008; Korinsak
et al. 2011; Singh et al. 2013). Indeed, the utilization of
genome-wide molecular markers for background
screening during backcrossing has been suggested as the best
method for improving low %RGC (Rajpurohit et al. 2011;
Suh et al. 2013).
The number of molecular markers used for
genomewide scanning, which ultimately determines cost vs.
precision, has varied from 44 to 205 SSR loci in rice. Four
groups have reported rice MAB projects involving
background selection: Group 1) used 4451 SSRs (Yi et al.
2009; Siangliw et al. 2003; Tomita 2009; Wongsaprom
et al. 2010), Group 2 used 6772 SSRs (Win et al. 2012;
Singh et al. 2013), Group 3 used 8497 SSRs (Siangliw
et al. 2007; Jantaboon et al. 2011), and Group 4 used
107205 SSRs (Rajpurohit et al. 2011; Korinsak et al.
2011; Suh et al. 2013). However, the majority of these
MAB projects utilized molecular markers during the
final stage of selection.
In conventional backcrossing, many portions of the donor
genomes are inserted into both the carrier and non-carrier
recipient chromosomes during early cycles. After
continued backcrossing, the donor genome segments are
gradually replaced by sequences from the recurrent parent at
varying rates. In most backcrossing programs, linkage drag
is responsible for long-lasting donor genome segments
remaining in the recurrent genome. However, there is no
difference in terms of linkage drag in pseudo-backcrossing
schemes. In pseudo-backcrossing BILs, there was only a
1% reduction in linkage drag from BC1 to pseudo-BC3,
and most of this reduction was due to the heterozygous
segments of the linkage drag. Our results also revealed
that the degree of linkage drag is independent of the size
of the target genes/QTLs. When comparisons were made
between the selection of single genes or single QTLs, the
degree of linkage drag was less for QTLs than for single
genes. Of the five single gene selections, SSIIa, Sub1, xa5,
Xa21 and TPS, only selection for Xa21 showed large,
persistent linkage drag. Persistent linkage drags when
selecting for BB genes have been reported in several backcross
breeding programs, such as during the transfer of
Xa4 + xa5 + Xa21 from indica IRBB57 into japonica
Mangeumbyeo (Suh et al. 2013), and pyramiding of the
BB resistance genes Xa21 and xa13 and a semi-dwarfing
gene (sd-1) from PR106-P2 into Type 3 Basmati (Rajpurohit
et al. 2011). The problem of persistent linkage drag when
selecting for Xa21 may due to the fact that Xa21 was
derived from IRBB21, which inherited the chromosomal
region containing Xa21 from the wild species Oryza
longistaminata through several cycles of backcrossing with
indica rice (Song et al. 1995). The degree of linkage drag
may depend on linkage disequilibrium surrounding the
target gene to be transferred. Genes inherited from wild
species in cultivated strains may be retained within long,
stable LD stretches that are difficult recombine. Therefore,
selection based on functional markers alone does not
guarantee linkage drag-free progeny. More successful
single-gene target selection has been reported when
markers flanking the gene of interest were also
selected for (Rajpurohit et al. 2011). In one of the most
comprehensive backcrossing projects, a single gene,
sd1, was integrated into the desirable variety Koshihikari
using eight cycles of MAB with 51 SSR markers
surrounding sd1 to completely eliminate linkage drag (Tomita
Grain yield performance of pseudo-BC BILs
In this study, we combined multiple resistance genes
from four donors using a new backcrossing method
involving pseudo-backcrossing. The results show that all
pseudo-BC BILs showed significant improvements in
resistance to BB, BL, BPH and Sub compared with the
pseudo-recurrent parent PinK3, as well as significant
improvements in grain yield (2168% over the donors,
but 726% lower than the recipient). The reduction in
grain yield in the pseudo-BC BILs should be interpreted
in several ways. First, there was an average of 7.6% and
22.5% of linkage drag and DGC, respectively, in the
recurrent genome. As these donors were inferior in grain
yield with respect to the pseudo-recurrent parent PinK3,
the high persistent %DGC could have disrupted the
optimal expression of high-yield genes in pseudo-BC BILs.
Second, there can be a slow recovery of the %RGC
during pseudo-backcrossing when MAS for the recurrent
background is not in place. Under such conditions,
reconstruction of the recurrent genome content by
recombining different pseudo-backcrossed lines is not
favorable, as different donor segments on both carrier and
non-carrier chromosomes have more chances to
recombine, creating new substitution lines that do not resemble
the recurrent parent. Third, the multiple donated genes/
QTLs from donors to the recipient act as a genetic load
against the fitness of the recurrent parent. The
overexpression of multiple resistance genes could counteract
the metabolic energy needs necessary for high yield.
Therefore, pseudo-backcrossing may be the fastest
method for gene/QTL pyramiding, although it may
not be the ideal breeding platform for creating elite
recurrent varieties. However, marker-assisted,
genomewide scanning can be implemented during early stages
to facilitate the reconstruction of favorable genomic
backgrounds at the end of the pseudo-backcrossing
scheme. Ultimately, the trade-offs must be considered
by the breeders. Ideally, new high-throughput,
lowcost, genome-wide scanning technologies should be
utilized in combination with skillful breeder selection.
For the whole project, more than 50,000 plants were
individually genotyped for at least one molecular marker. If
budget is allow, extensive background selection must be
We improved high-yield, aromatic rice varieties by
introducing desirable multiple traits by pyramiding five target
genes (Sub1A-C, SSIIa, xa5, Xa21 and TPS) and three
QTLs (qBph3, qBL1 and qBL11) from four resistance
donors. We redesigned the gene-pyramiding platform to
minimize the total project time span by integrating MAS
into pseudo-backcross breeding. Consequently, only seven
breeding cycles in four years were required to develop
new varieties exhibiting multiple resistance traits. Using
pseudo-backcrossing, approximately 77.48% of the
recurrent genome background was recovered. With additional
background genome selection, the recurrent genome
background can further improve the %RGC and optimize
the expression of introgressed QTLs.
Plant materials used
PinK3 is a high-yield, irrigated aromatic rice cultivar
developed by Rice Science Center, Kasetsart University,
Thailand (unpublished). However, this variety is
susceptible to flash flooding (Sub), bacterial leaf blight (BB),
leaf-neck blast (BL) and the brown planthopper (BPH).
The four donors used to transfer five genes and three
QTLs to the pseudo-recurrent parent PinK3 (Table 1)
were developed by the Rice Gene Discovery and Rice
Science Center, Kasetsart University, Thailand. The four
donors used to improve the abiotic and biotic stress
tolerances of PinK3 are listed in Table 1.
The pseudo-backcross platform is divided into three
steps. In the initial step, one round of backcrossing is
conducted to donate the favorable QTL allele to the
recipient background using marker-assisted backcrossing
(MAB). Each QTL-BC1F1 contains approximately 75% of
its recurrent genome content (RGC). In the second step,
the BC1F1-plus QTLs are used as pseudo-recurrent
parents, and pseudo-BC2F1 plants are formulated by
crossing between them. More BC1F1 plus new QTLs can be
crossed to successively generate pseudo-BCnF1 QTLs
and, thus, to continue streamline gene pyramiding. At
the end of this step, the BCnF1, which contains the new
QTLs at full heterozygosity at all target marker loci, are
self-pollinated to fix the target loci (Additional file 5). In
rice, this new platform allows breeders to stack more
QTLs in the shortest possible time (shorter than that
required by any other method).
Donors for gene pyramiding in rice
Four donors providing submergence tolerance, bacterial
leaf blight resistance, blast resistance, BPH resistance
and desired gelatinization temperature were introduced
into PinK3 (aroaro and WxAWxA) as the female
pseudorecurrent parent. The donor CholSub1 carries two target
traits: submergence tolerance and desired gelatinization
temperature (Sub1A-C and SSIIa); the donor Xa497
carries two functional genes for bacterial leaf blight
resistance (xa5 and Xa21); the donor RBPiQ carries two
QTLs for blast resistance (qBL1 and qBL11); and the
donor Bph162 carries two target traits for BPH
resistance (qBph3 and TPS) (Table 1). These four donors were
used for gene pyramiding.
Genomic DNA isolation
Rice seedlings from each segregating population were
grown in a 288-well plastic tray (representing three
96well plates). Young leaves from 14-day-old individual
plants were cut into small pieces and placed (~0.2 g
weight per sample) in a 2-ml 96-well plastic block. Leaf
tissues were ground in liquid N2 using a Tissue Striker II
(KisanBio, Seoul, South Korea). After grinding, 300 l
AgencourtChloropure lysis buffer was added to the
samples. Homogenized tissues were incubated in a 2-ml
96-well plastic block at 65C for 1 hour. The sample
blocks were then centrifuged at 4,000 rpm for 10
minutes. Lysates (containing at least 200 l) were
transferred to a new 2-ml 96-well plastic block using an
Automated Biomeck NX AP96 instrument (Beckman
Coulter, California, USA). The extraction was conducted
using the standard protocol of Agencourt Chloropure
for nucleic acid isolation from plants (Beckman Coulter,
Foreground selection was performed using two marker
systems. For SNP and functional markers, multiplex
genotyping was conducted using the SNPstream system
(Beckman Coulter, California, USA). The remainder of
the foreground markers were SSR markers flanking
specific QTLs. The SNP-based genotyping array was
High-throughput multiplex SNP genotyping
High-throughput genotyping was performed by
multiplex PCR, as described by Bell et al. (2002) with certain
modifications. In brief, the forward/reverse (1820 nt in
length) and SNP-specific (4045 nt in length) primers
were designed for each foreground locus (Primers can be
manually designed) to generate a product of 90180 nt in
size. The program selects the best Single Base Extension
(SBE)-primer based on sequence melting temperature
(Tm; C) and secondary structure. At the 5 end of the
SBE-primer sequences are 20-nt tags that are
complementary to the sequences of specific positional tags in the
SNPware (384-well) microarray format (Beckman Coulter,
California, USA) (Table 6).
A 10-l PCR reaction containing 5 l genomic DNA
(10 ng/l) and 5 l KAPA TaqHotStart PCR buffer (1 U
KAPA TaqHotStart (KapaBiosystems, MA, USA) (final
concentration of 1 KAPA TaqHotStart Buffer: 75 M
dNTPs, 5 mM MgCl2 and 50 nM 38-primer pool) was
performed in a 384-well PCR plate (Sorenson BioScience,
UT, USA). The following thermocycler touch-up PCR
cycle was used: 95C for 3 min, followed by 6 cycles of
95C for 30 seconds, 50C up to 55C (0.3 increment/
cycle) for 30 seconds, and 72C for 30 seconds; this was
followed by 34 cycles of 95C for 30 seconds, 55C for
30 seconds, 72C for 30 seconds and a final extension at
72C for 7 min. Subsequently, the temperature was held
Multiplex PCR assays were prepared separately based
on the SNP panel type (A/G, A/C, A/T, G/C, G/T and
C/T). Following PCR amplification, PCR products were
cleaned, and the SBE reactions were performed. Next,
multiplex SBE products from different panel types were
pooled prior to the hybridization step.
PCR clean up, SBE reactions, hybridization and
washing, SNPstream imaging and data analysis were
performed as described by Bell et al. (2002).
Evaluation of abiotic and biotic stress traits
The parents and pseudo-BC3F3BILs (PinK + 4) were
screened for submergence resistance traits. The
experiment was conducted under complete submergence in an
outdoor lagoon located at the Rice Science Center,
Kasetsart University, Kamphaeng Sean Campus, Thailand,
during the dry season of 2013. The experiment was
arranged using a randomized complete block design with
three replications. Sixteen three-week-old BC3F3BIL
seedlings and controls including PinK3, CholSub, Xa497,
RBPiQ and Bph162 were transplanted in three replicate
plots (plot size: 0.75 0.75 m2) at a spacing of 25 cm
25 cm. Two weeks after transplanting, the number of
seedlings was counted in each plot; then, the lagoon was
filled with water to a depth of 2 m. To impose severe
submergence stress, the seedlings were completely submerged
for 15 days; the water level was maintained at 11.2 m
above the leaf tip throughout the experimental period.
After this period, the lagoon was drained, and the
seedlings were re-exposed to air for 10 days (Jantaboon et al.
2011). The number of surviving plants was recorded. The
percentage of survival (PS) was calculated using the
Number of surviving plants
Total number of plants
9 Bph_Chr6_ 6 Bph3 17 T/C 287 1210* 10 Bph_Chr6_ 6 Bph3 22 T/C 300 3380*
11 Bph_Chr6_ 6 Bph3 19 A/G 296
12 TBGI055716 1 BLQ1 24 T/A 169
13 TBGI055578 1 BLQ1 42 A/G 93
14 TBGI055841 1 BLQ1 46 T/C 115
15 TBGI454069 11 BLQ11 14 A/G 142
16 TBGI453598 11 BLQ11 23 C/T 90
17 TBGI453126 11 BLQ11 32 C/T 92
18 TBGI454717 11 BLQ11 36 T/A 132
19 TBGI454800 11 BLQ11 38 G/C 113
*These primer designs were optimized manually.
1 Sub 9 A/G 111 ACGAGCCGACGACGACGA
2 BB 2 C/G 303 GGCCACCTTCGAGCTCTACC
3 BLQ11 40 T/C 141 AAAGCTAGGCTGCTAGTGCTG
4 GT 25 G/T 89 CCACTGCCTCGAGACGTA
5 AC 31 G/T 141 TTCACTTCTCTGCTTGTGTTGT
6 FR 41 A/G 400 AATCATGTATACCCCATCAA
7 Photo 13 C/G 152 TCCAAAGATTCCGACAACA
8 Bph3 27 T/G 264 AAGCGCTTATATTCAAGCAGAA
Brown planthopper (BPH) screening
A set of pseudo-BC3F3BILs (PinK + 4) and their parents
were screened for resistance against BPH using standard
seedbox screening (SSBS); the BPH population used was
collected from Ubon Ratchathani provinces (Jairin et al.
2007). The SSBS was conducted at the seedling stage
(10 days old) under greenhouse conditions following the
method described by Heinrichs et al. (1985). Damage
scores were recorded when the susceptible control, TN1,
died (9 days after infestation; 9DAI), using the standard
evaluation system (IRRI 1996).
Bacterial leaf blight screening
The Xoo isolate TXO156 was selected for this
experiment. The isolate was grown following the method
described by Win et al. (2012). The parents and
pseudoBC3F3BILs (PinK + 4) were grown in a greenhouse for
30 days before inoculation. The inoculation procedures
used were adapted from those described by Korinsak
et al. (2009a, b). Three to four fully expanded leaves of
each plant were inoculated. Lesion length (LL) was
measured at 1214 days after inoculation. The resistance
reaction was classified as resistant (R), moderately resistant
(MR), moderately susceptible (MS) and susceptible (S)
when the values of LL were 03 cm, 3.16 cm, 6.19 cm
and more than 9 cm, respectively (Yang et al. 2003; Lin
et al. 1996).
Leaf blast screening
Thailand Magnaporthe oryzae mixed isolates#2,
including THL710 (Mae Hong Son), THL282 (Phrae), THL906
(Yala), THL122 (Chiang Rai), THL757 (Mae Hong Son)
and THL603 (Surin) (Rice Gene Discovery, Thailand,
unpublished), which can damage the PinK3 form in the
mixed isolate pre-screening, was used in leaf blast
screening experiments. The inoculum was prepared and
the plants were inoculated following the method
described by Marchetti et al. (1987) with some
modifications. Pseudo-BC3F3BILs (PinK + 4) and their parents
were grown in polyvinyl trays containing paddy field soil
(four seedlings per line) following a three-replication
completely randomized design (CRD). The seedlings
were maintained in a greenhouse for 17 days before
inoculation, after which they were inoculated with mixed
isolates#2. Disease scoring was recorded at seven days
after inoculation on a 0 to 6 scale following the
procedure described by Roumen et al. (1997) and IRRI (2002).
The average score of each line was computed from the
disease score measured for 12 individual plants.
Recording of important agronomic traits
Traits measured included days to 100% flowering (DF100),
days to maturity (DM), number of tillers per plant (NTP),
plant height (PH), number of grains per panicle (NGP),
percent spikelet fertility (PSF), 1,000 grain weight (TGW)
and grain yield (GY); these traits were measured for rice
plants grown during field trials at Kasetsart University,
Kamphaeng Sean, Nakhon Pathom, Thailand.
Twenty-one-day-old seedlings were transplanted in
three replicates in 1 2 m2 plots using 25 25 cm2 plant
spacing. Agronomic traits were recorded for five randomly
selected plants grown in each plot. The DF was recorded
when 100% of the individual plants in each plot flowered.
NTP, PH, NGP and DM were measured at maturity, and
the results were averaged from five randomly selected
plants in each plot. PH was measured from the soil surface
to the neck of the panicle. The NGP was counted
manually for five panicles. To measure the GY in each plot, only
the inner rows (containing 21 plants) were used. Two
border rows on each side of the plot and the border plants
of each row were discarded. The GY recorded for each
plot was adjusted to 14% moisture content and then
extrapolated to units of kg per ha. TGW measurements were
replicated three times. Statistical analysis was performed
using the STATGRAPHICS plus 3.0 software package
Evaluation of grain quality
Grain quality was evaluated using grain harvested from the
trials field. Rice grains of the pseudo-BC3F3BILs (PinK + 4)
and their parents were harvested at physiological maturity
and sun-dried in a greenhouse. The dried grains were
stored at room temperature for one month prior to the
grain quality traits evaluation. Three hundred grams of
grains was sampled from each replicate. The grains were
mechanically dehulled and polished using a mini-polisher.
Four physical grain qualities, including percentages of
brown rice (BR), head rice (HR), grain length (GL/W) and
%cooking elongation increased (CE), were evaluated for
the polished rice. Ten grains of paddy rice were measured
using a Vernier caliper, and the GL/W ratio was
calculated. The polished rice grain length (PRL) was measured
using the same method. The cooking elongation of the
polished rice was determined by boiling 20 grains in 5 ml
of dH2O for ten minutes. Cooked grain lengths were
measured after air-drying the grains for 1 hour. Two chemical
grain qualities, amylose content (AC) and gel temperature
(GT), were evaluated following the procedures described
by Lanceras et al. (2000). GT is an indicator of the time
required for cooking. The GT was indirectly estimated based
on the alkali spreading value (ASV); higher values of ASV
represent increased spreading in alkali and therefore
represent lower values of GT; conversely, smaller values of
ASV indicate higher values of GT.
Genomic scanning of foreground and background
Using simple sequence repeat (SSR) markers spanned
across the genome, the effects of foreground selection on
linkage drag and genome background content were
estimated using the graphical genotyping software GGT 2.0
(Berloo 2007) based on a specific F2 population type
calculation. The effects of marker-assisted selection were
studied on BC1F2 and pseudo-backcross progeny after
recombining all target genes/QTLs (PinK + 4). Five selected
BC1F2 (Additional file 2) and nine pseudo-BC3F3BILs
(PinK + 4) (Additional file 3) representing nine
recombined families were analyzed for genetic background
recovery; 61 SSR markers showing clear polymorphism
between parents were used (Additional file 1). Twenty-six
markers distributed over six non-carrier chromosomes
(chromosomes 2, 3, 7, 8, 10 and 12), as well as 35 SSR
markers distributed over six carrier chromosomes
(chromosomes 1, 4, 5, 6, 9 and 11) were used for background
scanning. Furthermore, QTL-specific markers located
within each QTL were developed to estimate the risk of
target loss. Definitions of parameters describing %
recurrent genome content (%RGC) and % donor genome
content (%DGC) were calculated according to Xi et al.
(2006), (Suh et al. (2009) and Suh et al. (2013) with some
Donor segment link: homozygous allele similar to each
donor on the same arm of the carrier chromosome.
Heterozygous segment link: heterozygous allele on the
same arm of the carrier chromosome.
Donor segment unlink: homozygous allele similar to
each donor on the other arm of the carrier chromosome.
Heterozygous segment unlink: heterozygous allele on
the other arm of the carrier chromosome.
Donor segments on non-carrier chromosome:
homozygous allele similar to each donor on non-carrier
Heterozygous segments on non-carrier chromosomes:
heterozygous allele on non-carrier chromosomes.
Recurrent background: homozygous allele similar to
the pseudo-recurrent PinK3 allele.
Below is the link to the electronic supplementary material.
Additional file 1: Polymorphic SSR markers used for background
survey and QTL/functional markers for foreground selection using
physical distance (Pseudomoleculerelease7) on carrier and non-carrier
Additional file 2: The genomic composition (average per line) of
five selected BC1F2 lines resulting from four donors: CholSub1,
Xa497, RBPiQ and Bph162.
Additional file 3: The percentage of genome compositions (average
per line) of nine selected pseudo-BC3F3BILs.
Additional file 4: Average lesion length in centimeters of
pseudoBC3F3BILs(PinK + 4)as well as the donor and recurrent parents when
challenged with four Thai Xanthomonas oryzae pv. oryzae isolates.
Additional file 5: Pseudo-backcrossing scheme for multiple gene
pyramiding based on one single backcrossing to maintain the
percentage of recurrent genome content at 75% in the successive
pseudo-backcrossing phase. This novel platform facilitates the
introduction of additional genes/QTLs to be pyramided into the current
genotyping design. Once the desirable genotype is constructed, selfing,
MAS and phenotypic selection increase the likelihood of optimizing the
desirable pyramided lines.
%RGC: Recurrent genome content; %DGC: Donor genome content;
BCnFn: Pseudo-backcrossing; Sub: Submergence; BB: Bacterial leaf blight;
BL: Blast; BPH: Brown planthopper; MAB: Marker-assisted backcrossing.
The authors declare that they have no competing interests.
SR and SP performed the cross-hybridization and MAB screening experiments.
EC developed the SNP marker set used for MAB. WK and WS performed the
BPH infestation experiments and identified candidate genes for BPH resistance.
EP performed the submergence screening. SK and SK performed the BB and BL
screenings. The entire study was designed and coordinated by AV. SR drafted
the manuscript, and AV corrected the manuscripts draft. All authors read and
approved the final version of the manuscript.
This work was supported by the Agriculture Research Development Agency
(ARDA) (Grant No. P12/2552). SR gratefully acknowledges financial support
from the Royal Golden Jubilee (RGJ)-PhD program, Grant No. PHD0009
2546, from the Thailand Research Fund (TRF).
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