The mammary gland-specific marsupial ELP and eutherian CTI share a common ancestral gene
BMC Evolutionary Biology
The mammary gland-specific marsupial ELP and eutherian CTI share a common ancestral gene
Elizabeth A Pharo 0 1
Alison A De Leo 0 1
Marilyn B Renfree 1
Peter C Thomson 0
Christophe M Lefvre 0 1
Kevin R Nicholas 0 1
0 Cooperative Research Centre for Innovative Dairy Products
1 Department of Zoology, The University of Melbourne , Melbourne, Victoria 3010 , Australia
Background: The marsupial early lactation protein (ELP) gene is expressed in the mammary gland and the protein is secreted into milk during early lactation (Phase 2A). Mature ELP shares approximately 55.4% similarity with the colostrum-specific bovine colostrum trypsin inhibitor (CTI) protein. Although ELP and CTI both have a single bovine pancreatic trypsin inhibitor (BPTI)-Kunitz domain and are secreted only during the early lactation phases, their evolutionary history is yet to be investigated. Results: Tammar ELP was isolated from a genomic library and the fat-tailed dunnart and Southern koala ELP genes cloned from genomic DNA. The tammar ELP gene was expressed only in the mammary gland during late pregnancy (Phase 1) and early lactation (Phase 2A). The opossum and fat-tailed dunnart ELP and cow CTI transcripts were cloned from RNA isolated from the mammary gland and dog CTI from cells in colostrum. The putative mature ELP and CTI peptides shared 44.6%-62.2% similarity. In silico analyses identified the ELP and CTI genes in the other species examined and provided compelling evidence that they evolved from a common ancestral gene. In addition, whilst the eutherian CTI gene was conserved in the Laurasiatherian orders Carnivora and Cetartiodactyla, it had become a pseudogene in others. These data suggest that bovine CTI may be the ancestral gene of the Artiodactylaspecific, rapidly evolving chromosome 13 pancreatic trypsin inhibitor (PTI), spleen trypsin inhibitor (STI) and the five placenta-specific trophoblast Kunitz domain protein (TKDP1-5) genes. Conclusions: Marsupial ELP and eutherian CTI evolved from an ancestral therian mammal gene before the divergence of marsupials and eutherians between 130 and 160 million years ago. The retention of the ELP gene in marsupials suggests that this early lactation-specific milk protein may have an important role in the immunologically nave young of these species.
-
Background
Marsupials and eutherians diverged between 130 and 160
million years ago [1-3] and evolved very different
reproductive strategies [4-6]. Marsupials have an
ultrashort gestation ranging from 10.7 days for the stripe-faced
dunnart (Smithopsis macroura) [7] to 38 days for the
longnosed potoroo (Potorous tridactylus) [8] and deliver an
altricial young [5].
Organogenesis is completed after birth supported by a
long and physiologically complex lactation, during which
there is an increase in maternal mammary gland size and
milk production, and there are dramatic changes in milk
composition [5,9-13]. In contrast, eutherians have a long
pregnancy during which maternal investment is high
[14,15]. During eutherian lactation, milk composition
remains relatively constant apart from the initial
production of colostrum 2436 hr postpartum (pp) [16].
The tammar wallaby (Macropus eugenii) has a 26.5-day
pregnancy after embryonic diapause [17]. After giving
birth, the tammar produces milk for ~300 days until the
young is weaned. Phase 1 of lactation is comprised of
mammary development during pregnancy and
lactogenesis around parturition. At birth, the altricial young
(~400 mg) attaches to one of the four teats [5,9,13,18].
Lactation proceeds only in the sucked gland, whilst the
remaining three glands regress [5,9]. The young remains
permanently attached to the teat from the day of birth
until day 100 pp (Phase 2A) followed by detachment
from the teat and a period of intermittent sucking while
confined in the pouch between days 100200 pp (Phase
2B) [5,13,18]. The final phase is from day 200 to at least
day 300 when the young suckles variably and begins to
graze as well as maintaining a milk intake (Phase 3) [18].
These phases are highly correlated with changes in milk
composition and mammary gland gene expression
[10,13,19]. Milk protein genes such as -lactalbumin,
lactoglobulin (LGB), -casein, -casein and -casein are
induced at parturition and expressed throughout
lactation, whilst others are expressed and secreted in a
phasespecific manner [13]. Early lactation protein (ELP) is
expressed during Phase 2A only [13,20,21], whey acidic
protein (WAP) is Phase 2B-specific [22] and late
lactation protein A and B are characteristic to late Phase 2B/
Phase 3 and Phase 3 respectively [23,24].
The ELP gene was first identified in an Australian
marsupial, the brushtail possum (Trichosurus vulpecula)
[25]. ELP encodes a small precursor protein with a single
bovine pancreatic trypsin inhibitor (BPTI)-Kunitz
domain characteristic to serine protease inhibitors. ELP is
secreted in milk in multiple isoforms, which include an
~8 kDa pep (...truncated)