High-Resolution Linkage Analyses to Identify Genes That Influence Varroa Sensitive Hygiene Behavior in Honey Bees

Hunt GJ (2012) High-Resolution Linkage Analyses to Identify Genes That Influence Varroa Sensitive Hygiene Behavior in Honey Bees. PLoS ONE 7(11): e48276. doi:10.1371/journal.pone.0048276 High-Resolution Linkage Analyses to Identify Genes That Influence Varroa Sensitive Hygiene Behavior in Honey Bees Jennifer M. Tsuruda 0 Jeffrey W. Harris 0 Lanie Bourgeois 0 Robert G. Danka 0 Greg J. Hunt 0 Gro V. Amdam, Arizona State University, United States of America 0 1 Department of Entomology, Purdue University, West Lafayette, Indiana, United States of America, 2 Honey Bee Breeding, Genetics and Physiology Laboratory, United States Department of Agriculture - Agricultural Research Service , Baton Rouge, Louisiana , United States of America Varroa mites (V. destructor) are a major threat to honey bees (Apis melilfera) and beekeeping worldwide and likely lead to colony decline if colonies are not treated. Most treatments involve chemical control of the mites; however, Varroa has evolved resistance to many of these miticides, leaving beekeepers with a limited number of alternatives. A non-chemical control method is highly desirable for numerous reasons including lack of chemical residues and decreased likelihood of resistance. Varroa sensitive hygiene behavior is one of two behaviors identified that are most important for controlling the growth of Varroa populations in bee hives. To identify genes influencing this trait, a study was conducted to map quantitative trait loci (QTL). Individual workers of a backcross family were observed and evaluated for their VSH behavior in a mite-infested observation hive. Bees that uncapped or removed pupae were identified. The genotypes for 1,340 informative single nucleotide polymorphisms were used to construct a high-resolution genetic map and interval mapping was used to analyze the association of the genotypes with the performance of Varroa sensitive hygiene. We identified one major QTL on chromosome 9 (LOD score = 3.21) and a suggestive QTL on chromosome 1 (LOD = 1.95). The QTL confidence interval on chromosome 9 contains the gene 'no receptor potential A' and a dopamine receptor. 'No receptor potential A' is involved in vision and olfaction in Drosophila, and dopamine signaling has been previously shown to be required for aversive olfactory learning in honey bees, which is probably necessary for identifying mites within brood cells. Further studies on these candidate genes may allow for breeding bees with this trait using marker-assisted selection. - Pollination by honey bees (Apis mellifera) is an important part of modern agriculture, and honey bee health has been receiving increased attention recently from the public, beekeepers, and researchers. Honey bees face numerous challenges, including pesticides, pathogens, and parasites (such as Varroa mites, V. destructor) [13]. Varroa parasitism of honey bees is widely considered to be the greatest threat to beekeeping and has led to substantial colony losses worldwide [410]. These obligate ectoparasites live in the nest of honey bees and harm individuals and colonies. The mites require developing honey bees for their reproduction. Mated adult female mites enter brood cells and start laying eggs, one male and up to five female in worker brood cells but an average of 1.31.45 new, mature female offspring are produced [11,12]. The offspring feed on the hemolymph of the developing bee pupa and sibling mites mate with one another. When the adult bee emerges, mature female mites leave the worker cell and enter a phoretic stage while feeding on the hemolymph of adult bees [13]. The cycle is repeated when the female mite enters a new brood cell. When Varroa feed on hemolymph, the bees experience physical and physiological damage, protein levels decrease, and development can be abnormal [14,15]. One of the worst impacts of Varroa comes from its association with honey bee viruses mites can vector many honey bee viruses and some viruses can replicate within the mite [1620]. Untreated Varroa-infested colonies usually die after one to four years of mite infestation; however, there have been reports of untreated hives with mites surviving for up to six years [3,2126]. Although Varroa have been effectively controlled with several miticides, pesticide-resistant populations of mites have appeared [2739]. Miticides have significant drawbacks because they are soluble in the wax combs of the hive and can leave chemical residues in honey and wax, and synergism between chemicals can have negative effects on bee health [2,34,4045]. A more sustainable form of control is desirable and the beekeeping industry has already started to benefit from the recent development of stocks that show resistance to mites [46]. A few behavioral traits of bees have been shown to reduce Varroa populations. One important trait is Varroa sensitive hygiene (VSH). Broadly, hygiene in honey bees refers to the act of adult bees removing dead, diseased or parasitized brood from sealed cells [47,48]. Hygiene has been improved by breeding for bees that effectively remove free-killed brood (FKB). High hygiene bees also remove more Varroa than less hygienic bees [49,50]. VSH is a form of hygiene in which bees have heightened response to Varroa; greater frequencies of mites are removed by VSH bees than by FKB hygienic bees [51](Danka et al. unpub. obs.). Enhanced mite removal enables VSH bees to effectively slow growth of the Varroa population in a colony [5154]. VSH has a significant heritable component as evidenced by the response of the trait to selection in a USDA breeding program [52,53,5557]. Field studies have shown that bees with the VSH trait successfully reduce mite infestations while retaining performance in traits important to beekeepers [5860]. When infested brood is exposed to bees that exhibit high levels of VSH for one week, the mite reproduction decreases [52]. Immature mites may be killed due to uncapping and removal behavior [61]. When an infested pupa is removed from the colony, the adult female mite (and offspring) may be removed along with the pupa. If the adult female mites survive the removal of the host pupae, they usually attach to the bee that is removing the brood [62] but can also roam freely on the comb, where they are exposed to grooming behavior and can be detected and damaged via biting by the bees [49,63]. It has also been suggested that mites which are removed with pre-pupae and pupae are not likely to produce viable offspring if they invade new brood cells too soon after such events [64]. Thus, the effectiveness of VSH on reducing mite reproduction is due partly to interference with reproduction, and in part to the risks the mite faces once it is out of the safety of the brood cell [65]. Here, we investigate the genetic architecture of VSH. A companion paper takes a similar approach to study mite-grooming behavior, the other behavior that affects mite population growth [66,67]. The objective of the cu (...truncated)