The transcriptional basis of quantitative behavioral variation

What causes individuals to produce quantitatively different phenotypes? While substantial research has focused on the allelic changes that affect phenotype, we know less about how gene expression accompanies variable phenotypes. Here, we investigate the transcriptional basis of variation in parental provisioning using two species of burying beetle, Nicrophorus orbicollis and Nicrophorus vespilloides. Specifically, we used RNA-seq to compare the transcriptomes of parents that provided high amounts of provisioning behavior versus low amounts in males and females of each species. We found that there were no overarching transcriptional patterns that distinguish high from low caring parents, and no informative transcripts that displayed particularly large expression differences in females or males. However, we did find more subtle gene expression changes between high and low provisioning parents that are consistent across sexes as well as between the two species. Furthermore, we show that transcripts previously implicated in transitioning into parental care in N. vespilloides had high variance in the levels of transcription and were unusually likely to display differential expression between high and low provisioning parents. Thus, quantitative behavioral variation appears to reflect many transcriptional differences of small effect. We show that nuanced regulation of the same gene products that are required for the transition of one behavioral state to another are also those influencing variation within a behavioral state.\n\nAuthor SummaryBurying beetles in the genus Nicrophorus breed on vertebrate carcasses and provide advanced parental care to their offspring by regurgitating partially digested flesh. However, all adult beetles do not uniformly express this trait. Some provide a large amount of parenting to their offspring, and some only a little. Here, we investigate the genetic causes of why some Nicrophorus beetles feed their offspring more than others. We demonstrate that this difference is likely caused by many small changes in gene expression, rather than a few genes that have major effects. We also find that some of the same genes that help to turn on parental care behavior in burying beetles also seem to play a role in determining how much care a beetle gives. These results provide new angles on longstanding questions about the complexity of the mechanisms that underlie quantitative variation in populations.