Evolution of Growth Hormone Axis Genes Across Birds

Abstract

The growth hormone axis drives the tempo and mode of development across vertebrates, and birds are no exception. Because these genes are so tightly intertwined with major life history events such as development time to fledge, age of sexual maturity, terminal body size, clutch size, and lifespan, they may influence the patterns of diversity that I observe across the avian tree of life. As the order comprising more than 60% of living bird species, passerines are a remarkably successful, diverse taxon. Their sole unique, taxon-limited gene is a growth hormone gene I have named GH2 for its duplication from the ancestral avian GH growth hormone gene. I explore the molecular evolution of both avian GH genes across birds, finding that evolution of these genes is accelerated in passerines, and that they have sequence and genomic location differences which may enable differences in the roles of these two genes in passerine development and adulthood. To examine this possibility, I measure gene expression of GH1 and GH2 among tissues and across development from embryo to adult in the Zebra Finch (Taeniopygia guttata). I report that these two genes both show strong pituitary expression, but GH2 is expressed more than GH1 in extrapituitary tissues, especially the eye and ovary. One or both paralogs are silenced in other somatic tissues, indicating that expression is not leaky. This map of expression shows that contrary to the fate of most duplicated genes, GH2 has escaped degradation and serves important roles in the passerine Zebra Finch. Lastly, I sought to examine the growth hormone receptor (GHR) gene, as GH-GHR binding initiates a growth signaling cascade. I found that GHR has not been duplicated in passerines. I report on the phylogenetic pattern of presence and absence of a previously undescribed ancestral, female-limited GHR-W gene, which appears sporadically across several non-passerine avian taxa. These taxa are often noted for their larger female body size, a form of reverse sexual dimorphism. I find that the presence of GHR-W, inherited only by females, is highly correlated with the repeated evolution of larger female mass across the avian tree. Exploring the GH and GHR genes in the growth hormone axis leads to a greater understanding of the genetic diversity and clade-specific adaptations potentially underlying avian diversity.