Molecular evolution of bumble bee vitellogenin and vitellogenin-like genes.

Vitellogenin (Vg), a storage protein, has been significantly studied for its egg yolk precursor role in oviparous animals. Recent studies found that vitellogenin and its Vg-like homologs were fundamentally involved in many other biological processes in social insects such as female caste differences and oxidative stress resilience. In this study, we conducted the first large-scale molecular evolutionary analyses of vitellogenin coding genes () and genes of bumble bees, a primitively eusocial insect belonging to the genus Bombus. We obtained sequences for each of the four genes (, , , and ) from 27 bumble bee genomes (nine were newly sequenced in this study), and sequences from the two closest clades of , including five species and five . Our molecular evolutionary analyses show that in bumble bee, the conventional experienced strong positive selection, while the genes showed overall relaxation of purifying selection. In and however, all four genes were found under purifying selection. Furthermore, the conventional showed signs of strong positive selection in most subgenera in , apart from the obligate parasitic subgenus which has no caste differentiation. Together, these results indicate that the conventional , a key pleiotropic gene in social insects, is the most rapidly evolving copy, potentially due to its multiple known social functions for both worker and queen castes. This study shows that concerted evolution and purifying selection shaped the evolution of the gene family following their ancient gene duplication and may be the leading forces behind the evolution of new potential protein function enabling functional social pleiotropy.