Not only for egg yolk - functional and evolutionary insights from expression, selection and structural analyses of Formica ant vitellogenins

Vitellogenin, a storage protein, has been extensively studied for its egg-yolk precursor role, and it has been suggested to be fundamentally involved in caste differences in social insects. More than one vitellogenin copy has been reported in several oviparous species, including ants. However, the number and function of different vitellogenins, their phylogenetic relatedness and their role in reproductive queens and non-reproductive workers has been studied in few species only. We studied caste biased expression of vitellogenins in seven Formica ant species. Only one copy of conventional vitellogenin was identified in Formica species, and three vitellogenin homologues, derived from ancient duplications, which represent yet undiscovered vitellogenin-like genes. We show that each of these vitellogenin-like genes is present in all studied Hymenoptera and some of them in other insects as well. We show that after each major duplication event at least one of the vitellogenin-like genes has experienced a period of positive selection. This, combined with the observation that the vitellogenin-like genes have acquired or lost specific protein domains suggests sub- or neofunctionalization between vitellogenin and the duplicated genes. In contrast to earlier studies, vitellogenin was not consistently queen biased in its expression, and the caste bias of the three vitellogenin-like genes was highly variable among species. Furthermore, a truncated and Hymenoptera-specific vitellogenin-like gene, Vg-like-C, was consistently worker biased. Multispecies comparisons are essential for vitellogenin expression studies, and for gene expression studies in general, as we show that expression and also, putative functions cannot be generalized even among closely related species.