Insect polyploid adaptation for cell number and size varies in longstanding versus neopolyploid lines of the wasp Nasonia vitripennis

Polyploidisation, or whole genome duplication, is a major component of insect evolution. One immediate challenge is that more genomic content increases cell size, causing problematic gigantism. How this is managed, e.g. with a possible cell reduction mechanism to retain normal scaling, has not been investigated in invertebrates. This study examines the wing cells of the wasp Nasonia vitripennis across divergent ploidies of both male and female polyploids. Polyploid cell number and size vary depending on lineage age, the gene silencing method through which polyploidization occurred, and sex, with complex downstream implications for life history. • Polyploidisation is one of the most powerful drivers of eukaryotic evolution, including for insects, but is difficult to study in animals because of sterility effects. • The haplodiploid parasitoid wasp Nasonia vitripennis is a rare exceptional animal polyploid model because it is easy to use in the lab and can be readily polyploidized through multiple means. • One of the biggest challenges to polyploidy is gigantism because cells get larger from more nuclear content; how polyploidised animals cope with this is unclear. • In this study we examined the wing for polyploidized Nasonia for cell count and size for several polyploid lines: a long established Whiting polyploid line (WPL) and two newly generated lines (TRA and WOM) • There was evidence for cell number reduction to retain normal organ size, but effect varied by sex and lines, suggesting a greater stabilization effect in the older line. • These differences in wing morphology can also impact downstream life-history traits and thus evolutionary trajectories of different polyploid lines. Polyploidisation (whole genome duplication) is pervasive in eukaryotic evolution but poses significant challenges. Longstanding polyploid lines may have specialised coping mechanisms, but neopolyploids must overcome immediate impediments. In particular, larger polyploid cells disrupt development and physiological function. Some vertebrate polyploids have larger but fewer cells (cell reduction); in invertebrates, such coping mechanisms are unstudied. Here we study polyploid cellular and morphological responses by comparing wings of a longstanding polyploid line (Whiting Polyploid Line, WPL) and neopolyploid lines of the wasp Nasonia vitripennis . As with all hymenopterans, in wasps males are haploid and females are diploid. Polyploids are diploid males and triploid females. We created neopolyploid lines with RNA interference of female development genes transformer (TRA) and wasp-overruler-of-masculinisation (WOM). We analysed differences in wing cell counts, wing surface area, and wing cell size between these polyploid lines. There were sex-specific and line differences, with female WPL exhibiting no difference in cell count between the diploids and triploids, whereas the neopolyploid lines had significantly reduced cell counts in triploids. In males, both the WPL and WOM neopolyploid line had lower cell counts in diploids than in haploids, with a less pronounced effect in the TRA neopolyploid line. Wing surface area and cell size also varied, with the longstanding WPL having greater similarity between polyploids and non-polyploids than the newly generated TRA and WOM lines. Variation in cellular size and reduction between polyploid lines suggests greater stabilization and a possible signature of re-diploidisation in the long-standing line compared to the neopolyploid lines. We discuss implications for polyploid adaptation and evolution, including effects on reproductive success.