Where are we with CSD? The candidate regions and population dynamics of hymenopteran complementary sex determination

All Hymenoptera (bees, ants, wasps, sawflies) have haplodiploid sex determination. Unfertilized eggs develop into haploid males, and fertilized eggs develop into diploid females. Many species further have complementary sex determination (CSD), a mechanism by which hemizygosity results in normal male development, and heterozygosity results in normal female development. However, homozygosity (at one or multiple CSD loci) results in aberrant diploid males that are frequently reproductively challenged and threaten population health. CSD is broadly distributed taxonomically, but only the honey bee csd and a few candidate genomic regions in a few other species are known. Honey bee csd has been a flagship model for 20 years. Recent studies expand on mechanisms at multiple levels that maximize CSD diversity and functional heterozygosity, from molecular aspects to population dynamics. This demonstrates that a similar depth of knowledge for other CSD species is achievable, but there are technical limitations to discovering CSD loci and interpreting downstream population impacts. These include a lack of diploid males needed for loci identification (particularly for ml-CSD systems) and challenges to functional validation. Here, we suggest practical solutions. This review summarizes recent discoveries of CSD properties in the honey bee and new candidates in several species of bees, parasitoid wasps, and ants. Notably, there is both evidence of independent de-novo evolution (e.g. honey bees) and deep-level conservation (i.e. ants, mason bee, bumble bee) of CSD mechanisms. We then highlight ways to develop an integrative framework for understanding the implications of CSD knowledge for control of invasive populations and conservation of endangered populations.