Leaving X: How scale insects found success ditching a popular sex determination system

Reproductive biology is a key area of research with both basic and applied synergy. On the basic side, reproductive traits are some of the most highly variable across the tree of life and they play a key role in the diversification of organisms. On the applied side, a better understanding of reproduction helps inform management practices and risk assessment for pest species. Yet for all of this importance, much of life's reproductive diversity is poorly studied. Scale insects present an ideal opportunity to advance our understanding of the evolution of reproductive systems as members of this group employ a variety of sex determination from classical chromosomal sex determination to a bizarre form of haplodiploid and even simultaneous hermaphroditism. Using modern genomic methods, we reconstruct a high confidence molecular phylogeny and show that scale insects have lost the X chromosome two independent times: once in the lineage leading to hermaphroditic giant scales and a second time leading to the group of paternal genome elimination families including true mealybugs. We highlight unique features of these alternative reproductive systems with both evolutionary genetic and pest management implications.