Insights from the lack of an enigmatic trait: monomorphic sperm in evergreen bagworm moths and the evolution of sperm dimorphism

Reproductive traits contain some of the most bizarre and unintuitive innovations seen across the tree of life. Chief among these is the sperm dimorphism of butterflies and moths (Lepidoptera). Males make two types of sperm: traditional fertilizing sperm (eupyrene) and a second, non-fertilizing type (apyrene) that lacks a nucleus entirely. Despite decades of study, the function and evolution of this second sperm type has remained unclear. Here we explore the sperm biology of the evergreen bagworm moth, Thyridopteryx ephemeraeformis (Lepidoptera: Tineoidea: Psychidae), and find no evidence for non-fertilizing sperm in this species. We generate genomic, transcriptomic, and proteomic resources to characterize the apparently monomorphic sperm of this species and compare it to that of other Lepidoptera. The fertilizing sperm of the evergreen bagworm shows key differences from other studied species, especially a lack of proteolytic enzymes used to break down sperm bundles in other species. Combining this and other evidence, we offer new hypotheses for the evolution and function of this enigmatic reproductive trait. Notably, early diverging moths appear to produce far less non-fertilizing sperm in general than later diverging Lepidoptera. We infer that non-fertilizing sperm are not necessary in these taxa, perhaps because less robust packaging of fertilizing sperm enables greater mobility than in later diverging moths.