The endovirosis hypothesis: viral domestication and the origin of gamete asymmetry

This is the authors' abstract. We don't add key points for this paper.

Sexual reproduction is a defining feature of eukaryotic life, yet the origin of anisogamy, the functional asymmetry between a small, motile sperm and a large, resource-rich egg, remains incompletely resolved. Current evolutionary models explain how anisogamy is maintained and elaborated but provide comparatively little insight into the evolutionary emergence of the specialized genome-delivery machinery that distinguishes sperm from eggs. Viral genes and protein architectures have repeatedly been co-opted during eukaryotic evolution, including proteins involved in membrane fusion and genome delivery. Here we propose the Endovirosis hypothesis, in which functions associated with motility, genome packaging, and membrane fusion were progressively exapted from domesticated viral elements into a sperm-like lineage, whereas ancestral host functions of dormancy and nutrient storage were retained in an egg-like lineage. The framework is anchored by the conserved gamete fusogen HAP2/GCS1, whose viral-class structure motivates—but does not establish—a viral ancestry that remains to be tested. Accordingly , Endovirosis is presented as a testable evolutionary hypothesis rather than an established historical reconstruction. Unlike a functional-convergence model, Endovirosis predicts that viral-like features should be preferentially enriched in the motile genome-delivery gamete and exhibit a phylogenetically structured distribution across eukaryotes, whereas isogamous lineages retaining ancestral reproductive machinery provide an intrinsic negative control. We outline experimentally tractable and falsifiable predictions that distinguish viral exaptation from independent functional convergence.