Independent origin of sperm heteromorphism in the Drosophilidae

Sperm heteromorphism, the production multiple discrete sperm types (or "morphs"), occurs in diverse animal and plant taxa. This phenomenon represents a division of labour by male gametes, with one morph participating in fertilization and the other morph(s) specialized to perform some other reproductive function(s). One of the most intensively investigated systems is the Drosophila obscura species group, for which sperm heteromorphism evolved once in the common ancestor to the group, with all examined descendent species exhibiting sperm length heteromorphism. The absence of additional sperm heteromorphism origins among drosophilids has precluded comparative evolutionary analyses. Here, we report the discovery of a second origin within the Drosophilidae, with Chymomyza procnemis exhibiting the most extreme sperm length heteromorphism known (>15-fold difference). Males produced and transferred to females approximately twice as many short sperm as long sperm. Shortly after insemination, females ejected from their reproductive tracts most of the short morph sperm, yet relatively few of the long morph sperm, resulting in a preponderance of long morph sperm occupying the females' sperm-storage organs. By the start of egg laying, few-to-no short morph sperm were detectable in females. Although C. procnemis have very long eusperm and the highest relative investment (gonadosomatic index; GSI) in testes for any drosophilid species, a comparative analysis of 149 Drosophila species revealed no significant coevolutionary relationship between sperm heteromorphism and either sperm length or GSI. Continued investigations with Chymomyza will advance our understanding of how sperm heteromorphism arises, persists, and diversifies across lineages.