Figure legends
Figure 1 . Average fecundity, and 95% confidence interval, of females mated to males of the same subspecies or a different subspecies. A) Results are shown for crosses using Puerto Rico (D. w. willistoni ) and Saint Vincent (D. w. winge ) and B) Guadeloupe (D. w. willistoni ) and Uruguay (D. w. winge ) flies. Shared letters above bars indicate that the groups did not differ significantly.
Figure 2 . Expansion of the uterus (Ut) as a result of mating and content of the male transferred ejaculate. A) In virgin females the uterus (Ut) is compact between the gonopod long bristles (GLB) and the sperm storage organs (SR). B) Two minutes into copulation, the Uterus is semi-elongated, and the ejaculate (Ej) has been transferred. C) Six minutes into copulation, the uterus is fully elongated, an ejaculate plug (Ep) has formed in the posterior part of the uterus. D) Ejaculate removed from the uterus of a female mated to a fertile male. Sperm tails (black thin lines) are clearly visible. E) Mass of seminal fluid removed from the uterus of a female mated to a sterile hybrid male is devoid of sperm. SR= Seminal Receptacle; Spt= Spermathecae; An= Anus.
Figure 3 . Testes of fertile and sterile hybrid males. A) and B) Testes of a 5-days old fertile male and C) testes of a 5-days old sterile male. Arrowheads indicate the seminal vesicle and the thin arrow the vas deferens . D and E) Testes of a 5 and 10 days old sterile male showing enlargement of the seminal vesicle.
Figure 4 . Seminal vesicle and vas deferens of fertile and sterile hybrid males. A) Seminal vesicle of a sterile hybrid male showing vacuolar formations and sperm (arrow) displaced to the edge (left) and seminal vesicle of a fertile male with arrow pointing at sperm (right). B) Vas deferens (VD) of a fertile male (above) with sperm (arrow) present in its lumen vs . sperm (arrow) within the testes (T) of a sterile hybrid not entering the VD. C) VD of a fertile male (above) vs . sterile hybrid (bellow) showing an irregular shape and lack of lumen.