Production of fertile unreduced sperm by hybrid males of the Rutilus alburnoides complex (Teleostei, Cyprinidae): An alternative route to genome tetraploidization in unisexuals

The hybrid minnow Rutilus alburnoides comprises diploid and polyploid femal es and males. Previous studies revealed that diploid and triploid females e xhibit altered oogenesis that does not involve random segregation and recom bination of the genomes of the two ancestors, constituting unisexual lineag es. In the present study, we investigated die reproductive mode of hybrid m ales from the Tejo basin, using experimental crosses and flow cytometric an alysis of blood and sperm. The results suggest that diploid hybrids produce d fertile unreduced sperm, transmitting their hybrid genome intact to offsp ring. Triploid hybrids also produced unreduced sperm, but it was net possib le to obtain data concerning their fertility. Finally, tetraploid hybrids p roduced fertile diploid sperm, which exhibited Mendelian segregation. Tetra ploid R. alburnoides may reestablish biparental reproduction, as individual s of both sexes with the appropriate constitution for normal meiosis (two h aploid genomes from each parental species) are likely to occur in natural p opulations. Tetraploids probably have arisen from syngamy of diploid eggs a nd diploid sperm produced by diploid hybrid males. Diploid hybrid males may therefore play a significant role in the dynamics of the complex, starting the evolutionary process that may ultimately lead to a new sexually reprod ucing species.