Fertilizing ability of chicken sperm bearing the W chromosome

In the sex-reversed domestic fowl, spermatids and sperm carrying the female-specific W chromosome have been demonstrated, but whether the spermatids can become functional sperm and can fertilize the oocyte remains undetermined. In the present study, sex reversal was induced by injection of a nonsteroidal aromatase inhibitor (Fadrozole) into the air sac of the chicken egg on d 4 of incubation, and the chicks were reared to 18 mo old. A single elongated spermatid or sperm was isolated from the testis from either normal roosters or sex-reversed hens, and each was microinjected into a mouse oocyte and cultured for 24 h. Although injected oocytes were monitored on the stage of microscope, they were classified into groups by the number of pronuclei. Those that showed male and female pronuclei (2PN) were considered to have oocyte-activating potency. In the normal rooster group, most semen and testicular sperm induced 2PN, whereas only half of the elongated spermatids induced 2PN. In the sex-reversal group, most testicular sperm induced 2PN, whereas nearly half of the elongated spermatids induced 2PN in the oocytes. There was no pronucleus in the oocytes after microinjection of medium only. A second experiment confirmed the higher rate of oocyte activation by testicular sperm than testicular elongated spermatids. In this second experiment, individual oocytes injected with spermatids and sperm of sex-reversed hens were assayed by PCR to identify the W chromosome. Most spermatids and sperm carried Z chromosome, whereas a minority carried W chromosome. However, the sperm carrying W chromosome evoked 2PN with the same rate of oocyte activation as those carrying Z chromosome. From these results, it is concluded that the chicken elongated spermatids and sperm carrying W chromosome may possess a fertilizing ability similar to normal chicken sperm carrying the Z chromosome.