The ZW sex chromosomes of the European eel, Anguilla anguilla, and the American eel, A. rostrata, were examined with C-band and fluorescent staining to demonstrate the C-band heterochromatin. The W as well as Z chromosomes in both species are C-band negative except for a small amount of C-band heterochromatin in the centromeric region, in contrast to the W or Y elements of most other vertebrates. No fluorescing W-associated body is evident either in interphase nuclei or in metaphase plates. The ZW chromosomes of the two species have substantially similar size, morphology, and patterns of C-band heterochromatin. Karyologic and evolutionary implications are discussed.

When the Y chromosome of a Mus musculus domesticus male mouse (caught in Tirano, Italy) is placed on a C57BL/6J genetic background, approximately half of the XY (B6.YTIR) progeny develop into normal-appearing but infertile females. We have previously reported that the primary cause of infertility can be attributed to their oocytes. To identify the primary defect in the XY oocyte, we examined the onset and progress of meiotic prophase in the B6.YTIR fetal ovary. Using bromo-deoxyuridine incorporation and culture, we determined that the germ cells began to enter meiosis at the developmental ages and in numbers comparable to those in the control XX ovary. Furthermore, the meiotic prophase appeared to progress normally until the late zygotene stage. However, the oocytes that entered meiosis early in the XY ovary failed to complete the meiotic prophase. On the other hand, a considerable number of oocytes entered meiosis at late developmental stages and completed the meiotic prophase in the XY ovary. We propose that the timing of entry into meiosis and the XY chromosomal composition influence the survival of oocytes during meiotic prophase in the fetal ovary.