Both sexes of Microtus oregoni are gonosomic mosaics; the male with the OY/XY constitution, the female the XX/XO. The diploid chromosome number of 17 originally reported by Matthey (1956, 1958) holds true only for male germ cells (OY) and female somatic cells (XO). The male, which develops from an XY-zygote with 18 chromosomes, produces two kinds of gametes; one with the Y, the other with no sex element; the X has been eliminated from the spermatogonia. Thus in this species the sperm with no sex element at all is female-determining; the XO-zygote develops into a female lacking a paternal X-chromo-some.
Four age groups of the rainbow trout (Salmo irideus) were surveyed in an extensive cytological study. While the basic diploid complement of this species was found to consist of 104 chromosome arms, some elements united and dissociated early in embryonic life; thus within each individual fish, from two to seven distinct chromosome constitutions were identified, ranging from 2n = 58 to 2n = 65. Each somatic cell type tended to have a particular diploid chromosome number. Several multivalents were found in meiotic germ cells of both sexes. The Robertsonian type of chromosomal change which played an important part in the evolution of terrestrial vertebrates is seen within single individuals of this teleost fish.
In the chromosome complement of Bos taurus, the large X and small Y are both metacentric while the remaining 29 autosomal pairs are all acrocentric, a karyological characteristic which permits unequivocal differentiation of 2 A-XX cells from 2A-XY cells at mitotic metaphase. In this study, seven Holstein-Freisian calves less than one day old were utilized. One bull and one heifer, normal and single-born, served as controls. In the remaining three freemartins and two bull twins, blood chimerism was determined cytologically. In the testes of the two bull twins, 2 A-XX cells were also detected. In fact, twice as many 2 A-XX as 2A-XY cells were found in the histologically normal testis of one animal. Whether or not these 2 A-XX cells were germ cells capable of functioning as spermatogonia could not be ascertained. In the free-martin gonads, there was no evidence of propagation of 2 A-XY cells.
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