Search

The existing XYY meiotic data for mice present a very heterogeneous picture with respect to the relative frequencies of different sex chromosome associations, both at pachytene and diakinesis/metaphase I. Furthermore, where both pachytene and diakinesis/MI data are available for the same males, the frequencies of the different configurations at the two stages are very different. In the present paper we utilise “XYY” and “XY/XYY” mosaic mice with cytologically distinguishable Y chromosomes to investigate the factors responsible for this heterogeneity between different males and between the two meiotic stages. It is concluded (1) that the initial pattern of synapsis is driven by the relatedness of the three pseudoautosomal regions (PARs); (2) that the order and extent of PAR synapsis within radial trivalents are also affected by PAR relatedness and that this leads to chiasmata being preferentially formed between closely related PARs; (3) that trivalents with a single chiasma resolve into a bivalent + univalent by the diakinesis stage; (4) that although many spermatocytes with asynapsed sex chromosomes are eliminated between pachytene and diakinesis, those that survive this phase of elimination progress to the first meiotic metaphase (MI) and accumulate in large numbers, leading to an over-representation of those with univalents as compared to radial trivalents; and (5) that the arrested MI cells are eventually eliminated, so that very few “XYY” cells contribute products to MII.

Small mammals can survive cooling to o°, at which temperature their tissues are virtually anoxic. Thus hypothermia provides a method for studying the role of oxygen tension differences in determining genetic radiosensitivity of different mammalian germ-cell stages. Male mice were given 600 rad acute X-irradiation when hypothermic or at normal body temperature. In mice irradiated when hypothermic the yield of dominant lethal mutations from treated spermatozoa was slightly but not significantly reduced, while the incidence of cytologically identifiable reciprocal translocation configurations in primary spermatocytes derived from irradiated A type spermatogonia was halved (8.3% instead of 17.1%). The usual post-radiation sterile period after 600 rad was abolished in the males made hypothermic and the testis weights showed less reduction than in those given X-rays alone. This evidence for the protection of spermatogonia from somatic and genetic radiation injury by hypothermia shows that oxygen tension differences are important in determining the extent of both types of damage in these immature germ cells. With mature germ cells, however, these and previous findings agree in showing that hypothermia or other methods of oxygen deprivation provide little if any protection from genetic radiation damage to spermatozoa, presumably because they are normally hypoxic.

The study of a bovine freemartin and its male co-twin has shown that the sex-reversed gonads of the female are secreting testosterone. Therefore, we suggest that the masculinisation of the freemartin’s reproductive tract may be due to sex hormones secreted by its own gonads, rather than by those from its male co-twin. On the evidence at present available, we favour the view that the female gonad is sex-reversed by a humoral inductor substance coming from the male co-twin that causes retention of the medullary sex cords. A study of the male co-twin suggests that its gonads may also be abnormal in that they secrete reduced amounts of androgen. Both twins showed extensive 60, XX/60, XY chimaerism, with a metacentric marker chromosome associated with the XY cell line only. This was derived from the dam. The proportion of XY cells varied markedly between tissues in both the twins. We were unable to find any germ cells in the female gonad, and, with the exception of one possible female pachytene, all the germ cells examined in the male’s testes were XY.

The rate of induction of reciprocal translocations by fast neutron irradiation of mouse spermatogonia was studied by cytological examination of descendant spermatocytes. With acute irradiation (49–55 rad/min) the frequency of affected spermatocytes rose to a maximun of 8.7% at 100 rad, falling abruptly at higher doses to 1.6% at 220 rad. With chronic irradiation over 12 weeks, however, no such fall-off was found. Instead, the yield at 214-rad neutrons was 21.7%, while that at 62-rad was 3.3%, which is significantly less than expected on a linear hypothesis. If one assumes linearity at low doses, the neutron-X RBE for acute irradiation is about 4 and the neutron-γ RBE for chronic irradiation is about 20–25.