Spermatogenesis in house mouse in a Robertsonian polymorphism zone

Robertsonian (Rb) translocations can be important in speciation as a mechanism of postzygotic isolation between populations. Meiotic non-disjunction, gametogenic impairment, and association of impaired autosomal segments with sex chromosomes have been postulated as mechanisms responsible for reducing fertility in Rb mice. Quantitative histological studies needed to understand the role of Rb fusions in gametogenic impairment are scarce. Most research on Rb mice has analyzed meiotic non-disjunction of laboratory and wild-derived strains, which have complex or simple structural heterozygosity with large numbers of fusions. Using histological multilevel sampling, we examined spermatogenesis in mice from the Rb polymorphism area of Barcelona. We studied four chromosomal groups having: a) one Rb heterozygote fusion and 2n=39, b) one Rb heterozygote fusion and 2n=31, c) three Rb heterozygote fusions without monobrachial homology and with diploid number ranging from 2n=29 to 2n=37, and d) only Rb homozygote fusions with diploid number ranging from 2n=28 to 2n=30. Standard mice from the area surrounding the Rb zone were used as control. We analyzed morphological variables of the testes, relative frequency of stages in the seminiferous epithelium cycle, the ‘round spermatids:primary spermatocytes’ ratio, and other derived parameters. Our results reveal that structural homozygote mice and simple heterozygote mice having as few as one to three Rb fusions undergo greater germ cell death (GCD) than standard mice, suggesting that Rb fusions are related to increased GCD (in both the heterozygous and homozygous state) and may be the main cause of decreased gene flow between mice populations from this area.

A study of meiotic pairing, nondisjunction and germ cell death in laboratory mice carrying Robertsonian translocations

SummaryFrequencies of anaphase I nondisjunction, germ cell death and pairing abnormalities at pachytene were assessed in male mice singly heterozygous and homozygous for the Robertsonian (Rb) translocations: Rb (1.3)lBnr, Rb(ll. 13)4Bnr and Rb(10. ll)8Bnr. Rb homozygotes showed low frequencies of nondisjunction but substantial germ cell death. This germ cell death could not be attributed to problems at pachytene as Rb homozygotes showed no increase in pairing abnormalities over the (C3H/HeH×1O1/H)F1 controls. Instead genie factors are involved. Rb heterozygotes showed substantial frequencies of nondisjunction and even greater germ cell death than found in the homozygotes. Pachytene pairing abnormalities were observed and it appears that these, together with genie factors, cause physiological perturbation of meiocytes, thereby promoting germ cell death, with nondisjunction of the trivalent as a sublethal response.

Implications of the genetic divergence between European wild mice with Robertsonian translocations from the viewpoint of mitochondrial DNA

SUMMARYGenetic divergences between the wild mouse populations with various Robertsonian translocations from the Poschiavo Valley, Yugoslavia, Milan and the Apenninies, were estimated based on the mitochondrial (mt) DNAs. The mtDNAs isolated from the liver were analysed by agarose slab-gel electrophoresis after digestion with eight kinds of restriction endonucleases: BamHI, EcoRI, HindII, HindIII, PstI, HpaI, HpaII and BgII. These preparations were further used to make restriction maps, from which sequence divergence between each Rb variation was calculated to be 0·2–2·2%. These rather larger values appear to be in conflict with the present concept that the Rb variations occurred during the last several thousand years. Both, however, might be reconciled by assuming genetic introgression of the founder with a small number of Rb translocations into other subspecies populations genetically remote and the subsequent rapid accumulation of Rb translocations unique to each population due to an unknown mechanism occurring specifically in the intersubspecies hybrids between M. m. domesticus and the other M. m. subspecies. This was the case also in a new Rb (9.15) translocation obtained from Ogasawara Islands in Japan which was the intersubspecies hybrid between M. m. molossinus and M. m. domesticus.