Robertsonian-type chromosome variation in the tobacco mouse, M. poschiavinus, and in M. poschiavinus × M. musculus hybrids

Abstract

Cytogenetic studies were made in dark-colored wild mice, previously described as M. poschiavinus Fatio, 1869. These mice are found only at a single locality, a Swiss high-altitude valley, the Val Poschiavo/Grisons. The karyotype is characterized by the presence of 26 chromosomes, with seven pairs of meta-centrics, five pairs of acrocentric autosomes and the acrocentric sex-chromosomes (2n = 26; N.F. = 40). The F1 resulting from crossbreeds of M. poschiavinus and the laboratory mouse has invariably a somatic set of 33 chromosomes, 7 metacentrics and 26 aerocentrics (2n = 33; N.F. = 40). Meiotic diakinesis figures of the F1 show seven trivalents, five bivalents and one end-to-end-associated X-Y bivalent. M. poschiavinus, M. musculus and their hybrids seem to display an ideal Robertsonian variation. However, there is marked reduction of fertility in the F1 and in subsequent generations. The analysis of meiotic metaphase II in the male leads to the assumption that meiotic nondisjunction and embryonic aneuploidy are a major cause of the reduced fertility. The infrequent and small litters in the hybrid generations probably represent the viable products of only the balanced genetic combinations. The sequence of Robertsonian fusions that have led to the actual karyotypic conditions in M. poschiavinus seems to have built up an effective barrier, providing cytologic isolation of this species and securing its coexistence with M. musculus. The system of chromosomal variation in M. poschiavinus and its hybrids with the laboratory mouse provides an appropriate material for studies of phenotype variation and of genetic control of biochemical markers.