A comparative chromosome‐banding study in the silver fox, the blue fox, and their hybrids

The silver fox ( Vulpes fulvus Desm.) (2n = 34 to 42) and the blue fox ( Alopex lagopus L.) (2n = 48 to 50) represent different genera but, nevertheless, produce viable, though sterile, offspring. The variations in chromosome number are due to B chromosomes in the silver fox and a centric fusion translocation in the blue fox. Silver and blue foxes with 2n = 34 and 48, respectively, have two‐armed chromosomes exclusively. Chromosome‐banding techniques applied to pure silver and blue foxes as well as to hybrids showed whole chromosome arm homologies to be very common, there being few dissimilarities caused by pericentric inversion and tandem fusion translocation. Only a few chromosomes could not be homologized. The amounts of constitutive heterochromatin differed considerably, being sparse in the silver fox, but abundant, in the form of twenty chromosome arms, in the blue fox. The common ancestor of the two foxes probably had a karyotype with almost exclusively one‐armed chromosomes. The X and one autosomal pair were probably the only two‐armed chromosomes. The karyotypes of the genera Vulpes and Alopex evolved in independent directions by centric fusion translocations of one‐armed chromosomes. In karyotype evolution of Vulpes a tandem fusion of the ancestral two‐armed autosome and a one‐armed autosome occurred later. In Alopex two inversions occurred, as well as a large amount of constitutive heterochromatin in the form of new chromosome arms. The interspecific dissimilarities with regard to chromosome number and morphology are so extensive that no production of chromosomally balanced gametes can be expected. This would explain the sterility of the hybrids.