Cytosystematics, sex chromosome translocations and speciation in African mole-rats (Bathyergidae: Rodentia)
The Bathyergidae are subterranean rodents endemic to Africa south of the Sahara. They are characterised by divergent diploid numbers that range from 2n=40 in Fukomys mechowi to 2n=78 in F. damarensis. In spite of this variation there is limited understanding of the events that shaped the extant karyotypes and in an attempt to address this, and to shed light on the mode and tempo of chromosomal evolution in the African mole-rats, a detailed analysis of both the autosomal and sex chromosome components of the genome was undertaken. In addition to G- and Cbanding, Heterocephalus glaber (2n=60) flow-sorted painting probes were used to conduct cross-species chromosome painting among bathyergids. This allowed the detection of a balanced sex chromosome-autosome translocation in F. mechowi that involved a complex series of rearrangements requiring fractionation of four H. glaber autosomes and the subsequent translocation of segments to sex chromosomes and to the autosomal partners. The fixation of this rare rearrangement has probably been favoured by the presence of an intercalary heterochromatic block (IHB) that was detected at the boundary with the translocated autosomal segment. Male meiosis in Cryptomys, the Fukomys sister clade, was investigated by immunostaining of the SCP1 and SCP3 proteins involved in the formation of the synaptonemal complex. This allowed confirmation of a Y-autosome translocation that is shared by C. hottentotus subspecies. We discuss reduced recombination between Y and X2 that seems to be heterochromatin dependent in the C hottentotus lineage, and the implications this holds for the evolution of a meiotic sex chromosome chain such as has been observed in platypus. By extending cross-species chromosome painting to Bathyergus janetta, F. damarensis, F. darlingi and Heliophobius argenteocinereus, homologous chromosomal regions across a total of 11 species/subspecies and an outgroup were examined using cladistic and bioinformatics approaches. The results show that Bathyergus, Georychus and Cryptomys are karyotypically highly conserved in comparison to Heterocephalus, Heliophobius and Fukomys. Fukomys in particular is characterised by a large number of rearrangements that contrast sharply with the conservative Cryptomys. The occurrence and fixation of rearrangements in these species has probably been facilitated by vicariance in combination with life history traits that are particular to these mammals.