Chromosomal phylogeny and evolution of the African mole-rats (Bathyergidae).

The subterranean African mole-rats (Family Bathyergidae) show considerable variation in their diploid numbers, but there is limited understanding of the events that shaped the extant karyotypes. Here we investigate chromosomal evolution in specimens representative of six genera and an outgroup species, the cane rat Thryonomys swinderianus, using flow-sorted painting probes isolated from the naked mole-rat, Heterocephalus glaber (2n = 60). A chromosomal phylogeny based on the cladistic analysis of adjacent syntenies detected by cross-species chromosome painting was not consistent with that obtained using DNA sequences due, in large part, to the conserved nature of the Bathyergus, Georychus and Cryptomys karyotypes. In marked contrast, the Fukomys and Heliophobius species showed extensive chromosome reshuffling, permitting their analysis by a computational approach that has conventionally been employed in comparative genomic studies for retrieving phylogenetic information based on DNA sequence or gene order data. Using the multiple genome rearrangements (MGR) algorithm and chromosomal rearrangement data detected among F. damarensis, F. darlingi, F. mechowi and the sister taxa B. janetta and Heliophobius argenteocinereus, cytogenetic support for the monophyly of Fukomys and a sister association for F. darlingi + F. damarensis was retrieved, mirroring the published sequence-based topology. We show that F. damarensis, a lineage adapted to arid and climatically unpredictable environments in Southern Africa, is characterized by a large number of fissions the fixation of which has probably been favoured by environmental factors and/or its particular eusocial structure.

Dissection of a Y-autosome translocation in Cryptomys hottentotus (Rodentia, Bathyergidae) and implications for the evolution of a meiotic sex chromosome chain.

We describe the outcome of a comprehensive cytogenetic survey of the common mole-rat, Cryptomys hottentotus, based on G and C banding, fluorescence in situ hybridisation and the analysis of meiotic chromosomes using immunostaining of proteins involved in the formation of synaptonemal complex (SCP1 and SCP3). We identified the presence of a Y-autosome translocation that is responsible for a fixed diploid number difference between males (2n = 53) and females (2n = 54), a character that likely defines the C. hottentotus lineage. Immunostaining, combined with C banding of spermatocytes, revealed a linearised sex trivalent with X(1) at one end and X(2) at the other, with evidence of reduced recombination between Y and X(2) that seems to be heterochromatin dependant in the C. hottentotus lineage. We suggest that this could depict the likely initial step in the differentiation of a true neo-X, and that this may mimic an early stage in the mammalian meiotic chain formation, an evolutionary process that has been taken to an extreme in a monotreme mammal, the platypus.

Complex evolution of X and Y autosomal translocations in the giant mole-rat, Cryptomys mechowi (Bathyergidae).

Cross-species chromosome painting was used to determine homologous chromosomal regions between two species of mole-rat, the naked mole-rat, Heterocephalus glaber (2n = 60), and the giant mole-rat, Cryptomys mechowi (2n = 40), using flow-sorted painting probes representative of all but two of the H. glaber chromosomal complement. In total 43 homologous regions were identified in the C. mechowi genome. Eight H. glaber chromosomes are retained in toto in C. mechowi, and 13 produce two or more signals in this species. The most striking difference in the karyotypes of the two taxa concerns their sex chromosomes. The H. glaber painting probes identified a complex series of translocations that involved the fractionation of four autosomes and the subsequent translocation of segments to the sex chromosomes and to autosomal partners in the C. mechowi genome. An intercalary heterochromatic block (IHB) was detected in sex chromosomes of C. mechowi at the boundary with the translocated autosomal segment. We discuss the likely sequence of evolutionary events that has led to the contemporary composition of the C. mechowi sex chromosomes, and consider these in the light of prevailing views on the genesis of sex chromosomes in mammals.