Chromosomal evolution in the Vlei Rat Otomys irroratus.
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Proponents of the recognition concept of species hold that isolating mechanisms, including chromosome rearrangements, play no role in speciation while the more commonly accepted biological species concept proposes that isolation mechanisms are instrumental in the formation of new species. Moreover, some adherents of the biological concept of species, reject the hypothesis that chromosomal rearrangements can be instrumental in causing reproductive isolation and, hence, speciation. Evidence to the causative role played in speciation by chromosome changes can be obtained from cytogenetic investigations of sibling species, in parallel with analyses of gene products, DNA polymorphism and premating behaviour. This study reports the results of a cytogenetic investigation of 97 specimens of the vlei rat 0. irroratus, from 18 South African localities, and 11 samples of the Angoni vlei rat 0. angoniensis from two geographically distant populations. All 0. angoniensis individuals showed a constant karyotype with 56 acrocentric chromosomes but extensive variation was detected in 0. irroratus. Five cytotypes could be recognized within the latter. In the south-eastern parts of its South African range, 0. irroratus had a diploid number (2n) of 30 chromosomes in whicll all autosomes were acrocentric (cytotype A) while further east (cytotype A2), the diploid number was 30-32 with, again, acrocentric autosomes, A further acrocentric cytotype (AI) with 2n = 24-27 occupied the southern and south-eastern slopes of the Drakensberg range. A type with 2n = 28-30 (cytotype B), with eight pairs of biarmed autosomes, was found in the southern Cape region while in the Cape of Good Hope and in the north-eastern parts of South Africa, 0. irroratus had 2n = 28 with only four pairs of biarmed autosomes (cytotype C). Most of the numerical changes were due to variation in the number of copies of Bchromosomes which were small, biarmed and partly heterochromatic. C-banding analysis revealed that the short arms of bianned autosomes were totally heterochromatic. On the other hand, G-banding patterns of acrocentric autosomes were, with two exceptions (AI and A2 types), similar in all cytotypes while G~banding of the long arms VII of biarmed chromosomes matched the pattern of their homologues in acrocentric cytotypes. A potentially heterotic rearrangement was detected in the Al localities where a unique acrocentric autosome was identified as the product of a fusion between chromosomes 7 and 12. The geographic distribution of these groups of karyotypes correlated, by Discriminant Function Analysis, with bioclimatic regions of South Africa. The Al cytotype was shown to occupy the coldest and wettest region of the montane Drakensberg while the B type is found in the hot area of the eastern Cape with an unpredictable rainfall pattern: group C occupies regions of intermediate climate. Gene product analysis was carried out using the novel approach of subjecting liver homogenates to "Western blotting". This method was first assessed at supraspecilic level using specimens of various southern African rodents, and allowed the generation of phylogenies essentially similar to those produced by allozyme studies of the same taxa. At intraspecilic level, immunobloHing analysis did not reveal synapomorphies congruent with karyotype groups. This was interpreted, in conjunction with available allozyme data from the same populations, as evidence of low genetic differentiation between 0. irroratus cytotypes, A measure of genetic divergence was indicated in two populations from the Cape province and this was in agreement with existing data from allozyme electrophoresis and mitochondrial DNA polymorphism. The cytogenetic results were related to available data on breeding and premating behaviour concerning some of the O. irroratus populations investigated here. The presence of the 7/12 chromosome fusion in the Al cytotype correlated with a dramatic reproductive impairment of FI individuals originated from Al/A2 and Al/B cytotype crosses. Evidence of partial premating behavioural barriers has been reported by others, but information on premating behaviour between populations which are not chromosomally isolated is lacking. Therefore, it was not possible to establish if behavioural premating barriers preceded, or followed, the fixation of negatively heterotic chromosomal rearrangements. It was, nevertheless, suggested that the existence of such impaired mate recognition may be an example of reproductive character displacement which may have followed the fixation of the t(7: 12) typiVIII cal of the Al populations. In conclusion, the existence of chromosome changes in the AI, and possibly A2, populations accompanied by low genetic divergence and severely impaired hybrid reproductive success, are consistent with a hypothesis whereby chromosomal reproductive isolation causes speciation. Nonetheless, other speciation mechanisms mediated by genetic divergence and/or mate recognition failure, are possible in other populations where no chromosome changes of negatively heterotic potential were found.