Department of Botany and Zoology

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Browsing Department of Botany and Zoology by Author "Abrahams, Dale"

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    Revisiting chromosomal diversity in Otomys irroratus: meiotic behaviour and mitochondrial divergence
    (Stellenbosch : Stellenbosch University, 2018) Abrahams, Dale; Rambau, Ramugondo Victor; Ruiz-Herrera Moreno, Aurora; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: In this study, immunofluorescence allowed the evaluation of meiotic chromosomal behaviour of the vlei rat, Otomys irroratus, a species documented for its karyotypic variability. This species was recently reported to form a species complex, as high sequence divergence values of the mtDNA cyt b gene, revealed a cryptic sister species, O.auratus. Pericentric inversions, tandem- and Robertsonian fusions are found in O. irroratus s.l karyotypes, however these classes of rearrangements are often negatively heterotic. Immunofluorescence was used to provide insight into the persistence of these chromosomal rearrangements believed to be deleterious to the carrier. The aim was to uncover the mechanisms which override the deleterious consequences of chromosome rearrangements, and gain a better understanding of the chromosomal evolution in this species. Firstly, specimens were G- banded to identify chromosomal rearrangements. Secondly, specimens were immunostained to assess the influences of chromosomal rearrangements on meiotic configurations. Lastly specimens were sequenced for the mtDNA cyt b and COI genes to determine to which extent chromosomal rearrangements correlate with mtDNA clades. The cytogenetic screening revealed two general cytotypes. The first has karyotypes which consists of 4 – 9 bi-armed chromosomes (2n= 28 - 30/NA= 44 – 50) and an abundance of rearrangements that appears as either pericentric inversions or centromere shifts on OIR 1, 4, 6,7 and 10. From these, the rearrangement on OIR 7 was described for the first time. The immunofluorescence trials with the SYCP3 antibody, targeting the proteins of the lateral elements of the synaptonemal complex revealed no inversion loops, even in specimens indicated to carry inversions by G-banding data. Rearrangements therefore did not appear to hamper pairing of chromosomes during meiosis. In addition, one specimen was indicated to have two centromeric signals per chromosome, by the CENPC antibody, which targets centromeric proteins. This suggests that a centromeric shift took place recently with the second centromere likely being an evolutionary new centromere. The immunostaining data hence casts doubt onto pericentric inversions, and rather suggests centromeric shifts. Also these rearrangements does not seem to affect reproduction given the data gathered here. The second cytogenetic group (2n= 24, 30/FA= 32 - 37) has karyotypes with acrocentric chromosomes, while the fusion chromosome involving OIR 7, 8 and 12, as well as Rb fusion 1/6 are also present in this group. These cytotypes differ in chromosomal arms (NA), 2n, rearrangements present, and localities where they have been found and is consistent with the two main chromosomal blueprints described for O. irroratus s.l. The mtDNA sequence data revealed two large clades with 5.6% divergence (uncorrected p-distances) that corresponds to the two cytogenetic groups. These groups have the same geographical distribution as the two main cytotypes, i.e. clade 1 encompassing the Western and Eastern Cape, whereas clade 2 stretches north-eastwards from the Eastern Cape to Kwazulu-Natal. These groupings were retrieved with both the cyt b and the COI genes, indicating that the COI may be a useful marker for phylogeographies of small mammals. The sequence divergence values were compared among specimens with and without inversions and revealed that specimens without inversions are equally divergent from specimens with one or many inversions. In effect the data suggests that inversions are not a strong driver for mtDNA divergence in the species complex. The sequence and immunofluorescence data therefore substantiate the idea that these rearrangements exist as neutral polymorphisms.