 Reference Base  The complete mitochondrial DNA sequence of the white rhin... |
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Xu, Xiufeng; Arnason, U., 1997. The complete mitochondrial DNA sequence of the white rhinoceros, Ceratotherium simum, and comparison with the mtDNA sequence of the Indian rhinoceros, Rhinoceros unicornis.. Molecular Phylogenetics and Evolution 7 (2): 189-194, fig.1, tables 1-4
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World |
| Subject: |
Taxonomy - Evolution |
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White Rhino |
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DNA of Ceratotherium simum
Estimating the divergence time between the white and the indian rhinoceroses
The time of the evolutionary divergence between the families Equidae and Rhinocerotidae was recently estimated using the 60 MYBP dating of the split between Artiodactyla and Cetacea (Arnason and Gullberg, 1996) as an external reference. The application of this reference (A/C-60) to the molecular distance between the Equidae and the Rhinocerotidae gave an estimated divergence time of ca 50 AMP between these two perissodactyl families (Xu et al., 199.6b). The aa difference between the combined sequences of the 13 protein- coding genes of the Equidae and the Rhinocerotidae is 9.8%, while the corresponding difference between the genera Ceratotherium and Rhinoceros is 5.9%. Application of A/C-60 dates the divergence between the two rhinoceroses to 27.2 MYPB (95% confidence limits: ?3.8 MY). This dating conforms with the molecular difference between the Equidae and the Rhinocerotidae, suggesting that the relative rate of evolution has been similar in the two families.
Comparison of sequencies from the white, indian, and black rhinoceroses
The sequences of the mitochondrial cytochrome b (Irwin et al. 1991) and of two tRNA genes (tRNA-Phe and tRNA-Pro) and the control region (Jama et al, 1993) have been reported for the black rhinoceros. In both the control region and the two tRNA genes are fewer differences between the white and the black rhinoceroses than between either of these species and the Indian rhinoceros. Comparisons of repeat motifs from the control region of the three species are also consistent with a closer relationship between the white and the black rhinoceroses than between either of these species and Indian rhinoceros. With respect to the cytochrome b gene, however, the phylogenetic relationship among the three species varies depending on the mode of comparison. The pairwise difference be- tween the cytochrome b gene of the three species is shown in Table 4. Protpars analysis identified the white and the Indian rhinoceroses as sister taxa to the exclusion of the black rhinoceros (bootstrap value 81), whereas analysis of the nt sequences joined the white and the black rhinoceroses with a bootstrap value of 86.
Discussion
The family Rhinocerotidae includes four extant genera-two African (Ceratotherium and Diceros) and two Asian (Rhinoceros and Dicerorhinus). Systematic relationships among the four genera are controversial, with arguments, to a considerable extent, focussing on the phylogenetic position of the Sumatran rhinoceros, Dicerorhinus sumatrensis (an Asian two-horned rhinoceros). Hypotheses for the relationships of these genera have been proposed on the basis of both the number of horns and geographical distribution. According to the number of horns hypothesis (Simpson, 1945) the African genera plus the Sumatran rhinoceros have been joined in a two-horned group. The ?geographic split' hypothesis (Groves, 1983), which does not consider the number of horns, joins the two Asian genera into one group, separate from the African rhinoceroses. A third hypothesis (Prothero and Schoch, 1989), which takes into account both number of horns and geographic distribution, separates all extant rhinoceroses into three subtribes - one including the two African genera (Ceratotherium and Diceros), one including Dicerorhinus, and one including Rhinoceros. Restriction site mapping of the mitochondrial rRNA genes (Morales and Melnick, 1994) supports a subdivision of the living rhinoceroses according to the number of horns, consis- tent with Simpson (1945).
In the present study we have studied the basal divergence among extant rhinoceroses, irrespective of the mode of defining this distinction (number of horns or geographical distribution), by examining the molecular difference between the white rhinoceros, a two- horned rhinoceros which lives in Africa, and the Indian rhinoceros, a one-horned rhinoceros which lives in Asia. On the basis of this molecular comparison we estimate the basal rhinocerotid divergence to have been ca 27 MYBP. The external reference used in establishing this divergence time was the A/C-60 standard, the dating of 60 MYBP for the evolutionary divergence between Artiodactyla and Cetacea (Arnason and Gullberg, 1996). As discussed by Xu et al. (1996b) and Arnason et al. (1996) the divergence between Artiodactyla and Cetacea is probably the most distinct ordinal mammalian divergence that has been defined in both paleontological (Gingerich et al., 1994; Thewissen et al., 1994) and molecular (Arnason and Gullberg, 1996) terms, and the application of A/C-60 to the dating of the divergence between Equidae and Rhinocerotidae gives an estimated divergence time (Xu et al., 1996b) which conforms reasonably well with paleontological (Prothero and Schoch, 1989) findings.
Our dating of the basal divergence among extant rhinoceroses is somewhat earlier than some previously proposed datings of this divergence. Based on studies of variation at allozymic loci, Merenlender et al. (1989) calculated that the genetic distance between the white (Ceratotherium simum) and the black (Diceros bicornis) rhinoceroses was around one-third of that between either of these species and the Indian rhinoceros (Rhinoceros unicornis). The fossil records of Diceros and Ceratotherium indicate that they have coexisted for approximately seven million years and based on these findings Merenlender et al. (1989) dated the divergence between these genera and Rhinoceros to 26 MYBP. This dating is consistent with our dating of the divergence between the white and Indian rhinoceroses, obtained using A/C-60. It should be noted, however, that the dating by Merenlender et al. (1989) is dependent on a much closer relationship between Ceratotherium and Diceros than is suggested by comparison of the complete cytochrome b genes of the black, white, and Indian rhinoceroses (see Table 4).
Based on restriction site mapping, O'Ryan and Harley (1993) reported a 6.8% sequence divergence between the mtDNAs of the black and the white rhinoceroses. This figure is low compared with the total difference (9.9%, see table 4) between the cytochrome b genes of these species. Without calibration for differences in evolutionary rates it is difficult to use total
difference for calcultaing times of divergence and it has furthermore been shown (Ohland et al., 1995) that differences calculated on the basis of restriction analysis may differ markedly from actual differences. On the basis of restriction analysis, Morales and Melnick (1994) proposed an evolutionary rate of 0.3% per million years in the mitochondrial rRNA genes of rhinoceroses. Applying this rate to partial sequences of the 12S and 16S rRNA genes (ca 1.6 kilobases total length), and using as a reference the divergence between Equidae and Rhinocerotidae dated at 50 MYBP, they calculated that extant two-horned (such as the white rhinoceros) and one-horned (such as the Indian rhinoceros) rhinoceroses separated around 21.5 MYBP. This dating is somewhat later than our estimate of 27 MYBP, obtained using complete mitochondrial DNA molecules.
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