Monday, August 20, 2018

[Mammalogy • 2018] Multi-locus Phylogeny of the Tribe Tragelaphini (Mammalia, Bovidae) and Species Delimitation in Bushbuck: Evidence for Chromosomal Speciation Mediated by Interspecific Hybridization



in Hassanin, Houck, Tshikung,et al., 2018. 

Highlights
• Two species of bushbuck: Tragelaphus scriptus in NW Africa and T. sylvaticus in SE Africa.
• The two species have 2n = 57M/58F and 2n = 33M/34F chromosomes, respectively.
T. scriptus is related to T. angasii with mtDNA, and to T. sylvaticus with nuDNA.
 • Mitochondrial introgressive hybridization in the common ancestor of T. scriptus.
• Evidence for chromosomal speciation after an event of interspecific hybridization.


Abstract
The bushbuck is the most widespread bovid species in Africa. Previous mitochondrial studies have revealed a polyphyletic pattern suggesting the possible existence of two distinct species.

To assess this issue, we have sequenced 16 nuclear genes and one mitochondrial fragment (cytochrome b gene + control region) for most species of the tribe Tragelaphini, including seven bushbuck individuals belonging to the two divergent mtDNA haplogroups, Scriptus and Sylvaticus. Our phylogenetic analyses show that the Scriptus lineage is a sister-group of Sylvaticus in the nuclear tree, whereas it is related to Tragelaphus angasii in the mitochondrial tree. This mito-nuclear discordance indicates that the mitochondrial genome of Scriptus was acquired by introgression after one or several past events of hybridization between bushbuck and an extinct species closely related to T. angasii. The division into two bushbuck species is supported by the analyses of nuclear markers and by the karyotype here described for T. scriptus (2n= 57M/58F), which is strikingly distinct from the one previously found for T. sylvaticus (2n= 33M/34F). Molecular dating estimates suggest that the two species separated during the Early Pleistocene after an event of interspecific hybridization, which may have mediated massive chromosomal rearrangements in the common ancestor of T. scriptus.

Keywords: Spiral-horned antelopes, species complex, introgressive hybridization, chromosomes, cytogenetics


Figure 3. Bayesian divergence times (in million years ago, Mya) estimated using the nuclear concatenation of 16 genes (A) or the mitochondrial fragment (B). Divergence times were estimated with BEAST 2.4.7 (see main text for details). Taxa other than Tragelaphini were removed from the figures. Bold values at the nodes are mean ages. Grey bars and values between brackets represent the 95% Highest Posterior Density (HPD) interval.

 Alexandre Hassanin, Marlys L. Houck, Didier Tshikung, Blaise Kadjo, Heidi Davis and Anne Ropiquet. 2018. Multi-locus Phylogeny of the Tribe Tragelaphini (Mammalia, Bovidae) and Species Delimitation in Bushbuck: Evidence for Chromosomal Speciation Mediated by Interspecific Hybridization. Molecular Phylogenetics and Evolution. In Press.  DOI: 10.1016/j.ympev.2018.08.006 

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