Saturday, 6 January 2018

Calculating the role of Pleistocene refugia in explaining the distribution of modern Duiker populations.

Although not affected by extensive glaciations during the Pleistocene, Africa is thought to have had periods of much cooler and drier climate which correspond the the glaciations in Eurasia and North America. During these periods the tropical forests which cover much of West and Central Africa are thought to have shrunk into a series of refugia, smaller forest patches from which forest species spread to cover the remainder of the area in warmer and wetter periods. Many modern groups of animals and plants have distributions that are thought to derive from these refugia, with species, subspecies or populations thought to have derived from different refugia still having distinct distributions within modern continuous forests. However examination of the various groups of animals and plants found in African forests appear to suggest different numbers of refugia, with different studies suggesting different numbers of refugia providing different origins for modern populations in the same regions.

In a paper published in the journal BMC Evolutionary Biology on 6 September 2017, Stephan Ntie of the Department of Biology at the Université des Sciences et Techniques de Masuku, and the Department of Biological Sciences at the University of New Orleans, Anne Davis, also of the Department of Biological Sciences at the University of New Orleans, Katrin Hils of the Cheetah Conservation Fund and the Institute for Evolution and Ecology at the University of Tübingen, Patrick Mickala, also of the Department of Biology at the Université des Sciences et Techniques de Masuku, Henri Thomassen, also of the Institute for Evolution and Ecology at the University of Tübingen, Katy Morgan, also of the Department of Biological Sciences at the University of New Orleans, Hadrien Vanthomme of the Département Ecologie et Gestion de la Biodiversité at the Muséum National d’Histoire Naturelle, Mary Gonder of the Department of Biology at Drexel University, and Nicola Anthony, once again of the Department of Biological Sciences at the University of New Orleans, describe the results of a study into the populations of modern Duikers, Cephalophinae, in modern West-Central African rainforests, with a view to understanding if this distribution derives from Pleistocene refugia.

Duikers are small-to-medium-sized Antelope found in dense forests in sub-Saharan Africa. The group is thought to have originated in the Miocene, and is composed of specialist browsers (feeding on leaves, shoots and fruits) rather than grazers (feeding on grass) like other Antelope. There are currently sixteen species of Duiker, split into three genera, many of which have overlapping ranges, but which are able to co-exist due to different ecological roles and diets.

A Bay Duiker, Cephalophus dorsalis, photographed slightly outside the Akanda National Park in Gabon in July 2012. Lyse Primault/Wikimedia Commons.

Studies of other groups suggest there were probably four refugia in the West-Central Africa region, the Gulf of Guinea Refugia in the volcanic mountains between Cameroon and Nigeria, the South-western Cameroon Refugia in coastal Cameroon to the south of the Sanaga River, the Equatorial Guinea and northern Gabon Refugia in the coastal forests spanning the borders between Equatorial Guinea and Gabon and the Southern Gabon Refugia to the south of the Ogooué River in Gabon.

In order to assess the distribution of modern Duiker populations in the region, Ntie et al. collected samples of Duiker dung from the Cross River National Park in Nigeria, the Takamanda National Park, Campo Ma’an National Park, and Lobéké National Park in Cameroon, the Monte Alén National Park, Monte Mitra, and Bioko Island in Equatorial Guinea, the Monts de Cristal National Park, Birougou National Park, Gamba Complex of Protected Areas and Minkébé National Park in Gabon, the Nouabelé Ndoki National Park in the Republic of Congo, the Mbaéré-Bodingué National Park in the Central African Republic, and the Salonga National Park in the Democratic Republic of Congo. In addition museum specimens from the Mount Nimba Strict Nature Reserve in Guinea, Cape Province in South Africa, the Dja Faunal Reserve, and area around the town of Bamenda in Cameroon, Lefini Reserve, Odzala National Park and area around Brazzaville in the Republic of Congo, Mbaéré-Bodingué National Park in the Central African Republic, area around Mitzic in Gabon, and area around Kisangani in the Democratic Republic of Congo

These samples were analysed for traces of mitochondrial DNA (mtDNA). Because mitochondrial DNA is found in the mitochondria, organelles outside the cell nucleus, it is passed directly from mother to child without being sexually recombined each generation, enabling precise estimations of when individuals shared common ancestors, at least through the female line (it is also possible to trace direct ancestry through the male line, using DNA from the Y chromosome, which is passed directly from father to son without sexual recombination), enabling Ntie et al. to determine the species of Duiker that produced each stool, and sort the samples into discrete lineages with common ancestry within each species.

A Blue Duiker, Philantomba monticola. Derek Keats/Wikimedia Commons.

Ntie et al. were able to recover mtDNA from six species of Duiker, the Bay Duiker, Cephalophus dorsalis, the Peter's Duiker, Cephalophus callipygus, the Yellow-backed Duiker, Cephalophus silvicultor, the Black-fronted Duiker Cephalophus nigrifrons, the White-bellied Duiker Cephalophus leucogaster, and the Blue Duiker, Philantomba monticola, plus the Water Chevrotain, Hyemoschus aquaticus and the Sitatunga Tragelaphus spekii, which are ecologically similar Ruminants. However, only three species, the Bay Duiker,the Peter's Duiker and the Blue Duiker, were found in more than three locations, preventing the inclusion of any of the other species in the study.

Stool samples from the Bay Duiker were found at all areas except Cross Rivers in Nigeria and Cape Province in South Africa. However, there appeared to be no clear population structure within this species, suggesting that individuals of this species range widely between locations, and that if they ever were restricted to refugia all traces of this have long since been overwritten by breeding migration between populations.

Both the Peter's Duiker and the Blue Duiker were also present in most locations (neither was found in Guinea and the Peter's Duiker was also absent in Cape Province and the Central African Republic). In both these species two distinct lineages appeared to be present, with populations in Nigeria and northern Cameroon apparently separate from all other populations.

A Peter's Duiker, Cephalophus callipygus, in the Campo Ma’an National Park in Cameroon. Wikimedia Commons.

This strongly suggests that the Peter's Duiker and the Blue Duiker were reduced to at least two separate populations during the Pleistocene glaciations, one in the Gulf of Guinea Region and one to the south; though it cannot be determined if these southern populations were restricted to single forest fragments, or were found in more than one fragment but able to move between them (Duiker cannot survive and reproduce for long periods in open grassland, but are certainly capable of crossing it).

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