Wednesday, 29 April 2015

Cryptic diversity in West African Torrent Frogs.


The West African Torrent Frog, Odontobatrachus natator, is found in fast moving streams and waterways in the forests of Guinea, Sierra Leone Liberia, and western Côte d’Ivoire, part of the Upper Guinean Biodiversity Hotspot. While it has been known to science for over a hundred years, a recent study has shown it to be sufficiently genetically isolated from all other Frogs to merit placing it in its own family, the Odontobatrachidae, this being the first family of Vertebrates discovered which is entirely restricted to West Africa.

In a paper published in the journal BMC Evolutionary Biology on 19 April 2015, Michael Barej and Johannes Penner of the Museum für Naturkunde Berlin at the Leibniz Institute for Evolution and Biodiversity Science, Andreas Schmitz of the Department of Herpetology and Ichthyology at the Natural HistoryMuseum of Geneva and Mark-Oliver Rödel, also of the Museum für Naturkunde Berlin at the Leibniz Institute for Evolution and Biodiversity Science examine the genetic diversity of West African Torrent Frogs in order to determine if they were truly all one widespread species or a cluster of morphologically indistinguishable cryptic species.

A West African Torrent Frog, Odontobatrachus natator. Barej et al. (2014).

Cryptic species are species which resemble one-another closely but which are reproductively isolated. These have been known about for a long time in groups such as calling Frogs and Songbirds, where morphologically identical populations can be separated on the basis of different mating calls, but the true extent of hidden diversity in many groups did not become apparent until genetic tools became available to study wild populations in the 1990s. The discovery of cryptic species can have profound implications for conservation efforts, as what was thought to be a single, widespread species with a large population can suddenly be found to be a cluster of species, each with very limited populations and ranges. Cryptic diversity seems to be particularly common among groups such as small Amphibians and Reptiles, but has been found even in large charismatic animals such as Elephants, Giraffes, Baboons and Hammerhead Sharks.

Barejet al. found that the population of Odontobatrachus natator contains six distinct genetic lineages, and though they do not go so far as to name them as new species from this preliminary study, they do indicate that further studies are likely to result in the group being divided into a number of separate species.

Tree resulting from partitioned Bayes and ML analyses of mitochondrial genes 16S, 12S, cytb and nuclear genes RAG1, SIA and BDNF (outgroups not shown). Barej et al. (2015).

The most widespread of these is found in western Guinea, across Sierra Leone and as far to the east as eastern Guinea and eastern Liberia. This includes the area from which the species was first described (Sierra Leone, unspecified), and will therefore retain the name Odontobatrachus natator however the group is subdivided in future. This population is considered to be of ‘Least Concern’ under the terms of the International Union for the Conservation of Nature’s Red List of ThreatenedSpecies.

The most closely related outgroup to this (and therefore the group least likely to subsequently be described as a separate species) was a population found only on the Freetown Peninsula in Sierra Leone. This population is referred to as Odontobatrachus natator (Freetown Peninsula), and while closely related to the larger population, does appear to be separate on the basis of the genes studies and is separated from the main population by a wide area of unsuitable habitat, which may have allowed reproductive isolation to develop. If this population is recognized as a separate species then it would be recognized as ‘Critically Endangered’ under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species, due to the limited geographical range it inhabits.

Environmental niche modelling map of genetically confirmed records of the Operational Taxonomic Units Odontobatrachus natatorand Odontobatrachus natator (Freetown Peninsula). Barej et al. (2015).

In addition four other distinctive genetic lineages are identified, and provisionally named as ‘Operation Taxonomic Units’ (OTUs) 1 to 4. These can be further groups as (OTU 1 and OUT 4) which are most closely related to one-another and the sister group to (Odontobatrachus natatorand Odontobatrachus natator (Freetown Peninsula)), and (OUT 2 and OUT 3) which are more closely related to one-another, and form an outgroup to all the other populations.

OTU 1 is found in the Simandou Range and the Massif du Ziama in south-eastern Guinea, and environmental niche modelling suggests that it may also occur westwards into parts of Sierra Leone. This population is considered to be ‘Vulnerable’ under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

Environmental niche modelling map of genetically confirmed records of Operational Taxonomic Unit 1. Barej et al. (2015).

OTU 2 was found in western Guinea, though Environmental Niche Modelling suggested that it may also be found as far east as central Sierra Leone, though Barej et al. deem this unlikely. This population is considered to be ‘Vulnerable’ under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

Environmental niche modelling map of genetically confirmed records of Operational Taxonomic Unit 2. Barej et al. (2015).

OTU 3 was also found in wesertn Guinea, though Environmental Niche Modelling suggests that it may be found as far east as central Sierra Leone and as far west as parts of Guinea Bissau, though againBarejet al. deem this unlikely.This population is considered to be ‘Endangered’ under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

Environmental niche modelling map of genetically confirmed records of Operational Taxonomic Unit 3. Barej et al. (2015).

OTU 4 was found in the Nimba Mountains of southeaster Guinea and the surrounding area, as well as in northeastern Liberia and the Mont Sangbé National Park in western Côte d’Ivoire. Environmental niche modelling suggested that this population could extend across much of eatern Guinea and northern Liberia, and even into eastern Sierra Leone. This population is considered to be ‘Endangered’ under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

Environmental niche modelling map of genetically confirmed records of Operational Taxonomic Unit 4. Barej et al. (2015).

Although this is only a preliminary study, the degree of genetic separation between the populations suggests that they may have diverged during the Plio-Pleistocene. This is consistent with palaeoclimatic models of West Africa, which suggest while the region was not directly affected by glaciation during the Pleistocene, glacial intervals at higher latitudes led to a cooler drier climate in which forests withdrew and grasslands expanded. During such periods forest species would have been restricted to isolated refugia, where populations cut off from one-another geographically could become permanently separated by genetic drift.

See also….

Robber Frogs of the genus Eleutherodactylus are found from Texas to Guatemala and Belize and across the islands of the Caribbean. The genus was formerly the most specious of any genus of Vertebrate Animals (i.e. it contained more species than any other Vertebrate...

In the late 1960s and early 1970s a series of anthropological excavations were carried out at Minatogawa Fissure on southern Okinawa Island, producing a number of Late Pleistocene Human...


Fanged Frogs, Limnonectes spp., are unique among Frogs in that the males are typically larger than the females, and frequently fight for territories, the females then mating with the males perceived as having the...

 

 

 

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