Name: Cavadas JMS

Biodeserts supervisors: Brito JC, Godinho R

Title: Population structure and gene flow in desert environments: an application of molecular tools to isolated populations of crocodiles and monitor lizards in the mountains of Mauritania

Institution: University of Porto

Status: Completed



There is widespread consensus that habitat change, human-mediated transformation and climate change are fragmenting habitats and eroding global biodiversity. In extreme areas, such as deserts and arid regions, water-dependent taxa could be particularly vulnerable to isolation. The deserts and arid regions of Africa, specifically the Sahara and the Sahel, have complex environmental histories punctuated by sudden and dramatic “regime shifts” in climate and ecological conditions, which have induced large variations in geomorphic processes, as well as in the distribution and diversity of species. Currently, relict populations of Nile crocodiles (Crocodylus niloticus) and Nile monitor lizards (Varanus niloticus) persist in isolated populations restricted to the mountains of Mauritania (Tagant, Afollé, and Assaba). The occurrence of suitable corridors for dispersal could inter-connect some of these isolated habitats and may allow gene flow and probably a complex metapopulation system. However, there is lack of knowledge on the genetic variability and structure of the isolated populations.

The present study aimed at determining the genetic variability, population structuring, and gene flow levels between Mauritanian subpopulations of C. niloticus and V. niloticus. A total of 94 C. niloticus and 10 V. niloticus samples were analysed with mitochondrial and microsatellite markers (only crocodiles). Diversity indexes, phylogenetic trees and networks were calculated for mitochondrial markers. Microsatellites were analysed for structuring, Hardy-Weinberg equilibrium, allelic patterns, pairwise population genetic distances, gene flow, correlation between genetic and geographic distances, recent bottlenecks, and estimates of effective population sizes. Geostatistical tools were also used to analyse spatial patterns in the distribution of genetic variability from microsatellite data.

Results showed genetic differentiation of African Varanus, possible misidentification of V. albigularis, and genetic structure of Mauritanian Varanids. For C. niloticus, it was found two highly divergent clades in Africa, corresponding to West and East Africa, and a pattern of spatially structured genetic variability within Mauritania. Differentiated crocodile populations were distributed in Assaba and Afollé mountains, and Gabbou and Gorgol river basins. No deviations from the Hardy-Weinberg equilibrium were found and no clear spatial distribution of the allelic diversity was observed. Gene flow between particular populations was detected. No correlation between genetic and geographic distances was found. Recent bottlenecks and effective population sizes estimates were not conclusive. Spatial analyses further allowed understanding the spatial structure of genetic variability.

African Varanus differentiation seems well supported and a significant contribution to knowledge about genetic diversity of the genus is given. Western and Eastern C. niloticus clade are also coherent with literature and may support the recognition of two species. Within Mauritania, distribution of genetic variability is mostly supported by recent fieldwork findings and remote sensing analyses. This study represents a baseline for the recognition of management units for the conservation of biodiversity in the fragile humid habitats of the Sahara-Sahel arid regions.