Genetic patterns of a range expansion: The spur-thighed tortoise Testudo graeca graeca in southeastern Spain

Abstract

In the present work we analyzed the genetic structure of the populations of the terrestrial tortoise Testudo graeca graeca in southeastern Spain, identified as a recent range expansion from North Africa. The study and interpretation of the species' genetic spatial pattern could provide clues to the processes related to the species' arrival and, because of its endangered status, is especially useful in implementing appropriate management measures. We used microsatellite markers to analyze 17 populations located in the coastal region of the species' range in southeastern Spain, and an external group of Algerian tortoises. Three genetic units with a high level of spatial coherence and moderate levels of admixture resulted from a cluster analysis, and an isolation-by-distance pattern covering the entire study area was detected. These results suggest that southeastern Spanish populations show a complex spatial genetic pattern resulting from their isolation from North African populations and their natural dispersal in this region. Finally, our work shows that conservation actions such as captive breeding, introductions or translocations, may have played a relevant role in the modification of the genetic structure of some populations in southeastern Spain. Therefore, these types of conservation measures should be carried out with more caution.

Surfing in tortoises? Empirical signs of genetic structuring owing to range expansion

“…Moreover, the finding of a spatially coherent pattern of genetic variation within the recent population supported the natural dispersal of the species within the newly colonized territory from a single point of entry located in the central-southern part of the current range [9,10].…”

“…In this range expansion context, we find a spatially structured genetic pattern within the Spanish range that shows several signatures predicted by genetic surfing: (i) genetic diversity significantly decreases with increasing distance to the probable arrival area for the species; (ii) more clinal patterns of allele frequencies involving different unlinked loci are present in the expanded population; (iii) some of these clinal patterns correspond to African rare alleles that are frequent in some areas of southeastern Spain; and (iv) spatial differentiation is stronger in the more recently established range than in the original one. Furthermore, Graciá et al [9,10] found regions of relative genetic uniformity within southeastern Spain, both using mitochondrial and microsatellite markers. Notably, some mitochondrial haplotypes appeared fixed in different regions of the studied area.…”

“…We used five previously used polymorphic microsatellite markers (GenBank accession numbers: AF517235, AF517229, AF517239, AY822049 and AY822051; table 1), and only those samples coming from sites with at least seven individuals were included in the subsequent analyses [9,10].…”

The uncertainty of Late Pleistocene range expansions in the western Mediterranean: a case study of the colonization of south‐eastern Spain by the spur‐thighed tortoise, Testudo graeca

“…Fritz et al, 2009;Salinas et al, 2011;Vamberger et al, 2011). However, for the south-eastern Spanish tortoises an incipient evolutionary isolation has recently been suggested by the discovery of unique haplotypes and an explicit genetic pattern with spatial coherence (Fritz et al, 2009;Graciá et al, 2011;Salinas et al, 2011), suggesting a more ancient origin for this expansion.…”

Distribution of Testudo graeca in the western Mediterranean according to climatic factors

“…In contrast to large scale distribution and species-habitat relationships, phylogeography of North African populations of T. graeca has recently received notable attention (Álvarez et al, 2000;Fritz et al, 2007Fritz et al, , 2009Graciá et al, 2011;Salinas et al, 2011). Fritz et al (2007Fritz et al ( , 2009 showed that T. graeca comprises six major genetic clades, that diverged 4.2 to 1.8 Ma ago; five of them from the Caucasus region and the Middle East, while the sixth clade corresponds to the western Mediterranean populations.…”