Gene flow across secondary contact zones of the Emys orbicularis complex in the Western Mediterranean and evidence for extinction and re-introduction of pond turtles on Corsica and Sardinia (Testudines: Emydidae)

Abstract

European pond turtles represent a phylogeographically deeply structured complex of distinct taxa. Here, we use mitochondrial DNA sequences (cytochrome b gene) and eight polymorphic microsatellite loci to investigate genetic differentiation and gene flow of Sicilian, Corsican and Sardinian pond turtles and of subspecies involved in two secondary contact zones in the Pyrenean region and Southern Italy. Mitochondrial and microsatellite differentiation is largely concordant in populations from the core regions of the distribution ranges of the studied taxa. Both marker systems provide no evidence for gene flow between Sicilian pond turtles (Emys trinacris) and Southern Italian subspecies of E. orbicularis. By contrast, in the contact zones limited gene flow occurs between distinct subspecies of E. orbicularis. Although the Southern Italian contact zone is significantly older than the Pyrenean contact zone of Holocene age, patterns of asymmetric introgression are similar. Introgressive hybridization leads to the exchange of mitochondria, but microsatellite data indicate only a few individuals with mixed ancestry. This suggests that incipient isolating mechanisms maintain largely discrete nuclear genomic gene pools. Furthermore, this implies that Southern Italy acted as a hotspot rather than as a melting pot of genetic diversity during the last glacial. Pond turtles from Corsica and Sardinia are not differentiated from continental populations of the subspecies E. o. galloitalica, neither in the mitochondrial nor in the quickly evolving microsatellite markers. As the fossil record argues for a continuous presence of pond turtles on both islands since the Middle Pleistocene, this suggests that the native island populations became extinct and the extant turtles were later introduced by prehistoric settlers. The lack of genetic differentiation of pond turtles from Corsica and Sardinia supports the view that the subspecies described from these islands are not valid.

Where are you from, stranger? The enigmatic biogeography of North African pond turtles (Emys orbicularis)

“…Identical haplotypes marked with asterisks are identical in their cyt b sequences, but differ elsewhere. Outgroups removed for clarity (branching patterns correspond to expectations; see Wiens et al 2010) lineages VI (Morocco, Iberian Peninsula) and IX (Algeria, Tunisia) plus lineage V. The latter lineage is distributed along the Mediterranean coast of northern Spain through southern France to western and southern Italy and corresponds to the subspecies galloitalica (introduced populations also occur on Corsica and Sardinia; Lenk et al 1999;Fritz et al 2005Pedall et al 2011). This well-supported Western Mediterranean clade is sister to another clade containing several mtDNA lineages (I, II, VII, VIII, X) occurring in the eastern part of the distribution range of E. orbicularis, and the successive sister group is constituted by lineage IV having a circum-Adriatic distribution.…”

“…A Western Mediterranean clade includes the two Fig. 2 Parsimony network for mitochondrial haplotypes of Emys orbicularis (mtDNA lineages I and II, IV-X sensu Lenk et al 1999;Fritz et al , 2009a (Pedall et al 2011) and on the Balearic Islands, where lineage II is found as well (Lenk et al 1999) Fig. 4 Inferred population structure of European pond turtles from North Africa and the Doñana National Park (Spain) for K=2 (top) and K=4 (bottom) using 15 microsatellite loci.…”

“…The mitochondrial haplotypes of these Moroccan turtles closely resemble haplotypes endemic in Spain and Portugal. Pond turtles from other parts of the range represent nine distinct genetic lineages (Lenk et al 1999;Fritz et al , 2009a, two of which penetrate the Iberian Peninsula in the Pyrenean region and hybridize there with the Iberian lineage (Lenk et al 1999;Pedall et al 2011). In phylogenetic and parsimony network analyses, the Moroccan haplotypes came consistently out as basal to the Iberian haplotypes, suggesting that the latter are derived from the North African haplotypes.…”

“…While the knowledge of the phylogeography and taxonomy of European and Sicilian pond turtles has increased steadily during the past 25 years (Fritz 1996(Fritz , 1998(Fritz , 2001(Fritz , 2003Lenk et al 1999;Fritz et al 2004Fritz et al , 2005Fritz et al , 2006aFritz et al , 2009aSommer et al 2007Sommer et al , 2009Velo-Antón et al 2007, 2008, 2011Pedall et al 2011), the North African populations remain poorly studied. For instance, several studies accumulated a large, nearly range-wide data set of more than 1,550 sequences of the mitochondrial cytochrome b (cyt b) gene for phylogeographic purposes (Lenk et al 1999;Fritz et al 2004Fritz et al , 2005Fritz et al , 2006aFritz et al , 2009aSommer et al 2009;Velo-Antón et al 2011).…”

The good, the bad and the ugly: Emys trinacris, Placobdella costata and Haemogregarina stepanowi in Sicily (Testudines, Annelida and Apicomplexa)

“…Although it is morphologically similar to E. orbicularis sensu lato (s.l.) (see Fritz et al 2006), molecular taxonomic studies have unambiguously revealed the presence of significant differences between the two species, which are adelphotaxa (Fritz et al 2005, Pedall et al 2011, Vamberger et al 2015. In spite of the biogeographical and evolutionary importance of this chelonian species and the reduction of its populations caused by habitat destruction, pollution and pathogens, E. trinacris was listed in the IUCN Red List as 'Data Deficient' (van Dijk 2009), thus highlighting the need for further studies aimed at its characterising and monitoring of its health status.…”