Phylogeography of the littoral prawn species Palaemon elegans (Crustacea: Caridea: Palaemonidae) across the Mediterranean Sea unveils disparate patterns of population genetic structure and demographic history in the two sympatric genetic types II and III

In this study, we examine the spatial distribution of genetic diversity of the littoral prawn species complex Palaemon elegans across the Mediterranean, with an emphasis in the eastern Mediterranean, and compare patterns of phylogeographic structure between the two genetic types II and III across potential gene flow barriers. For this purpose, a total of 293 mtDNA sequences of the cytochrome oxidase subunit-1 (Cox1) gene, including a newly generated dataset of 114 sequences, were obtained from 32 Mediterranean locations. The retrieved differences in genealogy and biogeographic patterns between both genetic types, as revealed by the outcome of a TCS statistical parsimony procedure, suggest different colonisation patterns in type II and type III within the Mediterranean. Furthermore, particular gradual transitions of both genetic types were unveiled across different geographic scales. Our results showed not only a marked latitudinal cline in the southeastern Mediterranean, but also a remarkable transition of the proportion of both types along the relatively small scale of the Italian coast. Detailed population genetic investigations and phylogeographic examinations within each genetic type datasets of P. elegans revealed evident and contrasting patterns of genetic structure in type II and type III. While the one-level analysis of molecular variance (AMOVA) resulted in significant genetic differentiation for both examined datasets, the outcome of multidimensional scaling (MDS), PERMUT, spatial analysis of molecular variance (SAMOVA) and two level-AMOVA analyses showed an obvious lack of phylogeographic structure within type II, versus significant population structuring within type III across known gene flow barriers in the Mediterranean. Notably, the outcome of isolation by distance (IBD) and isolation by environment (IBE) analyses within the type III dataset showed that both geographic and environmental distances were significant predictors of genetic distances. The remarkable difference in biogeographic patterns and demographic history [revealed mainly by Bayesian skyline plots (BSP) analysis] between both types suggests the likely involvement of different processes, including larval behaviour and/or residual effects of different phases of evolutionary history in the Mediterranean Sea.