Patterns of genetic variation of a Lessepsian monogenoidean parasite

After the opening of the Suez Canal in 1869 and after the further changes of the water regime of the canal and the near area of the Eastern Mediterranean Sea, the biogeographical barrier existing until that moment, between the Red Sea and the Mediterranean Sea, has been, as time goes by, weakened, allowing the contact between two different biota that was separated for 12 Million years. This work is focused on the mechanisms that set the spread and the colonization of Mediterranean Sea from Glyphidohaptor plectocirra, monogenoidean parasite of rabbit fish Siganus rivulatus (Siganidae), checking also the relationship between the spread of the host species, known to bibliography, and of the parasite, experimentally analysed. Based on available literature, several genetic studies of Lessepsian species often demonstrate the absence of a genetic bottleneck in a wide plethora of taxa, from plants to fish, but information regarding the genetic responses of their parasites in the newly colonized ecosystems is still lacking. In detail, in this work, the genetic flow relationships between five populations was estimated, three from the Red Sea (Nabq and Ismalia, Egypt, and Eilat, Israel) and two from Mediterranean Sea (Rhodes Island, Greece and Tel Aviv, Israel) by sequencing a portion of the mitochondrial CoxI gene. G. plectocirra specimens were extracted from gill arches of S. rivulatus and morphologically identified by analysing haptor sclerotized structures. After the morphological identification of specimens, two DNA regions were amplified, one mitochondrial, about 688 bps long, coding for the Cytochrome Oxidase I (COI) and one nuclear, 681 bps long, coding for the terminal region of the 18S gene, that include the entire ITS1 and a portion of the 5.8S gene, was then amplified using primers S1. The 681 bp alignment of 60 rDNA sequences found in all populations showed only two polymorphic sites, which distinguished only three haplotypes (data not shown). Thus, the low variability of this marker at the species level in the monogenoidean parasites was confirmed, and any downstream analysis was performed only on the CoxI gene. For CoxI marker about 197 sequences were obtained for a total of 58 haplotypes, showing 65 polymorphic sites (27 parsimony informative), and no gaps were observed. In the original populations were identified more than 30 haplotypes, half of which was private; while in the Mediterranean population were identified about 20 haplotypes, just under half of which was private. In general the values of haplotype and nucleotide diversity were higher within the origin population. The maximum parsimony network obtained showed absence of geographic structure, and is characterized by a star shape: by an ancestral haplotype are derived a large number of derived haplotypes, which are very similar to each other. Indeed, despite the large number of variants, derived haplotypes differ from the central one by a low number of mutations and, with very few exceptions, are equally low represented; in most of the cases derived haplotypes were identified in just one individual. The analysis of molecular variance showed that the Mediterranean populations may be considered a subgroup with reduced variability but that the genetic variance is attributable mostly to differences within individual populations. The maximum parsimony network, with star shape, is characterized by one central haplotype from which all the others derive. The analysis of molecular variance confirm that the genetic variance is attributable for 97% to differences within the populations, supporting that the Mediterranean populations are a subgroup of Red Sea, even if they show some private haplotypes. An analysis of Bayesian inference showed a significant unidirectional genetic flow, from Red Sea to Mediterranean Sea. While it even showed a large genetic dimension of Mediterranean populations that confirms the presence of different colonization source, not sampled, in Red Sea. Despite evidence of a slight decrease in the genetic diversity of Mediterranean populations, a simulation analysis based on coalescent theory demonstrated the absence of significant bottlenecks, but there was directional selection along a cline moving further from the Suez Canal. The absence of bottlenecks was congruent with that described for G. plectocirra hosts Siganus rivulatus and Siganus luridus, and reflected a common history of high propagule pressure during initial colonization, and constant or repeated gene flow from the Red Sea to the Mediterranean area. However, directional selection was peculiar to the parasites and likely originated from parasite genotype  environment interactions. Finally, an anisotropic contribution of Red Sea populations to the Lessepsian invasion was demonstrated.