Mitochondrial DNA based diversity over several Mediterranean regions in holothurian species.

Sea cucumbers are deposit feeding members of marine benthic communities which play a key role in sea floor dynamics by processing and bioturbating the sediment. Many of these species are considered economically relevant as luxury food and for preparations in traditional medicine in Asia. This importance has caused an over-exploitation of many natural stocks, especially in the Mediterranean area where the excessive fishing has caused a biodiversity loss among populations with a negative impact on the marine ecosystem. For these reasons, few initiatives are recently evaluating sea cucumbers as novel aquaculture species. The aim of this study was to evaluate the level of genetic diversity within two sympatric Holothuria sea cucumber species, H. polii and H. tubulosa, across different Mediterranean areas, before considering the implementation of conservation actions and as a first step to establishing farmed stock populations. We sequenced informative portions of cox1 and 16S mitochondrial genes in a total of 240 holothurians (120 H. polii and 120 H. tubulosa). Sea cucumbers have been sampled from three sites across Mediterranean Sea on different seabed typologies (sand, rock, Posidonia meadows). Genetic divergence (p-distance) within and between species has been calculated and two Neighbour Joining (NJ) phylogenetic trees have been generated from reduced datasets, including six samples per species per site and three outgroups (H. scabra, H. forskali and H. mammata). While the average p-distance was similar between and within species (0.10 and 0.09, respectively), the phylogenetic analyses showed a clear differentiation between the two species. Moreover, the outgroup H. mammata clustered as a sister group of H. tubulosa clade, as expected. Evidences of heteroplasmy in mtDNA in few holothurians were detected, suggesting that some hybridization events might have occurred between these sympatric species. The NJ trees showed no differentiation between site populations, indicating active gene flows among Mediterranean areas. The results provide genetic markers useful for species identification and an overview of the genetic variability in three Mediterranean populations of H. polii and H. tubulosa. Further studies are needed to disentangle the origin of the mtDNA heteroplasmy reported in a few samples. These data will be useful to establish farmed stocks with high level of genetic variability.