Cytochrome b marker reveals an independent lineage of Stenella coeruleoalba in the Gulf of Taranto

Abstract

Heterogeneity in geomorphological and hydrographical conditions throughout the Mediterranean Sea could be the driving factors behind the significant differences between putative sub-populations, although the existence of a large panmictic population of striped dolphin Stenella coeruleoalba (Meyen 1833) in this marine region could not be excluded. However, understanding the ecological implications of such genetic differentiation is difficult, as inferences about gene flow are usually made on evolutionary time scales and not along the ecological time frame over which most management and conservation practices are applied. In fact, as stated by the IUCN Red List, in the case of species assessed as vulnerable, the degree of genetic exchange between populations within a biogeographic region and its ecological implications represent a fascinating challenge that should be very deeply explored. This is even more significant in the Gulf of Taranto (Northern Ionian Sea, Central-eastern Mediterranean Sea), where the geomorphological and hydrographic characteristics support the hypothesis of a separated striped dolphin population genetically diverging from its original Mediterranean counterpart. To assess this hypothesis, a genetic analysis was carried out on DNA fragments of the mitochondrial cyt b gene to explore the evolutionary origin of S. coeruleoalba in the investigated area and its genetic diversity in comparison with available sequences from other Mediterranean and Atlantic populations. Results were discussed indicating ecological implications and suggesting conservation objectives. Moreover, a delphinid systematic was also suggested.

Fig 1. Map of the Gulf of Taranto (Northern Ionian Sea, Central-eastern Mediterranean Sea) indicating the survey area.

Red triangles indicate the fifteen sampling groups sites (see Table 1).

Fig 2. Median-joining network of cytochrome b haplotypes from S. coeruleoalba.

Circle size is proportional to the number of individuals exhibiting that haplotype (H_1 represents a single animal and H_10 represents 18 animals). Colours indicate the geographic origin of the samples. Mutation events separating haplotypes are indicated as "hatch marks". Black circles indicate missing, intermediate haplotypes.

Fig 3. NJ tree inferred from Delphinidae cyt b gene sequences.

Evolutionary analysis was conducted using MEGA7 [42]. The optimal tree, with a sum of branch length = 0.45049170, is shown. The tree is drawn to scale with branch lengths in the same units as those of the evolutionary distances used to infer phylogenetic trees. The evolutionary distances were computed using the p-distance method [44] and are in the units of the number of base differences per site. The analysis involved 95 nucleotide sequences. Codon positions included were 1st+2nd+3rd+Noncoding. All positions containing gaps and missing data were eliminated. There were 421 positions in the final dataset. The different coloured circles represent the distribution of the phylogenetic groups. Red squares indicate the S. coeruleoalba haplotypes of the Gulf of Taranto (this study). Nodes labelled A-E indicate the clades discussed further in the text.