Witnessing The Rise Or Fall Of A Species: Introgressive Hybridization Of Dolphins In The Greek Seas (Stenella coeruleoalba And Delphinus delphis)

Under some circumstances, it is possible for members of two different species to mate and produce viable offspring. This phenomenon is called hybridization and is far more common in natural populations than previously thought, as suggested by the findings of numerous scientific studies.

Hybrids can either inherit morphological characteristics from both their parents (morphological intermediates) or might more resemble one of their parents, rendering their detection by scientists challenging/daunting. This task becomes even more laborious when studying species that are difficult to observe, such as marine mammals. Such problems are alleviated when scientists seek DNA evidence for hybrid detection and molecular biology tools come into play. At the genetic level, hybrids always carry material from both parents (bi-parentally inherited markers), making their detection by genetic means highly accurate. Cases of hybridization that lead to viable offspring that are able to reproduce have implications on the genetic variability within individuals upon which natural selection can operate and, at a broader scale, on fundamental evolutionary processes such as speciation.

A challenging task that follows a hybrid detection is to define the conditions under which this phenomenon is favored and decipher their evolutionary implications/advantages, considering at the same time demographic and life history traits as well as habitat disturbance as promoting factors. Studying hybridization in natural populations offers a unique opportunity for the study of incipient evolution.

Cetaceans are marine mammals (whales, dolphins, and porpoises) that occur in mixed groups, thus providing excellent opportunities for interspecific sexual interaction and hybridization potential. The fact that almost 20% of cetacean species hybridize both in the wild and in captivity, as shown in a recent review by Crossman and colleagues (2016), is indicative of the incompleteness of pre- and post-mating barriers to interbreeding in this group. This high percentage is due to lack of incompatibilities in the number and arrangements of chromosomes attributed to the groups’ slow evolutionary rate and recent evolutionary radiation. In turn, these might be indicative of cetacean’s greater potential with respect to other mammals, to hybridize and generate viable and fertile progeny.

The Greek Seas host a large diversity of marine habitats where 11 cetacean species are present. The Gulf of Corinth (GOC) constitutes a unique and very interesting area where three sympatric dolphin species form permanent mixed-species groups, providing excellent opportunities for interspecific sexual interaction and the potential for hybridization. This became even more interesting when, during the previous year’s observations on dolphins in the GOC, there were reports on individuals with unusual pigmentation patterns that were intermediate between two inhabitant species of dolphins: namely, the striped dolphin (Stenella coeruleoalba) and the short-beaked common dolphin (Delphinus delphis). This phenomenon could either be the result of interspecific hybridization between the two aforementioned species or might represent another incident of the high morphological variability in the pigmentation pattern of the striped dolphin.

Recently, a scientific team, which includes researchers from Greek institutions (Natural History Museum of Crete-University of Crete, Institute of Marine Biology, Biotechnology and Aquaculture-HCMR, and Pelagos Cetacean Research Institute), attempted to shed light on whether natural hybridization is the cause and understand the evolutionary processes that were responsible for the presence of these intermediate morphs. In order to accomplish that, they collected skin samples from individuals of the two species of dolphins as well as from the intermediate morphs and conducted a DNA-based study by means of two types of genetic markers (bi-parentally inherited high resolving microsatellite DNA markers and maternally transmitted mitochondrial DNA sequences) serving as two independent sources of information.

By employing several distinct analysis approaches, they were not only able to genetically discriminate the two species but also to discover 15 individuals as putative hybrids. The intermediate morphs also carried DNA of both parental species (in respect of the genome that is inherited by both parents,) i.e., they were admixed, displaying unique variation indicative of recent hybridization events between the two species. This led scientists to conclude that the two species do hybridize. Furthermore, there is evidence that there is no reproductive isolation between hybrids and their parental species since the hybrids are fertile and able to reproduce not only with other hybrids but also with each of the two parental species (stemming from the discovery of hybrids resulting from backcrosses with each of the parental species). Another striking fact is that although the two dolphin species are known to have different ecological and dietary needs, those distributed in the GOC do display common characteristics with D. delphis having adapted their behaviors, habitat preference, and, likely, dietary needs to coexist with S. coeruleoalba.

Based on the findings of the study, it is highly probable that crosses between males of D. delphis with females of S. coerueoabla are more common than other crosses mostly due to the low abundance of potential conspecific mates for D. delphis, jeopardizing its genetic integrity (a phenomenon called Hubb’s principle or “desperation hypothesis”). This is something already reported for porpoises. Such a pattern could also be affected by the occurrence of organochlorine pollutants and/or polychlorinated biphenyls, which is of high conservation concern. New evidence provided by the study led to an inevitable rise of new scientific questions. The new challenge that researchers will need to address next is whether hybridization will lead to the generation of a new species and/or the permanent diminishing of an existing species in the study area, which occurs in low density. In any case, scientists urge that the evaluation of the genetic status of common dolphins in the Mediterranean is deemed indispensable.