Mixing of porpoise ecotypes in southwestern UK waters revealed by genetic profiling

Contact zones between ecotypes are windows for understanding how species may react to climate changes. Here, we analysed the fine-scale genetic and morphological variation in harbour porpoises (Phocoena phocoena) around the UK by genotyping 591 stranded animals at nine microsatellite loci. The data were integrated with a prior study to map at high resolution the contact zone between two previously identified ecotypes meeting in the northern Bay of Biscay. Clustering and spatial analyses revealed that UK porpoises are derived from two genetic pools with porpoises from the southwestern UK being genetically differentiated, and having larger body sizes compared to those of other UK areas. Southwestern UK porpoises showed admixed ancestry between southern and northern ecotypes with a contact zone extending from the northern Bay of Biscay to the Celtic Sea and Channel. Around the UK, ancestry blends from one genetic group to the other along a southwest--northeast axis, correlating with body size variation, consistent with previously reported morphological differences between the two ecotypes. We also detected isolation by distance among juveniles but not in adults, suggesting that stranded juveniles display reduced intergenerational dispersal. The fine-scale structure of this admixture zone raises the question of how it will respond to future climate change and provides a reference point for further study.

. Microsatellite loci used in this study.       (c) Geographic distribution of the parental pure populations from the southern and northern ecotype of harbour porpoise and the hybrid. Pure individuals were identified with Structure analysis as individuals having their multilocus genotypes assigned with > 80% probability to their respective cluster or as hybrids if the probability was < 80% (see figure 2). Only the Iberian (IB) population from the southern ecotype is shown. Hybrids (HYB) porpoises are geographically restricted to the northern side of the Bay of Biscay, Celtic Sea, and SW UK, with some individuals found in the western side of the channel and in Scotland. Figure S7. (a) Geographic locations of the harbour porpoises sampling (n=591) based on GPS coordinates or reported discovery location. Locations have been subdivided into 6 regions around UK and color-coded accordingly. Genetic structure of harbour porpoises in UK waters at K=2, as estimated by Structure, is displayed as the posterior admixture estimates averaged per regions. Panel (b) shows the individual admixture proportions. Each individual is represented by a column and the probability of that individual belonging to each cluster is indicated by coloured segments. Admixture proportions from Structure are based on the highest probability run (of ten) at that value of K=2. Figure S8. Allelic richness estimated for a minimum sample size of n=2 and its variance using ADZE. Spatially interpolated surfaces were computed using an inverse distance weighted method on a gridded space of 1º. Red circle show each sample location and the black dots show the pixel where n ≥ 2 for which Ra was estimated on the local sampling.

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Figure S1. Geographic distribution of the sampling of harbour porpoise stratified by age class or by sex.    Structure analysis as individuals having their multilocus genotypes assigned with > 80% probability to their respective cluster or as hybrids if the probability was < 80% (see figure 2). Only the Iberian (IB) population from the southern ecotype is shown. Hybrids (HYB) porpoises are geographically restricted to the northern side of the Bay of Biscay, Celtic Sea, and SW UK, with some individuals found in the western side of the channel and in Scotland.

(a) (b) (c)
10/11 Figure S7. (a) Geographic locations of the harbour porpoises sampling (n=591) based on GPS coordinates or reported discovery location. Locations have been subdivided into 6 regions around UK and color-coded accordingly. Genetic structure of harbour porpoises in UK waters at K=2, as estimated by Structure, is displayed as the posterior admixture estimates averaged per regions. Panel (b) shows the individual admixture proportions. Each individual is represented by a column and the probability of that individual belonging to each cluster is indicated by coloured segments.
Admixture proportions from Structure are based on the highest probability run (of ten) at that value of K=2.

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Figure S8. Allelic richness estimated for a minimum sample size of n=2 and its variance using ADZE. Spatially interpolated surface were computed using an inverse distance weighted method on a gridded space of 1º. Red circle show each sample location and the black dots show the pixel where n ≥ 2 for which Ra was estimated on the local sampling.