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Mar Biol
2016 Jan 01;163:36. doi: 10.1007/s00227-015-2801-y.
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Genetic diversity of the NE Atlantic sea urchin Strongylocentrotus droebachiensis unveils chaotic genetic patchiness possibly linked to local selective pressure.
Norderhaug KM
,
Anglès d'Auriac MB
,
Fagerli CW
,
Gundersen H
,
Christie H
,
Dahl K
,
Hobæk A
.
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We compared the genetic differentiation in the green sea urchin Strongylocentrotus droebachiensis from discrete populations on the NE Atlantic coast. By using eight recently developed microsatellite markers, genetic structure was compared between populations from the Danish Strait in the south to the Barents Sea in the north (56-79°N). Urchins are spread by pelagic larvae and may be transported long distances by northwards-going ocean currents. Two main superimposed patterns were identified. The first showed a subtle but significant genetic differentiation from the southernmost to the northernmost of the studied populations and could be explained by an isolation by distance model. The second pattern included two coastal populations in mid-Norway (65°N), NH and NS, as well as the northernmost population of continental Norway (71°N) FV. They showed a high degree of differentiation from all other populations. The explanation to the second pattern is most likely chaotic genetic patchiness caused by introgression from another species, S. pallidus, into S. droebachiensis resulting from selective pressure. Ongoing sea urchin collapse and kelp forests recovery are observed in the area of NH, NS and FV populations. High gene flow between populations spanning more than 22° in latitude suggests a high risk of new grazing events to occur rapidly in the future if conditions for sea urchins are favourable. On the other hand, the possibility of hybridization in association with collapsing populations may be used as an early warning indicator for monitoring purposes.
Fig. 1. Map of the study area including sampling stations and a simplified illustration of the dominating currents along the coast from the Danish Belt Sea to the Barents Sea. Green arrows indicate the northbound coastal current, and red arrows indicate ocean currents from the NE Atlantic. Red circles and white boxes show the position and codes of the sample stations. See Table 1 for explanation of the codes. Oslofjorden (IO and D2), Lysefjorden (LY), Salangen (SI and SY) and Isfjorden (KW) are sill fjords, whereas the fjords represented by NH, NS VI and FV are open fjords/coastal areas. DV is located in the Danish Straits. The squares indicates groups (South and North) used in Migrate analysis
Fig. 2. Allele frequencies by populations shown for locus Strdro-1356. See Supplement 1 for the other 7 loci
Fig. 3. Clustering of 8-locus genotypes with Structure in two groups (K = 2 preferred by both log probability of K and by the Evanno method). Group 1, coloured in orange, gathers most individuals and populations, whereas group 2, coloured in blue, is found in NS, NH and about half of FV individuals
Fig. 4. Genetic versus geographical distance. Scaled differentiation (F
ST/(1 − F
ST)) for each population compared with the southernmost Belt Sea (DV) population. Based on the Structure results shown in Fig. 3, NH and FV are both split in two, NH1, NH2, FV1 and FV2. Statistics are based on the linear regression between the two variables, with the intercept forced to 0 (Danish population, DV)
Fig. 5. UPGMA (unweighted pair group means analysis) tree for the COI sequences using Kimura 2-parameter substitution model. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (10,000 replicates) is shown next to the branches. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. In addition to the 12 S. strongylocentrotus individuals sequenced for this study shown by full blue circles, 12 sequences were obtained from GenBank: 4 S. strongylocentrotus, 4 S. pallidus and 4 S. purpuratus sequences. All access numbers are indicated in the figure
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