Population Structure as Revealed by mtDNA and Microsatellites in Northern Fur Seals, Callorhinus ursinus
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Population Structure as Revealed by mtDNA and Microsatellites in Northern Fur Seals, Callorhinus ursinus B. R. Dickerson1, R. R. Ream1, S. N. Vignieri2, P. Bentzen3 and G. A. Antonelis4 1Molecular Ecology Research Laboratory, National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, Seattle, WA 98115, USA 2Departmentof Organismic and Evolutionary Biology, Harvard University, Museum of Comparative Zoology, 26 Oxford St., Cambridge, MA 02138, USA 3Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada 4Pacific Islands Fisheries Science Center, National Marine Fisheries Service, NOAA, 2570 Dole St., Honolulu, Hawaii, USA Introduction The northern fur seal, Callorhinus ursinus, is a widely 132 distributed philopatric species that breeds on a limited 165 Results 70 number of islands across the North Pacific Ocean (Fig. 1). 1 The species has undergone large fluctuations in abundance Microsatellites during the past two centuries. To date there have been no - Maximum likelihood tests looking at number of potential populations using molecular studies of genetic variation or population structure. an admixture model showed the islands best grouped into one population. 71 11 The goal of this project was to examine population structure - FST and RST were extremely low and insignificant following Bonferroni in northern fur seals, throughout their entire range, using corrections: 41 15 83 13 both mtDNA and microsatellites. 33 - All populations FST = 0.0004, RST = -0.0041 87 74 66 - Population pairs FST ≤ 0.0043 , RST ≤ 0.0373 134 31 -Tests for isolation by distance were also insignificant. 10 172 75 1 Substitution mtDNA -Tests for differentiation between the 6 regional population groups (Robben I., 136 Kuril Islands (Lovushki and Srednev), Commander Islands (Bering and Medny), Bogoslof I., Pribilof Islands (St Paul and St George) and San Miguel I.) were not significant (AMOVA, p = 0.87). - Pairwise comparisons of conventional FST values revealed little differentiation but ΦST values were higher and showed some difference between the Russian animals and those in the eastern range (Table 1). Figure 3. Minimum spanning network of 112 core mitochondrial DNA haplotypes of northern fur seals samples for this study. Branch lengths are the minimum number of steps between haplotypes. The size of the circle representing the individual haplotypes corresponds to the abundance of that haplotype. - Although not significant there does appear to be a trend toward isolation Numbers identify the most abundant haplotypes. Dashed lines represent alternative groupings. by distance (p = 0.27; Figure 2). - Phylogeny analysis results (neighbor joining tree) were inconclusive with 0.25 few nodes (7%) supported by >60% in the bootstrap analysis and no geographic concordance. 0.2 - The minimum spanning network showed 4 distinct maternal lineages, although there was no geographic concordance (Figure 3). Frequency 0.15 observed Figure 1. Distribution of northern fur seal breeding sites. - The nucleotide frequency mismatch distribution matched that of a model expected of sudden expansion (Figure 4). 0.1 0.05 Table 1. mtDNA based population differentiation and structure for population pairs (estimates of FST above diagonal Methods and ΦST below diagonal). Bold indicates significant P-values (P < 0.05). 0 Robben Lovushki Srednev Bering Medny Bogoslof St Paul St George San Miguel 0 5 10 15 20 25 Flipper punches were taken from 728 individuals during the 1993-1998 Pairwise Differences Robben -0.001 -0.001 -0.002 -0.001 -0.003 0.000 -0.003 0.000 and 2005 summer breeding seasons from nine islands: Bering Island (N Lovushki 0.013 0.000 0.001 0.001 0.001 0.002 0.002 0.001 = 55), Bogoslof Island (N = 99), Lovushki Island (N = 61), Srednev Rock (N = 50), Medny Island (N = 56), Robben Island (N = 50), St. George Srednev 0.018 -0.003 0.002 0.002 0.002 0.004 0.000 0.002 Figure 4. Observed pairwise mismatch distribution of mtDNA in northern fur seals compared with the distribution of a model of sudden population Island (N = 100), St. Paul Island (N = 113) and San Miguel Island (N = Bering 0.017 0.010 0.02 0.000 -0.002 0.000 -0.001 0.002 expansion. 144; Fig. 1). Medny 0.010 -0.007 0.006 0.012 -0.001 0.002 0.000 0.002 Bogoslof 0.010 -0.001 0.015 0.02 -0.007 0.002 -0.001 0.002 Discussion To test for genetic variability and population differentiation between St Paul 0.019 -0.002 -0.002 0.017 -0.001 0.007 0.003 0.005 islands, seven microsatellite loci and 381 bp from the mtDNA control St George 0.012 -0.003 0.006 0.025 -0.008 -0.002 0.000 0.000 region were used. Allelic variability for the microsatellite loci were high San Miguel 0.024 -0.000 0.011 0.024 -0.004 0.009 0.003 -0.001 Our microsatellite analysis did not show any structure across the range but based on with heterozygosities ranging from 0.25 to 0.93. mtDNA haplotype pairwise comparisons of ΦST values there does appear to be some separation variability was also high with overall haplotypic diversity at 0.994 and between the Russian and eastern populations based on the mtDNA analysis. Since nucleotide diversity moderate at 2.4%. From the 619 individuals we were mtDNA is maternally inherited movement between populations by males does not able to sequence we found 87 variable sites resulting in 332 haplotypes influence an examination of population structure. So, under conditions of high male of which 227 were represented by a single individual. migration and low female migration you would expect to see higher levels of structure 0.03 when looking at mtDNA compared to microsatellites which are inherited in the Estimates of FST , ΦST (mtDNA), and RST (microsatellites), were Mendialian fashion. These results suggest that as in other pinniped species males calculated and pairwise comparisons and AMOVA were used to asses 0.025 are probably more apt to immigrate than females. Although, given the low levels of population structure. The maximum likelihood program Structure was 0.02 structure that were found and the observed levels of philopatry in northern fur seals, used to estimate the most likely number of populations the islands these results cannot be explained by male migration alone. The shapes of the 0.015 minimum spanning network (Figure 3) and the pairwise mismatch distribution (Figure represented based on the microsatellite results. Isolation by distance ΦST \1- ΦST was examined for both markers and a neighbor joining tree, nucleotide 0.01 4) suggest that suggest that the population has recently (evolutionarily) undergone mismatch distribution and minimum spanning network were created for rapid population expansion, possibly as a result of populations expansion and the 0.005 the mtDNA data. subsequent recolonization of modern day breeding sites following the retreat of the 0 Pleistocene ice sheets and in a more recent time frame, expansion since the 2 4 6 8 10 cessation of hunting pressures. -0.005 -0.01 The results do not prove that population structure is absent in the northern fur seal. The extensive geographic separation of breeding islands and the foraging habitat -0.015 used around these islands, differences in population dynamics, high levels of Ln Distance (km) philopatry, the slight heterogeneity observed in microsatellite allele frequencies over all populations, and low levels of differentiation in mtDNA contradict this notion. When Figure 2. Isolation by distance based on mitochondrial DNA analysis in northern fur seals. Relationship between genetic distance, pairwise comparisons of examining population structure it is important to keep in mind that changes in F- rookeries (ΦST) and the natural log of the geographic distance between the rookery statistics increase very slowly after reductions in migration occur and even the use of pairs. markers with high mutation rates still result in a long lag time between demographic and genetic differentiation. It is important, particularly when using neutral markers, to look at more than just genetic differentiation to determine structure. Overall, further investigation into the population structure present in this species may allow for the The recommendations and general content presented in this poster do not necessarily represent the views or identification of the population processes which are actually occurring, be they due to official position of the Department of Commerce, the National Oceanic and Atmospheric Administration, or the migration (both resent and historic), mating strategy, high population numbers, or a National Marine Fisheries Service. complicated small-scale polymorphic pattern.