Genetic differentiation between sampling years

Analyzing the female bowhead whales sampled in 2005-2012, i.e. the years with reasonable numbers of samples, the analysis of molecular variance (AMOVA) of both the mitochondrial DNA and the microsatellites revealed that <<5% of the total molecular variation was due to differences among sampling years. The global FST-values and the global exact test of

population differentiation between sampling years were significant based on mitochondrial haplotypes. However, the same analyses conducted on the microsatellites did not reveal any significant differences between the sampling years. The pairwise tests between sampling years yielded diverging answers as well. When the mitochondrial haplotypes were investigated, eight pairs of sampling years showed a significant pairwise FST-value, which all

included the sampling years 2007, 2008 and 2010. On the other hand, all of the pairs resulting in significant pairwise FST-values in the microsatellite analysis contained the sampling year

2005. Additionally, 2005 was shown to differ significantly from other sampling years through rare faction of private allelic richness, but having a small sample size (N=17), no clear conclusions can be deduced. Further, none of the pairs of sampling years exhibited a significant P-value for the pairwise exact test of differentiation in the microsatellite analysis, but as the mitochondrial haplotypes were used, eight pairs of sampling years gave significant

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results. However, these were not the same pairs that had significant FST-values, with the

exception of 2007-2008, 2007-2009 and 2008-2010.

No simple conclusions regarding genetic differentiation among the sampling years can be inferred. Yet, the discrepancies resulting from the mitochondrial and microsatellite analyses may indicate a slight differentiation among the sampling years, which only the tests utilizing the mitochondrial haplotypes are able to detect. Other studies have as well remarked the strength of long mitochondrial fragments to reveal faint differences. Bachmann et al. (2010) found a weak genetic differentiation between the HB-FB and the BB-DS stocks when the mitochondrial D-loop region was investigated, which is contrasting the mitochondrial analysis of Alter et al. (2012). As suggested by the IWC (2012), more samples and the longer fragment of the mitochondrial DNA marker used by Bachmann et al. (2010) may have enhanced the analytical power to deduce minute differences between populations. The authors on their part argue that the genetic differentiation may only be a differentiation between Disko Bay and Foxe Basin and not the HB-FB and BB-DS stocks per se, and that there are similarities among the two stocks both in haplotype diversity and shared haplotypes. The same argumentation could be proposed for the results in this thesis as well. Even though the global FST (=0.02178)

significantly deviated from 0 analyzing the mitochondrial haplotypes among the females in sampling years 2005-2012, the value was small and pairwise comparisons found significant FST-values only for a subset of the pairs of sampling years. The sample sizes varied greatly,

and the haplotype diversity was high (>0.85) for every sampling year. As the mitochondrial DNA is inherited maternally in a haploid mode (Dawid and Blackler 1972; Hutchison et al. 1974; Wilson et al. 1985), the effective population size is only ¼ when compared with nuclear markers (Birky et al. 1983), and the sample sizes may thus be too small to yield reliable answers. On the other hand, the genetic drift is stronger in the mitochondrial than in the nuclear genome (Birky et al. 1983), and subtle differentiation may thus only be recognized by the mitochondrial marker. Despite the contradictive answers, it is important not to dismiss the hint on differentiation obtained through mitochondrial analyses as merely noise, but to acknowledge the possibility that some slight divergence may have been traced.

Although it is believed that bowhead whales mostly migrate solitary, pulses of migrating whales are observed, which likely is mediated through acoustic communication (Würsig and Clark 1993). If such groups of co-migrating whales are established along kin lines, as suggested by Rooney et al. (1999), the population could be subdivided into family groups. Given that these groups do not visit Disko Bay annually, such a substructure could influence the analyses of genetic differentiation among the years. This was in general not detected. However, a population substructure of female family groups visiting the bay at different years would render a result of more differentiation in the mitochondrial DNA than what would be detected through the microsatellites (see for instance O’Corry-Crowe et al. 2003). This kind of substructure would be consistent with the discrepancy in the mitochondrial and nuclear markers recognized in this study, even though the variability in the results remains unexplained. Exploratory analyses of mitochondrial haplotypes among the males yielded only a significant value for the exact differentiation test between 2005 and 2009, while global FST,

global exact differentiation test and pairwise FST were not significant, likely suffering from

insufficient sample sizes (data not shown). More samples of both sexes are needed in order to elucidate the hypothesis of genetic differentiation among sampling years further.

Matrilineal philopatry and male-biased dispersal could be additional explanation models for stronger differentiation in mitochondrial DNA than in nuclear markers (Rueness et al. 2003). Maternally directed philopatry to specific feeding aggregations has been observed for the migratory humpback whale (Megaptera novaeangliae; Palsbøll et al. 1995). As strong site

39 fidelity occurs in bowhead whales as well (Finley 1990), a subdivision of the stock could be indicated, which further could be traced through mitochondrial analyses. Whether this is the cause of the observed significant differences among the sampling years in the present study is not known, but it provides an additional explanation for the obtained results.

The FIS-values for the individual sampling years were only significantly deviating from 0 for

2010, and there was thus no indication of migration of closely related family groups. If the Disko Bay aggregation was substructured temporarily and/or along kin lines, heterozygote deficit or Hardy-Weinberg disequilibrium might be expected over all loci and in several if not all sampling years (Rousset and Raymond 1995; Rooney et al. 1999; see table 11). F-statistics have lately been criticized, and careful interpretation is needed due to the likely deviation from the assumption of mutation-drift equilibrium in bowhead whale stocks (Pearse and Crandall 2004; McLeod et al. 2012). However, FST is still used as a descriptive measurement

in this study, but needs to be interpreted in combination with the other obtained results. As errors in the microsatellite scoring would alter the allele frequencies and thus influence the applicable microsatellite analyses, it is important to be aware of them although eradication of such errors may be impossible (Bonin et al. 2004). For this thesis however, no obvious patterns in genetic differentiation congruent with a reproductive cycle of 3 or 4 years could be inferred, although a tendency of slight differentiation among the years could be implied from the analyses of the mitochondrial haplotypes.