Population structure of bowhead whales (balaena mysticetus) in Disko Bay, west Greenland

Skin biopsies of bowhead whales (Balaena mysticetus) were sampled during the spring aggregation in Disko Bay, West Greenland, over a period of 13 years, and analyzed regarding gender and genetic diversity at the mitochondrial D-loop region and at 11 microsatellite loci. By identifying recaptures through matching sex, mitochondrial haplotype and microsatellite genotype, individual interannual revisits to the bay were confirmed. These were further utilized to provide a mark-recapture population size estimate that applies to the source of the local aggregation in Disko Bay, which yielded 1219 bowhead whales (SE=278, 95%CI: 673-1765) and a corresponding estimate of 1087 female bowhead whales (SE=290, 95%CI: 518-1656) for 2012. Given that each adult female bowhead whale in the stock(s) between eastern Canada and West Greenland visits the sampling area during the reproductive cycle, the latter estimate is assumingly valid for the adult female proportion of the bowhead whales in these waters. A skewed sex ratio in the Disko Bay aggregation was observed, where females constitute an estimated proportion of 79% of the bowhead whales in the sampling area. As recent observations and early whaling records state that few calves are found in the area, Disko Bay is believed to serve as a feeding and mating ground, where adult females regain fat depots for their next calving period. The cyclicity in the female returns to the bay was thus assessed, which may arise from a multi-year migration pattern in relation to the female reproductive cycle. Although no conclusive results were obtained, a calving interval of four years would be most consistent with the data. Further, to test whether there was any substructuring of the stock in which different demes visit the bay in different years, the sampling years were analyzed with respect to both mitochondrial haplotypes and microsatellite genotypes. Global FST-value and an exact test of population differentiation were significant when based on the mitochondrial haplotypes across sampling years. When the microsatellites were investigated however, no global differentiation was detected. Slight differentiation was yet found among a few pairs of sampling years in both instances, although not coinciding among the markers. Hence, no obvious substructuring could be inferred from the data. The computer program STRUCTURE was additionally applied to the female bowhead whales sampled each year, finding that six clusters in the aggregation was the most likely number of clusters given the data. This was likely spurious and resulting from between-year recaptures, linked loci and close relatedness between the whales. In line with a star-shaped haplotype network and a sudden increase in the abundance in Disko Bay around the turn of this millennium, a population expansion was consequently implied. Although a recovery of the bowhead whale stocks after the extensive whaling during the 18th and 19th century is evident, close monitoring of the species is recommended in order to understand and manage it properly. This thesis is contributing to an extended dataset of bowhead whales in Disko Bay, of which most of its biology remains to be unveiled

Major histocompatibility complex and mate choice in two passerine birds

All organisms need to fight pathogens to avoid infectious diseases. In vertebrates, two classes of polymorphic genes from the major histocompatibility complex (MHC) are crucial for triggering immune responses against specific intracel-lular (MHC class I; MHCI) and extracellular (MHC class II; MHCII) pathogens. Whenever possible, females should choose mates that render an optimal MHC constitution in the offspring. Such genetic effects of mate choice can be elu-cidated by studying species where females engage in extra-pair mating. This thesis explores MHC-based extra-pair mate choice in two songbirds, using new sequencing technology. The hypervariable songbird MHC has been difficult to geno-type, but new methodology offers great improvements. For the bluethroat (Luscinia svecica), evidence for non-random, self-referential female choice of extra-pair males based on an intermediate optimum in MHCII diversity was found. For the willow warbler (Phylloscopus trochilus), females with few MHCI variants were more likely to engage in extra-pair mating. This thesis sheds light on the variation in MHC-based mate choice, and demonstrate an adaptive benefit of fe-male extra-pair mating in songbirds

Elevated phytohaemagglutinin‐induced skin‐swelling response at an intermediate number of MHC class II alleles in bluethroat nestlings

Genes of the major histocompatibility complex (MHC) are crucial for adaptive immunity, and theory predicts that the optimal number of alleles is intermediate rather than maximal. Support for such an intermediate optimum in MHC allele count has mainly been demonstrated in studies on parasite load and reproductive traits. In this study we tested if an intermediate number of MHC class II (MHCII) alleles based on positively selected sites was correlated with an increased skin-swelling response to two consecutive injections of phytohaemagglutinin (PHA) in bluethroat Luscinia svecica nestlings. We found that nestlings that were closer to the population mean in their number of alleles had a higher response to the first injection, supporting an intermediate, optimal number of alleles. After the second PHA injection, we found that the skin swelling was associated with MHCII allele count only at the among-brood level, and with paternity (extra-pair offspring had a larger swelling than within-pair offspring). We therefore conclude that the PHA response is partly correlated with MHCII allele count, but that additional factors must be involved in both the PHA response and in female extra-pair mate choice in this species

Extra‐pair mating in a passerine bird with highly duplicated major histocompatibility complex class II: Preference for the golden mean

Genes of the major histocompatibility complex (MHC) are essential in vertebrate adaptive immunity, and they are highly diverse and duplicated in many lineages. While it is widely established that pathogen‐mediated selection maintains MHC diversity through balancing selection, the role of mate choice in shaping MHC diversity is debated. Here, we investigate female mating preferences for MHC class II (MHCII) in the bluethroat (Luscinia svecica), a passerine bird with high levels of extra‐pair paternity and extremely duplicated MHCII. We genotyped family samples with mixed brood paternity and categorized their MHCII alleles according to their functional properties in peptide binding. Our results strongly indicate that females select extra‐pair males in a nonrandom, self‐matching manner that provides offspring with an allelic repertoire size closer to the population mean, as compared to offspring sired by the social male. This is consistent with a compatible genes model for extra‐pair mate choice where the optimal allelic diversity is intermediate, not maximal. This golden mean presumably reflects a trade‐off between maximizing pathogen recognition benefits and minimizing autoimmunity costs. Our study exemplifies how mate choice can reduce the population variance in individual MHC diversity and exert strong stabilizing selection on the trait. It also supports the hypothesis that extra‐pair mating is adaptive through altered genetic constitution in offspring

Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single-nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer-winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among-male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone. The final version of this research has been published in Journal of Avian Biology. © 2016 Wile