Heterozygosity at neutral and immune loci is not associated with neonatal mortality due to microbial infection in Antarctic fur seals

Abstract

Numerous studies have reported correlations between the heterozygosity of genetic markers and fitness. These heterozygosity–fitness correlations (HFCs) play a central role in evolutionary and conservation biology, yet their mechanistic basis remains open to debate. For example, fitness associations have been widely reported at both neutral and functional loci, yet few studies have directly compared the two, making it difficult to gauge the relative contributions of genome‐wide inbreeding and specific functional genes to fitness. Here, we compared the effects of neutral and immune gene heterozygosity on death from bacterial infection in Antarctic fur seal ( Arctocephalus gazella ) pups. We specifically developed a panel of 13 microsatellites from expressed immune genes and genotyped these together with 48 neutral loci in 234 individuals, comprising 39 pups that were classified at necropsy as having most likely died of bacterial infection together with a five times larger matched sample of healthy surviving pups. Identity disequilibrium quantified from the neutral markers was positive and significant, indicative of variance in inbreeding within the study population. However, multilocus heterozygosity did not differ significantly between healthy and infected pups at either class of marker, and little evidence was found for fitness associations at individual loci. These results support a previous study of Antarctic fur seals that found no effects of heterozygosity at nine neutral microsatellites on neonatal survival and thereby help to refine our understanding of how HFCs vary across the life cycle. Given that nonsignificant HFCs are underreported in the literature, we also hope that our study will contribute toward a more balanced understanding of the wider importance of this phenomenon.

Little evidence for inbreeding depression for birth mass, survival and growth in Antarctic fur seal pups

“…However, we did not find any effects of individual or maternal inbreeding on pup birth mass or survival after controlling for the confounding effects of pup sex and breeding season. The clear absence of inbreeding depression for these traits in the current study as well as in two previous studies 53,56 implies that viability selection against inbred pups is weak or absent during the first three months of life. This is in contrast to the situation with harbour seals 14 , grey seals 61 and California sea lions 62 , where microsatellite heterozygosity is strongly associated with neonatal survival.…”

“…These pups were weighed at birth and subsequently recaptured and weighed again at a mean age of 49 days (range = 12–89 days). Both the pups and their mothers were genotyped at 39 microsatellites that have previously been shown to be in Hardy-Weinberg equilibrium and linkage equilibrium in the study population 53,54 . The second dataset comprised 100 mother-offspring pairs (i.e.…”

“…Our study complements and builds upon two previous studies of inbreeding depression in Antarctic fur seal pups. The first of these found no effects of individual or maternal heterozygosity at nine microsatellites on pup birth mass and survival 56 and the second found no effects of individual heterozygosity at 48 microsatellites on neonatal mortality due to bacterial infection 53 . However, both of these studies had important limitations.…”

Intronic primers reveal unexpectedly high major histocompatibility complex diversity in Antarctic fur seals

“…The relatively high allelic diversity that we report in the current study is not necessarily unexpected for a polygynous, colonially breeding pinniped where a major cause of adult (but not neonatal) mortality is bacterial infection 63 , 73 . To investigate whether natural selection could be responsible for maintaining this diversity, it would be necessary to test for associations between MHC genotype and fitness components such as survival, body size and reproductive success, which are already known to be influenced by an individual’s genotype in this species 45 , 46 , 48 , 60 .…”

Genotyping of selection-significant polymorphisms of cattle of the Western Siberia

“…Molecular genetic methods for controlling genetic diversity are currently the subject of scientific discussion [12,13,14]. The most informative method for monitoring heterozygosity has not yet been determined.…”