No abstract

Sexual selection has been proposed as one mechanism to explain the maintenance of high allelic diversity in MHC genes that control the extent of resistance against pathogens and parasites in natural populations. MHC-based sexual selection is known to involve olfactory mechanisms in fish, mice, and humans. During mate choice, females of the three-spined stickleback (Gasterosteus aculeatus) use an odor-based selection strategy to achieve an optimal level of MHC diversity in their offspring, equipping them with optimal resistance toward pathogens and parasites. The molecular mechanism of odor-based mate-selection strategies is unknown. Because peptide ligands for MHC class I molecules function as individuality signals in mice, we hypothesized that female sticklebacks might assess the degree of MHC diversity of potential partners by means of the structural diversity of the corresponding peptide ligands in perceived odor signals. We show that structurally diverse MHC ligands interact with natural odors of male sticklebacks to predictably modify MHC-related mate choice. For a mating pair with suboptimal numbers of MHC alleles, peptides increase the attractiveness of male water, whereas for a mating pair with superoptimal numbers, attractiveness is decreased. Our results suggest that female sticklebacks use evolutionarily conserved structural features of MHC peptide ligands to evaluate MHC diversity of their prospective mating partners.sexual selection ͉ olfaction ͉ immunogenetics T he maintenance of high allelic diversity in MHC genes that control the extent of resistance against pathogens and parasites in natural populations has been explained by sexual selection (1, 2). MHC-based sexual selection is known to involve olfactory mechanisms in fish (3, 4), mice (5-7), and humans (8, 9). In natural populations of the three-spined stickleback (Gasterosteus aculeatus), individuals with an intermediate number of different MHC alleles are the most frequent genotype (3, 10) that both under field (10) and experimental conditions (11, 12) best resist natural parasites. During mate choice, female sticklebacks use an odor-based selection strategy to achieve this optimal level of MHC diversity in their offspring (3, 4). Therefore, male sticklebacks are postulated to produce and release MHC-related odors that provide information about the individual's composition of MHC alleles. It is also clear that, although the mechanism is as yet unknown, female sticklebacks are capable of assessing the degree of MHC diversity of their prospective mating partners. Because genes of the MHC complex are highly polymorphic and encode structurally related but distinct MHC molecules (13,14), natural chemosignals that function in MHC-related mate choice must be expected also to be structurally polymorphic, and the extent of their structural diversity should be a function of the number of structurally different MHC molecules expressed by an individual. With regard to the nature of this signal, MHC molecules or their fragments, degradation products of t...

Microbiota provide their hosts with a range of beneficial services, including defense from external pathogens. However, host-associated microbial communities themselves can act as a source of opportunistic pathogens depending on the environment. Marine poikilotherms and their microbiota are strongly influenced by temperature, but experimental studies exploring how temperature affects the interactions between both parties are rare. To assess the effects of temperature, temperature stress and infection on diversity, composition and dynamics of the hemolymph microbiota of Pacific oysters (Crassostrea gigas), we conducted an experiment in a fully-crossed, three-factorial design, in which the temperature acclimated oysters (8 or 22 1C) were exposed to temperature stress and to experimental challenge with a virulent Vibrio sp. strain. We monitored oyster survival and repeatedly collected hemolymph of dead and alive animals to determine the microbiome composition by 16s rRNA gene amplicon pyrosequencing. We found that the microbial dynamics and composition of communities in healthy animals (including infection survivors) were significantly affected by temperature and temperature stress, but not by infection. The response was mediated by changes in the incidence and abundance of operational taxonomic units (OTUs) and accompanied by little change at higher taxonomic levels, indicating dynamic stability of the hemolymph microbiome. Dead and moribund oysters, on the contrary, displayed signs of community structure disruption, characterized by very low diversity and proliferation of few OTUs. We can therefore link short-term responses of host-associated microbial communities to abiotic and biotic factors and assess the potential feedback between microbiota dynamics and host survival during disease.

Parasite mediated selection may result in arms races between host defence and parasite virulence. In particular, simultaneous infections from multiple parasite species should cause diversification (i.e. balancing selection) in resistance genes both at the population and the individual level. Here, we tested these ideas in highly polymorphic major histocompatibility complex (MHC) genes from three-spined sticklebacks (Gasterosteus aculeatus L.). In eight natural populations, parasite diversity (15 different species), and MHC class IIB diversity varied strongly between habitat types (lakes vs. rivers vs. estuaries) with lowest values in rivers. Partial correlation analysis revealed an influence of parasite diversity on MHC class IIB variation whereas general genetic diversity assessed at seven microsatellite loci was not significantly correlated with parasite diversity. Within individual fish, intermediate, rather than maximal allele numbers were associated with minimal parasite load, supporting theoretical models of self-reactive T-cell elimination. The optimal individual diversity matched those values female fish try to achieve in their offspring by mate choice. We thus present correlative evidence supporting the 'allele counting' strategy for optimizing the immunocompetence in stickleback offspring.

Summary1. Our study addresses the role of non-genetic and genetic inheritance in shaping the adaptive potential of populations under a warming ocean scenario. We used a combined experimental approach [transgenerational plasticity (TGP) and quantitative genetics] to partition the relative contribution of maternal vs. paternal (additive genetic) effects to offspring body size (a key component of fitness), and investigated a potential physiological mechanism (mitochondrial respiration capacities) underlying whole-organism growth/size responses. 2. In very early stages of growth (up to 30 days), offspring body size of marine sticklebacks benefited from maternal TGP: offspring of mothers acclimated to 17°C were larger when reared at 17°C, and offspring of mothers acclimated to 21°C were larger when reared at 21°C. The benefits of maternal TGP on body size were stronger and persisted longer (up to 60 days) for offspring reared in the warmer (21°C) environment, suggesting that maternal effects will be highly relevant for climate change scenarios in this system. 3. Mitochondrial respiration capacities measured on mature offspring (F1 adults) matched the pattern of TGP for juvenile body size, providing an intuitive mechanistic basis for the maternal acclimation persisting into adulthood. Size differences between temperatures seen at early growth stages remained in the F1 adults, linking offspring body size to maternal inheritance of mitochondria. 4. Lower maternal variance components in the warmer environment were mostly driven by mothers acclimated to ambient (colder) conditions, further supporting our tenet that maternal effects were stronger at elevated temperature. Importantly, all parent-offspring temperature combination groups showed genotype 9 environment (G 9 E) interactions, suggesting that reaction norms have the potential to evolve. 5. To summarize, TGP and G 9 E interactions work in concert to mediate impacts of ocean warming on metabolic capacity and early growth of marine sticklebacks. TGP can buffer short-term detrimental effects of climate warming and may buy time for genetic adaptation to catch up, therefore markedly contributing to the evolutionary potential and persistence of populations under climate change.