MHC-mediated mate choice increases parasite resistance in salmon

Natural (parasite-driven) and sexual selection are thought to maintain high polymorphism in the genes of the major histocompatibility complex (MHC), but support for a link between mate choice, MHC variation and increased parasite resistance is circumstantial. We compared MHC diversity and Anisakis loads among anadromous Atlantic salmon (Salmo salar L.) returning to four rivers to spawn, which had originated from natural spawning (parents allowed to mate freely) or artificial crosses (parents deprived from the potential benefits of mate choice). We found that the offspring of artificially bred salmon had higher parasite loads and were almost four times more likely to be infected than free-mating salmon, despite having similar levels of MHC diversity. Moreover, the offspring of wild salmon were more MHC dissimilar than the offspring of artificially crossed salmon, and uninfected fish were more dissimilar for MHC than infected fish. Thus, our results suggest a link between disassortative mating and offspring benefits and indicate that MHC-mediated mate choice and natural (parasite-driven) selection act in combination to maintain MHC diversity, and hence fitness. Therefore, artificial breeding programmes that negate the potential genetic benefits of mate choice may result in inherently inferior offspring, regardless of population size, rearing conditions or genetic diversity.     

MHC-based mate choice combines good genes and maintenance of MHC polymorphism

Polymorphic genes of the major histocompatibility complex (MHC) are regarded as essential genes for individual fitness under conditions of natural and sexual selection. To test this hypothesis, we investigated the ultimate individual fitness trait — that of reproductive success. We used three-spined sticklebacks ( Gasterosteus aculeatus ) in seminatural enclosures, located in natural breeding areas where the experimental fish had been caught. During their reproductive period, fish were exposed continuously to their natural sympatric parasites. By genotyping almost 4000 eggs with nine microsatellites, we determined parenthood and inferred female mating decision. We found that with reference to their own MHC profile, female sticklebacks preferred to mate with males sharing an intermediate MHC diversity. In addition, males with a specific MHC haplotype were bigger and better at fighting a common parasite ( Gyrodactylus sp.). This translated directly into Darwinian fitness since fish harbouring this specific MHC haplotype were more likely to be chosen and had a higher reproductive output. We conclude that females also based their mating decision on a specific MHC haplotype conferring resistance against a common parasite. This identifies and supports 'good genes'. We argue that such an interaction between host and parasite driving assortative mating is not only a prerequisite for negative frequency-dependent selection — a potential mechanism to explain the maintenance of MHC polymorphism, but also potentially speciation.     

Ecological immunogenetics of life-history traits in a model amphibian

Major histocompatibility complex (MHC) genes determine immune repertoires and social preferences of vertebrates. Immunological regulation of microbial assemblages associated with individuals influences their sociality, and should also affect their life-history traits. We exposed Xenopus laevis tadpoles to water conditioned by adult conspecifics. Then, we analysed tadpole growth, development and survivorship as a function of MHC class I and class II peptide-binding region amino acid sequence similarities between tadpoles and frogs that conditioned the water to which they were exposed. Tadpoles approached metamorphosis earlier and suffered greater mortality when exposed to immunogenetically dissimilar frogs. The results suggest that developmental regulatory cues, microbial assemblages or both are specific to MHC genotypes. Tadpoles may associate with conspecifics with which they share microbiota to which their genotypes are well adapted.     

Synergistic and antagonistic interaction between different branches of the immune system is related to melanin‐based coloration in nestling tawny owls

When exposed to parasites, hosts often mount energetically expensive immune responses, and this may alter resource allocation between competing life history traits including other components of the immune system. Here, we investigated whether a humoral immune challenge towards a vaccine reduces or enhances the cutaneous immune responses towards an injection of lipopolysaccharid (LPS, innate immunity) and phytohaemagglutinin (PHA, T‐cell immunity) in nestling tawny owls in interaction with the degree of plumage melanin‐based coloration. The humoral immune challenge enhanced the response to LPS similarly in differently coloured nestlings. In contrast, the same humoral immune challenge enhanced immune response to PHA in dark reddish melanic nestlings while reducing it in pale reddish melanic nestlings. Our results highlight that both antagonistic and synergistic interactions can take place among branches of immune system, and that the sign and magnitude of these interactions can vary with immune responses involved and the degree of melanin‐based coloration.     

Detecting the signature of selection on immune genes in highly structured populations of wild sheep (Ovis dalli)

The confounding effects of population structure complicate efforts to identify regions of the genome under the influence of selection in natural populations. Here we test for evidence of selection in three genes involved in vertebrate immune function - the major histocompatibility complex (MHC), interferon gamma (IFNG) and natural resistance associated macrophage polymorphism (NRAMP) - in highly structured populations of wild thinhorn sheep (Ovis dalli). We examined patterns of variation at microsatellite loci linked to these gene regions and at the DNA sequence level. Simple Watterson's tests indicated balancing selection at all three gene regions. However, evidence for selection was confounded by population structure, as the Watterson's test statistics from linked markers were not outside of the range of values from unlinked and presumably neutral microsatellites. The translated coding sequences of thinhorn IFNG and NRAMP are fixed and identical to those of domestic sheep (Ovis aries). In contrast, the thinhorn MHC DRB locus shows significant evidence of overdominance through both an excess of nonsynonymous substitution and trans-species polymorphism. The failure to detect balancing selection at microsatellite loci linked to the MHC is likely the result of recombination between the markers and expressed gene regions.     

Evaluation of two approaches to genotyping major histocompatibility complex class I in a passerine-CE-SSCP and 454 pyrosequencing

Genes of the highly dynamic major histocompatibility complex (MHC) are directly linked to individual fitness and are of high interest in evolutionary ecology and conservation genetics. Gene duplication and positive selection usually lead to high levels of polymorphism in the MHC region, making genotyping of MHC a challenging task. Here, we compare the performance of two methods for MHC class I genotyping in a passerine with highly duplicated MHC class I genes: capillary electrophoresis-single-strand conformation polymorphism (CE-SSCP) analysis and 454 GS FLX Titanium pyrosequencing. According to our findings, the number of MHC variants (called alleles for simplicity) detected by CE-SSCP is significantly lower than detected by 454. To resolve discrepancies between the two methods, we cloned and Sanger sequenced a MHC class I amplicon for an individual with high number of alleles. We found a perfect congruence between cloning/Sanger sequencing results and 454. Thus, in case of multi-locus amplification, CE-SSCP considerably underestimates individual MHC diversity. However, numbers of alleles detected by both methods are significantly correlated, although the correlation is weak (r = 0.32). Thus, in systems with highly duplicated MHC, 454 provides more reliable information on individual diversity than CE-SSCP.     

Evolution of MHC class I genes in the European badger (Meles meles)

The major histocompatibility complex (MHC) plays a central role in the adaptive immune system and provides a good model with which to understand the evolutionary processes underlying functional genes. Trans-species polymorphism and orthology are both commonly found in MHC genes; however, mammalian MHC class I genes tend to cluster by species. Concerted evolution has the potential to homogenize different loci, whereas birth-and-death evolution can lead to the loss of orthologs; both processes result in monophyletic groups within species. Studies investigating the evolution of MHC class I genes have been biased toward a few particular taxa and model species. We present the first study of MHC class I genes in a species from the superfamily Musteloidea. The European badger (Meles meles) exhibits moderate variation in MHC class I sequences when compared to other carnivores. We identified seven putatively functional sequences and nine pseudogenes from genomic (gDNA) and complementary (cDNA) DNA, signifying at least two functional class I loci. We found evidence for separate evolutionary histories of the α1 and α2/α3 domains. In the α1 domain, several sequences from different species were more closely related to each other than to sequences from the same species, resembling orthology or trans-species polymorphism. Balancing selection and probable recombination maintain genetic diversity in the α1 domain, evidenced by the detection of positive selection and a recombination event. By comparison, two recombination breakpoints indicate that the α2/α3 domains have most likely undergone concerted evolution, where recombination has homogenized the α2/α3 domains between genes, leading to species-specific clusters of sequences. Our findings highlight the importance of analyzing MHC domains separately.     

Effects of gravel size and peat material concentrations on embryo survival and alevin emergence of brown trout,Salmo trutta L.

Embryo survival and alevin emergence pattern of brown trout were studied in simulated redds with different homogeneous gravel sizes and different concentrations of peat material. Optimal survival (95%) occurred in 18 mm gravel and survival decreased with decreasing gravel size. High concentrations (40%) of peat material resulted in low survival (65%). The proportion of premature emerging alevins increased in finer gravels and at high peat concentrations. Premature alevins had a large yolk sac and are probably very vulnerable to predators.     

Molecular characterization of swine leucocyte antigen class II genes in outbred pig populations

The highly polymorphic swine leucocyte antigen (SLA) genes are among the most important determinants of swine immune responses to disease and vaccines. Accurate and effective SLA genotyping methods are required to understand how SLA gene polymorphisms affect immunity, especially in outbred pigs with diverse genetic backgrounds. In this study, we present a simple and rapid molecular‐based typing system for characterizing SLA class II alleles of the DRB1, DQB1 and DQA loci. This system utilizes a set of 47 sequence‐specific PCR primers developed to differentiate alleles by groups that share similar sequence motifs. We applied this typing method to investigate the SLA class II diversity in four populations of outbred pigs (n = 206) and characterized a total of 19 SLA class II haplotypes, six of which were shared by at least three of the sampled pig populations. We found that Lr‐0.1 (DRB1*01XX–DQB1*01XX–DQA*01XX) was the most prevalent haplotype with a combined frequency of 16.0%, followed by Lr‐0.2 (DRB1*02XX–DQB1*02XX–DQA*02XX) with 14.6% and Lr‐0.15b (DRB1*04XX–DQB1*0202–DQA*02XX) with 14.1%. Over 70% of the pigs (n = 147) had at least one copy of one of these three haplotypes. The PCR‐based typing system described in this study demonstrates a reliable and unambiguous detection method for SLA class II alleles. It will be a valuable tool for studying the influence of SLA diversity on various immunological, pathological and physiological traits in outbred pigs.     

Effects of fish farm effluents on egg-to-fry development and survival of brown trout in artificial redds

Egg-to-fry development and survival of brown trout Salmo trutta were compared in two rivers of the Pyrenean piedmont, the Nive d'Arnéguy, with few human activities, and the Nive des Aldudes, with many anthropogenic activities including 11 fish farms and two sewage treatment plants (STPs). Survival was estimated between spawning (early December) and emergence time (early March) by means of capsules, 7 cm³ in volume, filled with green eggs then inserted into the gravel of artificial redds at spawning sites. In the Nive des Aldudes, three sets of conditions were studied: a fish farm near the springs, a fish farm and an STP in a village, and a series of fish farms and an STP in a village. In each situation, two artificial redds were created upstream and two others downstream from the fish farms. In the Nive d'Arnéguy, four sites were equipped: each of the two downstream sites with two artificial redds, and the two upstream sites with one redd. Substratum characteristics (proportion of fine particles) and the quality of surface and interstitial water (oxygen content, ammonia and nitrite nitrogen) were periodically measured. There was no redd substratum difference between sites upstream and downstream of fish farms. Survival to fry emergence was higher in the Nive d'Arnéguy (63·5%) than in the Nive des Aldudes (47·7%). In this latter river, the nitrogen released from the STPs was 0·5% that from fish farms. Fish farming impaired survival close to fish farm effluents (31·6%), as compared to survival upstream (63·6%), and induced a development delay during the yolk-sac fry stage. These differences were mainly linked to a drop in the dissolved oxygen content in interstitial water induced by the nitrogen flow in surface and interstitial waters.     

Disentangling the roles of natural selection and genetic drift in shaping variation at MHC immunity genes

The major histocompatibility complex (MHC) forms an integral component of the vertebrate immune response and, due to strong selection pressures, is one of the most polymorphic regions of the entire genome. Despite over 15 years of research, empirical studies offer highly contradictory explanations of the relative roles of different evolutionary forces, selection and genetic drift, acting on MHC genes during population bottlenecks. Here, we take a meta-analytical approach to quantify the results of studies into the effects of bottlenecks on MHC polymorphism. We show that the consequences of selection acting on MHC loci prior to a bottleneck event, combined with drift during the bottleneck, will result in overall loss of MHC polymorphism that is ～15% greater than loss of neutral genetic diversity. These results are counter to general expectations that selection should maintain MHC polymorphism, but do agree with the results of recent simulation models and at least two empirical studies. Notably, our results suggest that negative frequency-dependent selection could be more important than overdominance for maintaining high MHC polymorphism in pre-bottlenecked populations.     

APPLIED ISSUES: Effects of habitat rehabilitation on brown trout (Salmo trutta) in boreal forest streams

1. Degradation of stream habitat because of anthropogenic activities (e.g. channelisation) has had a dramatic impact on fluvial environments and their biota, and as a consequence, increasing effort has been directed towards the restoration of degraded rivers. However, a major problem is that the success (or failure) of restoration has been rarely tested using a well‐designed monitoring programme to allow reliable detection of an impact, if any exists. We used a spatially and temporally replicated, balanced Before‐After‐Control‐Impact design to assess the impact of stream habitat rehabilitation on the densities and growth of brown trout of three age‐classes in North Finnish forest streams. 2. Three separate sections in each of six streams were selected for the study. After 3 years of pre‐rehabilitation monitoring, two randomly selected sections in each stream were restored; one using large woody debris and boulders and the other using only boulders. A third section remained as an unmodified control. Monitoring of fish densities continued for 3 years after rehabilitation. 3. Rehabilitation clearly increased streambed complexity, but did not have detectable effects on brown trout stocks in either of the rehabilitation schemes (LWD or stones), except for age‐2+ and older fish which decreased in abundance compared to control reaches. A severe drought after rehabilitation in late summer 2002 reduced densities of trout to a low level in all streams, overriding any local effects of rehabilitation. Rehabilitation structures seemed to provide some safeguard against drought for age‐2 and older, but not for the younger age‐classes. 4. Our results add to the growing body of literature suggesting that large‐scale regional factors may overwhelm local management efforts. To be successful in the future, stream rehabilitation schemes must include drought refuge areas for fish and other stream biota.     

Effects of six trace metals on calcium fluxes in brown trout (Salmo trutta L.) in soft water

Summary Calcium fluxes were measured simultaneously in brown trout fry maintained in an artificial soft water medium of [Ca] 20 mol·l^-1 and pH 5.6, and exposed to each of six trace metals (Al, Cu, Fe, Ni, Pb, and Zn). The trace metal concentrations represented typical and maximum levels found in acid waters experiencing declining fishery status. In the absence of trace metals, evidence is presented which suggests that ca. 91% of Ca taken up from the external medium was by extraintestinal active transport. Calcium efflux was stimulated by both concentrations of Al, Cu, Fe, and Pb. Efflux was also stimulated by [Ni] 170 nmol·l^-1 and [Zn] 3000 nmol·l^-1. In some cases, response to increased efflux was stimulation of influx. Lack of stimulation of influx resulted in negative net Ca fluxes. Net Ca losses were recorded at both concentrations of Al, Pb, and Ni, lower concentrations only of Fe, and higher concentrations only of Cu and Zn.Abbreviations J _in influx - J _net net flux - J _out efflux Henceforward in this paper, chemical elements are referred to by their chemical symbols rather than by full names     

Effects of stocking on the genetic diversity of brown trout populations of the Adriatic and Danubian drainages in Switzerland

This study focuses on genetic variation of brown trout Salmo trutta populations of the Adriatic and Danubian drainages in Switzerland. The allozyme and other protein loci data show a major replacement of native stocks from the Adriatic drainages by introduced hatchery trout of Atlantic basin origin. In most samples, diagnostic alleles for the Adriatic form of Salmo trutta f. fario and for the marbled trout Salmo trutta marmoratus are found at very low frequencies (f<0.15). Taking into account previous genetic studies on brown trout of this basin, the Danubian samples are not heavily contaminated with foreign alleles. The results are consistent with records of local stocking activities which account in part for the high introgression rates of Atlantic alleles into local populations of the Adriatic drainages. In addition, introgression is enhanced by a decrease of natural reproduction which is caused by a deterioration of trout habitats through human activities. Furthermore, a third mechanism is proposed that may contribute to the high introgression rates observed: if Atlantic trout are introduced, the reproductive barriers between the two native forms, marbled trout and Adriatic fario respectively, break down. Atlantic trout apparently hybridize with both native forms and generate gene flow between them. In some parts of Adriatic drainages in Switzerland, the patterns of introgression and hybridization are further complicated by introduction of trout from the Danubian system. Alleles of the marbled trout are also found in the samples of the Danubian drainage system. These are due to stocking activities across the watershed.     

Effects of angling on population structure of brown trout,Salmo trutta L., in mountain streams of Northern Spain

Effects of angling exploitation on brown trout populations were assessed by comparing fished sections with close ones unfished for at least 20 years, in mountain streams of Asturias (Northern Spain). Both the fish size and age structure significantly differed among sections in the expected direction according to their exploitation status. The main effects were a significant decrease in age structure complexity (diversity), life span, and percent individuals above the legal limit size in the exploited stocks versus the unexploited ones. Trout above the minimum length limit for fishing (18 cm) averaged 19.47% of the fish caught in the unfished sections (sd = 4.01; n = 5), and 4.72% (sd = 3.46; n = 4) in those subjected to angling. Furthermore, fish older than 4 years represented 39.84% (sd = 8.53) and 1.19% (sd = 1.60) of the catch, respectively. Effects on recruitment (density of young fishes) and growth rates (length at age 1 + to 3 +) were not absolutely consistent, though maximum values were associated with fished sections.     

Effects of Colonization of a Bacterial Endophyte,Azospirillum sp. B510, on Disease Resistance in Rice

Agriculturally important grasses contain numerous diazotrophic bacteria, the interactions of which are speculated to have some other benefits to the host plants. In this study, we analyzed the effects of a bacterial endophyte, Azospirillum sp. B510, on disease resistance in host rice plants. Rice plants (Oryza sativa cv. Nipponbare) were inoculated with B510 exhibited enhanced resistance against diseases caused by the virulent rice blast fungus Magnaporthe oryzae and by the virulent bacterial pathogen Xanthomonas oryzae. In the rice plants, neither salicylic acid (SA) accumulation nor expression of pathogenesis-related (PR) genes was induced by interaction with this bacterium, except for slight induction of PBZ1. These results indicate the possibility that strain B510 is able to induce disease resistance in rice by activating a novel type of resistance mechanism independent of SA-mediated defense signaling.     

Effects of Various Elicitors on the Transcription of a β-1,3-endoglucanase Gene in Citrus Fruit

Very little is yet known regarding the molecular mechanisms involved in pathogen defense responses in citrus fruit. Recently, a basic β-1,3-endoglucanase (EC 3.2.2.39) belonging to the pathogenesis-related (PR) group of proteins, has been purified from Citrus sinensis (L) Osbeck cv. `Valencia' orange callus. Specific antibodies raised against the purified protein were used to screen `Valencia' callus and flavedo cDNA expression libraries, and to isolate its corresponding cDNA, designated gns1. The gns1 gene encodes a predicted polypeptide of 336 amino acids with a molecular mass of 37.3 kDa and a basic pI of 9.19, and shares 55–65% identity with several other plant β-1,3-endoglucanase proteins. Hereby, we show that the expression of the gns1 gene is markedly induced by wounding and inoculation with Penicillium digitatum (Pers. Fr.) Sacc., and following treatments with various elicitors that induce fruit resistance against P. digitatum . These treatments include UV irradiation, application of jasmonic acid (JA), β-aminobutyric acid (BABA), Candida oleophila antagonist yeast cells and hot water rinsing and brushing. Overall, based on various RNA gel blot hybridizations, we assume that gns1 is most likely to be part of the molecular mechanisms involved in pathogen defense responses in citrus fruit. *     

Effects of sea trout stocking on the population genetics of landlocked brown trout, Salmo trutta L., in the Conwy River system, North Wales, U.K.

The effect of the introduction of fry of anadromous sea trout, Salmo trutta L., on the genetic integrity of landlocked brown trout populations was evaluated. Samples were taken from six brown trout populations from streams above impassable waterfalls in the Conwy river system (North Wales, U.K.) in 1989 and 1990. Three of these streams had no known stocking history and three had been stocked with sea trout fry from the lower Conwy system over the last few years. Representatives of these sea trout were collected from two streams in the lower Conwy system and from a hatchery. Allele frequencies at 13 loci, six of which were polymorphic, were determined by starch gel electrophoresis.
The stocked populations were intermediate in their allele frequencies between unstocked brown trout and sea trout samples. A principal component analysis suggested significant numbers of hybrids in all of the stocked streams. This shows that some of the introduced sea trout did not migrate down the falls to the sea, but stayed in fresh water and hybridized with the local population. The significance of this finding for the conservation of the genetic resource of brown trout stocks is discussed.