Examining the role of parasites in limiting unidirectional gene flow between lake and river sticklebacks.

Parasites are important selective agents with the potential to limit gene flow between host populations by shaping local host immunocompetence. We report on a contact zone between lake and river three-spined sticklebacks (Gasterosteus aculeatus) that offers the ideal biogeographic setting to explore the role of parasite-mediated selection on reproductive isolation. A waterfall acts as a natural barrier and enforces unidirectional migration from the upstream river stickleback population to the downstream river and lake populations. We assessed population genetic structure and parasite communities over four years. In a set of controlled experimental infections, we compared parasite susceptibility of upstream and downstream fish by exposing laboratory-bred upstream river and lake fish, as well as hybrids, to two common lake parasite species: a generalist trematode parasite, Diplostomum pseudospathaceum, and a host-specific cestode, Schistocephalus solidus. We found consistent genetic differentiation between upstream and downstream populations across four sampling years, even though the downstream river consisted of ~10% first-generation migrants from the upstream population as detected by parentage analysis. Fish in the upstream population had lower genetic diversity and were strikingly devoid of macroparasites. Through experimental infections, we demonstrated that upstream fish and their hybrids had higher susceptibility to parasite infections than downstream fish. Despite this, naturally sampled upstream migrants were less infected than downstream residents. Thus, migrants coming from a parasite-free environment may enjoy an initial fitness advantage, but their descendants seem likely to suffer from higher parasite loads. Our results suggest that adaptation to distinct parasite communities can influence stickleback invasion success and may represent a barrier to gene flow, even between close and connected populations.

Cryptic haplotype-specific gamete selection yields offspring with optimal MHC immune genes.

Females choose specific mates in order to produce fitter offspring. However, several factors interfere with females' control over fertilization of their eggs, including sneaker males and phenotypically unpredictable allele segregation during meiosis. Mate choice at the individual level thus provides only a poor approximation for obtaining the best genetic match. Consequently, postcopulatory sperm selection by female oocytes has been proposed as a mechanism to achieve complementary combinations of parental haplotypes. Here, using controlled in vitro fertilization of three-spined stickleback eggs, we find haplotype-specific fertilization bias toward gametes with complementary major histocompatibility complex (MHC) immunogenes. The resulting zygote (and thus offspring) genotypes exhibit an intermediate level of individual MHC diversity that was previously shown to confer highest pathogen resistance. Our finding of haplotype-specific gamete selection thus represents an intriguing mechanism for fine-tuned optimization of the offspring's immune gene composition and an evolutionary advantage in the Red Queen dynamics of host-parasite coevolution.

Effects of environmental variation on host-parasite interaction in three-spined sticklebacks (Gasterosteus aculeatus).

Recent research provides accumulating evidence that the evolutionary dynamics of host-parasite adaptations strongly depend on environmental variation. In this context, the three-spined stickleback (Gasterosteus aculeatus) has become an important research model since it is distributed all over the northern hemisphere and lives in very different habitat types, ranging from marine to freshwater, were it is exposed to a huge diversity of parasites. While a majority of studies start from explorations of sticklebacks in the wild, only relatively few investigations have continued under laboratory conditions. Accordingly, it has often been described that sticklebacks differ in parasite burden between habitats, but the underlying co-evolutionary trajectories are often not well understood. With the present review, we give an overview of the most striking examples of stickleback-parasite-environment interactions discovered in the wild and discuss two model parasites which have received some attention in laboratory studies: the eye fluke Diplostomum pseudospathacaeum, for which host fish show habitat-specific levels of resistance, and the tapeworm Schistocephalus solidus, which manipulates immunity and behavior of its stickleback host to its advantage. Finally, we will concentrate on an important environmental variable, namely temperature, which has prominent effects on the activity of the immune system of ectothermic hosts and on parasite growth rates.

Transcriptome profiling of immune tissues reveals habitat-specific gene expression between lake and river sticklebacks.

The observation of habitat-specific phenotypes suggests the action of natural selection. The three-spined stickleback (Gasterosteus aculeatus) has repeatedly colonized and adapted to diverse freshwater habitats across the northern hemisphere since the last glaciation, while giving rise to recurring phenotypes associated with specific habitats. Parapatric lake and river populations of sticklebacks harbour distinct parasite communities, a factor proposed to contribute to adaptive differentiation between these ecotypes. However, little is known about the transcriptional response to the distinct parasite pressure of those fish in a natural setting. Here, we sampled wild-caught sticklebacks across four geographical locations from lake and river habitats differing in their parasite load. We compared gene expression profiles between lake and river populations using 77 whole-transcriptome libraries from two immune-relevant tissues, the head kidney and the spleen. Differential expression analyses revealed 139 genes with habitat-specific expression patterns across the sampled population pairs. Among the 139 differentially expressed genes, eight are annotated with an immune function and 42 have been identified as differentially expressed in previous experimental studies in which fish have been immune challenged. Together, these findings reinforce the hypothesis that parasites contribute to adaptation of sticklebacks in lake and river habitats.

Genomics of divergence along a continuum of parapatric population differentiation.

The patterns of genomic divergence during ecological speciation are shaped by a combination of evolutionary forces. Processes such as genetic drift, local reduction of gene flow around genes causing reproductive isolation, hitchhiking around selected variants, variation in recombination and mutation rates are all factors that can contribute to the heterogeneity of genomic divergence. On the basis of 60 fully sequenced three-spined stickleback genomes, we explore these different mechanisms explaining the heterogeneity of genomic divergence across five parapatric lake and river population pairs varying in their degree of genetic differentiation. We find that divergent regions of the genome are mostly specific for each population pair, while their size and abundance are not correlated with the extent of genome-wide population differentiation. In each pair-wise comparison, an analysis of allele frequency spectra reveals that 25-55% of the divergent regions are consistent with a local restriction of gene flow. Another large proportion of divergent regions (38-75%) appears to be mainly shaped by hitchhiking effects around positively selected variants. We provide empirical evidence that alternative mechanisms determining the evolution of genomic patterns of divergence are not mutually exclusive, but rather act in concert to shape the genome during population differentiation, a first necessary step towards ecological speciation.

Extensive copy-number variation of young genes across stickleback populations.

Duplicate genes emerge as copy-number variations (CNVs) at the population level, and remain copy-number polymorphic until they are fixed or lost. The successful establishment of such structural polymorphisms in the genome plays an important role in evolution by promoting genetic diversity, complexity and innovation. To characterize the early evolutionary stages of duplicate genes and their potential adaptive benefits, we combine comparative genomics with population genomics analyses to evaluate the distribution and impact of CNVs across natural populations of an eco-genomic model, the three-spined stickleback. With whole genome sequences of 66 individuals from populations inhabiting three distinct habitats, we find that CNVs generally occur at low frequencies and are often only found in one of the 11 populations surveyed. A subset of CNVs, however, displays copy-number differentiation between populations, showing elevated within-population frequencies consistent with local adaptation. By comparing teleost genomes to identify lineage-specific genes and duplications in sticklebacks, we highlight rampant gene content differences among individuals in which over 30% of young duplicate genes are CNVs. These CNV genes are evolving rapidly at the molecular level and are enriched with functional categories associated with environmental interactions, depicting the dynamic early copy-number polymorphic stage of genes during population differentiation.

Genome-wide patterns of standing genetic variation in a marine population of three-spined sticklebacks.

Since the end of the Pleistocene, the three-spined stickleback (Gasterosteus aculeatus) has repeatedly colonized and adapted to various freshwater habitats probably originating from ancestral marine populations. Standing genetic variation and the underlying genomic architecture both have been speculated to contribute to recent adaptive radiations of sticklebacks. Here, we expand on the current genomic resources of this fish by providing extensive genome-wide variation data from six individuals from a marine (North Sea) stickleback population. Using next-generation sequencing and a combination of paired-end and mate-pair libraries, we detected a wide size range of genetic variation. Among the six individuals, we found more than 7% of the genome is polymorphic, consisting of 2599111 SNPs, 233464 indels and structural variation (SV) (>50 bp) such as 1054 copy-number variable regions (deletions and duplications) and 48 inversions. Many of these polymorphisms affect gene and coding sequences. Based on SNP diversity, we determined outlier regions concordant with signatures expected under adaptive evolution. As some of these outliers overlap with pronounced regions of copy-number variation, we propose the consideration of such SV when analysing SNP data from re-sequencing approaches. We further discuss the value of this resource on genome-wide variation for further investigation upon the relative contribution of standing variation on the parallel evolution of sticklebacks and the importance of the genomic architecture in adaptive radiation.

Permanent genetic resources added to Molecular Ecology Resources Database 1 February 2012 - 31 March 2012.

This article documents the addition of 171 microsatellite marker loci and 27 pairs of single nucleotide polymorphism (SNP) sequencing primers to the Molecular Ecology Resources Database. Loci were developed for the following species: Bombus pauloensis, Cephalorhynchus heavisidii, Cercospora sojina, Harpyhaliaetus coronatus, Hordeum vulgare, Lachnolaimus maximus, Oceanodroma monteiroi, Puccinia striiformis f. sp. tritici, Rhea americana, Salmo salar, Salmo trutta, Schistocephalus solidus, Sousa plumbea and Tursiops aduncus. These loci were cross-tested on the following species: Aquila heliaca, Bulweria bulwerii, Buteo buteo, Buteo swainsoni, Falco rusticolus, Haliaeetus albicilla, Halobaena caerulea, Hieraaetus fasciatus, Oceanodroma castro, Puccinia graminis f. sp. Tritici, Puccinia triticina, Rhea pennata and Schistocephalus pungitii. This article also documents the addition of 27 sequencing primer pairs for Puffinus baroli and Bulweria bulwerii and cross-testing of these loci in Oceanodroma castro, Pelagodroma marina, Pelecanoides georgicus, Pelecanoides urinatrix, Thalassarche chrysostoma and Thalassarche melanophrys.

Gene flow between species of Lake Victoria haplochromine fishes.

The haplochromine cichlid fishes of Lake Victoria (LV), East Africa, are a textbook example of adaptive radiation-a rapid divergence of multiple morphologically distinguishable forms from a few founding lineages. The forms are generally believed to constitute a "flock" of several hundred reproductively isolated species in a dozen or so genera. This belief has, until now, not been subjected to a test, however. Here, we compare genetic variation at 11 loci in 10 haplochromine populations of 6 different species. Although the genetic diversity in the populations is quite high, using a variety of statistical tests, we find no evidence of genetic differentiation among the populations of LV haplochromines. On genetic distance trees, populations of the same species intermingle with those of different species. At the molecular level, the species are indistinguishable from one another. Genetic comparisons with closely related species in 2 crater lakes indicate that the species within LV continue exchanging genes. These observations have important implications for phylogenetic reconstruction. The approach used in this study is applicable to other instances of adaptive radiation.

Identification of chemokines and a chemokine receptor in cichlid fish, shark, and lamprey.

Chemokines are small, inducible, structurally related proteins that guide cells expressing the right chemokine receptors to sites of immune response. They have been identified and studied extensively in mammals, but little is known about their presence in other vertebrate groups. Here we describe seven new chemokines in bony fish and one in a cartilaginous fish, as well as one chemokine receptor in a jawless vertebrate. All eight chemokines belong to the SCYA (CC) subfamily characterized by four conserved cysteine residues of which the first two are adjacent. The chemokine receptor is of the CXCR4 type. Phylogenetic analysis does not reveal any clear evidence of orthology of fish and human chemokines. Although the divergence of the subfamilies began before the fish-tetrapod split, much of the divergence within the subfamilies took place separately in the two vertebrate groups. The existence of a chemokine receptor in the lamprey indicates that chemokines are apparently also present in the Agnatha.