Showing papers by "Roger K. Butlin published in 2010"

Repeated evolution of reproductive isolation in a marine snail: unveiling mechanisms of speciation.

Abstract: Distinct ecotypes of the snail Littorina saxatilis, each linked to a specific shore microhabitat, form a mosaic-like pattern with narrow hybrid zones in between, over which gene flow is 10-30% of within-ecotype gene flow Multi-locus comparisons cluster populations by geographic affinity independent of ecotype, while loci under selection group populations by ecotype The repeated occurrence of partially reproductively isolated ecotypes and the conflicting patterns in neutral and selected genes can either be explained by separation in allopatry followed by secondary overlap and extensive introgression that homogenizes neutral differences evolved under allopatry, or by repeated evolution in parapatry, or in sympatry, with the same ecotypes appearing in each local site Data from Spain, the UK and Sweden give stronger support for a non-allopatric model of ecotype formation than for an allopatric model Several different non-allopatric mechanisms can, however, explain the repeated evolution of the ecotypes: (i) parallel evolution by new mutations in different populations; (ii) evolution from standing genetic variation; and (iii) evolution in concert with rapid spread of new positive mutations among populations inhabiting similar environments These models make different predictions that can be tested using comprehensive phylogenetic information combined with candidate loci sequencing

Why is adaptation prevented at ecological margins? New insights from individual-based simulations.

Abstract: All species are restricted in their distribution. Currently, ecological models can only explain such limits if patches vary in quality, leading to asymmetrical dispersal, or if genetic variation is too low at the margins for adaptation. However, population genetic models suggest that the increase in genetic variance resulting from dispersal should allow adaptation to almost any ecological gradient. Clearly therefore, these models miss something that prevents evolution in natural populations. We developed an individual-based simulation to explore stochastic effects in these models. At high carrying capacities, our simulations largely agree with deterministic predictions. However, when carrying capacity is low, the population fails to establish for a wide range of parameter values where adaptation was expected from previous models. Stochastic or transient effects appear critical around the boundaries in parameter space between simulation behaviours. Dispersal, gradient steepness, and population density emerge as key factors determining adaptation on an ecological gradient.

Population genomics and speciation.

Abstract: The process of speciation begins with genomically-localised barriers to gene exchange associated with loci for local adaptation, intrinsic incompatibility or assortative mating. The barrier then spreads until reproductive isolation influences the whole genome. The population genomics approach can be used to identify regions of reduced gene flow by detecting loci with greater differentiation than expected from the average across many loci. Recently, this approach has been used in several systems. I review these studies, concentrating on the robustness of the approach and the methods available to go beyond the simple identification of differentiated markers. Population genomics has already contributed significantly to understanding the balance between gene flow and selection during the evolution of reproductive isolation and has great future potential both in genome species and in non-model organisms.

An EST-based genome scan using 454 sequencing in the marine snail Littorina saxatilis.

Abstract: Genome scans have been used in the studies of ecological speciation to find genomic regions (‘outlier loci’) showing reduced gene flow between divergent populations ⁄ species. High-throughput sequencing (‘454’) offers new opportunities in this field via transcriptome sequencing. Divergent ecotypes of the marine gastropod Littorina saxatilis represent a good example of incipient ecological speciation. We performed a 454-based genome scan between H and M ecotypes of L. saxatilis from the British Isles using cDNA of pooled individuals. Allele frequencies were calculated for 2454 single nucleotide polymorphisms (SNPs), within 572 contigs, and 7% of loci were detected as outliers. Functional annotation of the contigs containing outlier SNPs showed that they included shell matrix and muscle proteins (lithostathine, mucin, titin), proteins involved in energetic metabolism (arginine kinase, NADH dehydrogenase) and reverse transcriptases. Follow-up investigations into these proteins and unannotated outliers will be a promising route in the study of ecological speciation in L. saxatilis.

The adaptive value of phenotypic plasticity in two ecotypes of a marine gastropod

Abstract: Few surveys have concentrated on studying the adaptive value of phenotypic plasticity within genetically-distinct conspecific ecotypes. Here, we conduct a test to assess the adaptive value that partial phenotypic plasticity may have for survival in the marine gastropod Littorina saxatilis. This species has evolved canalized ecotypes but, nevertheless, the ecotypes show some phenotypic plasticity for the traits under divergent selection between wave-exposed and high-predation habitats. We exposed juveniles of each ecotype to several environmental treatments under laboratory conditions in order to produce shape variation associated with plasticity. The two ecotypes from different treatments were then transplanted to the wave-exposed habitat and the survival rate was monitored. Ecotype explained the largest distinction in survival rate while treatment caused variation in survival rate within the ecotype released into its parental habitat which was correlated with plastic changes in shell shape. Snails that had experienced a treatment mimicking the environment of the transplantation location survived with the highest rate, while individuals from the contrary experimental treatment had lower survivorship. We conclude that the partial plastic response shown in Littorina saxatilis has a significant impact on fitness, although this remains small compared to the overall adaptive difference between ecotypes.

Reduction in post-invasion genetic diversity in Crangonyx pseudogracilis (Amphipoda: Crustacea): a genetic bottleneck or the work of hitchhiking vertically transmitted microparasites?

Abstract: Parasites can strongly influence the success of biological invasions. However, as invading hosts and parasites may be derived from a small subset of genotypes in the native range, it is important to examine the distribution and invasion of parasites in the context of host population genetics. We demonstrate that invasive European populations of the North American Crangonyx pseudogracilis have experienced a reduction in post-invasion genetic diversity. We predict that vertically transmitted parasites may evade the stochastic processes and selective pressures leading to enemy release. As microsporidia may be vertically or horizontally transmitted, we compared the diversity of these microparasites in the native and invasive ranges of the host. In contrast to the reduction in host genetic diversity, we find no evidence for enemy release from microsporidian parasites in the invasive populations. Indeed, a single, vertically transmitted, microsporidian sex ratio distorter dominates the microsporidian parasite assemblage in the invasive range and appears to have invaded with the host. We propose that overproduction of female offspring as a result of parasitic sex ratio distortion may facilitate host invasion success. We also propose that a selective sweep resulting from the increase in infected individuals during the establishment may have contributed to the reduction in genetic diversity in invasive Crangonyx pseudogracilis populations.

Myotis alcathoe Confirmed in the UK from Mitochondrial and Microsatellite DNA

Abstract: We describe the recent discovery of Alcathoe bat, Myotis alcathoe, at locations in the south and north of England. First described in 2001 from individuals caught in Greece, the species' known range has steadily been extended across much of Europe. Its presence in the UK was confirmed by mtDNA (ND1) and microsatellite analysis of wing biopsy samples collected between 2003–2009, primarily at swarming sites. Morphological and echolocation call data are consistent with the genetic interpretation. The presence of M. alcathoe at sites 350 km apart, and the probability that it makes up a significant proportion of the small Myotis bats at these sites, suggest it is a resident species. Preliminary assessment of its distribution in the UK will be facilitated by its distinctive echolocation call, an FM sweep that typically terminates at 43–46 kHz, above those of other Myotis species.