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17 results for “Littorina saxatilis”
Data from: Genetic architecture of repeated phenotypic divergence in Littorina saxatilis ecotype evolution
<p>Chromosomal inversions have been shown to play a major role in local adaptation by suppressing recombination between alternative arrangements and maintaining beneficial allele combinations. However, so far, their importance relative to the remaining genome remains largely unknown. Understanding the genetic architecture of adaptation requires better estimates of how loci of different effect sizes contribute to phenotypic variation. Here, we used three Swedish islands where the marine snail Littorina saxatilis has repeatedly evolved into two distinct ecotypes along a habitat transition. We estimated the contribution of inversion polymorphisms to phenotypic divergence while controlling for polygenic effects in the remaining genome using a quantitative genetics framework. We confirmed the importance of inversions but showed that contributions of loci outside inversions are of similar magnitude, with variable proportions dependent on the trait and the population. Some inversions showed consistent effects across all sites, whereas others exhibited site-specific effects, indicating that the genomic basis for replicated phenotypic divergence is only partly shared. The contributions of sexual dimorphism as well as environmental factors to phenotypic variation were significant but minor compared to inversions and polygenic background. Overall, this integrated approach provides insight into the multiple mechanisms contributing to parallel phenotypic divergence. </p>
Data from: Genetic architecture of repeated phenotypic divergence in Littorina saxatilis ecotype evolution
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Data from: Shared and non-shared genomic divergence in parallel ecotypes of Littorina saxatilis at a local scale
Parallel speciation occurs when selection drives repeated, independent adaptive divergence that reduces gene flow between ecotypes. Classical examples show parallel speciation originating from shared genomic variation, but this does not seem to be the case in the rough periwinkle (Littorina saxatilis) that has evolved considerable phenotypic diversity across Europe, including several distinct ecotypes. Small 'wave' ecotype snails inhabit exposed rocks and experience strong wave action, while thick-shelled, 'crab' ecotype snails are larger and experience crab predation on less exposed shores. Crab and wave ecotypes appear to have arisen in parallel and recent evidence suggests only marginal sharing of molecular variation linked to evolution of similar ecotypes in different parts of Europe. However the extent of genomic sharing is expected to increase with gene flow and more recent common ancestry. To test this we used de novo RAD-sequencing to quantify the extent of shared genomic divergence associated with phenotypic similarities among ecotype-pairs on three close islands (< 10 km distance) connected by weak gene flow (Nm~0.03) and with recent common ancestry (< 10,000 years). After accounting for technical issues, including a large proportion of null alleles due to a large effective population size, we found ~8-28% of positive outliers were shared between two islands and ~2-9% were shared among all three islands. This low level of sharing suggests that parallel phenotypic divergence in this system is not matched by shared genomic divergence despite a high probability of gene flow and standing genetic variation.
Data from: Multiple chromosomal rearrangements in a hybrid zone between Littorina saxatilis ecotypes
Both classical and recent studies suggest that chromosomal inversion polymorphisms are important in adaptation and speciation. However, biases in discovery and reporting of inversions make it difficult to assess their prevalence and biological importance. Here, we use an approach based on linkage disequilibrium among markers genotyped for samples collected across a transect between contrasting habitats to detect chromosomal rearrangements de novo. We report 17 polymorphic rearrangements in a single locality for the coastal marine snail, Littorina saxatilis. Patterns of diversity in the field and of recombination in controlled crosses provide strong evidence that at least the majority of these rearrangements are inversions. Most show clinal changes in frequency between habitats, suggestive of divergent selection, but only one appears to be fixed for different arrangements in the two habitats. Consistent with widespread evidence for balancing selection on inversion polymorphisms, we argue that a combination of heterosis and divergent selection can explain the observed patterns and should be considered in other systems spanning environmental gradients.
Negative frequency dependent selection maintains shell banding polymorphisms in two marine snails (Littorina fabalis and L. saxatilis)
<p>The presence of shell bands is common in gastropods. The marine snails, <i>Littorina fabalis</i> and <i>L. saxatilis</i>, are<i> </i>both polymorphic for this trait. Such polymorphism would be expected to be lost by the action of genetic drift or directional selection, but it appears to be widespread at relatively constant frequencies. This suggests it is maintained by balancing selection on the trait or on a genetically linked trait. Using long time-series of empirical data, we compared potential effects of genetic drift and negative frequency-dependent selection, in the two species. The contribution of genetic drift to changes in the frequency of bands in <i>L. fabalis</i> was estimated using the effective population size estimated from microsatellite data, while the effect of genetic drift in <i>L. saxatilis</i> were derived from previously published study. Frequency-dependent selection was assessed comparing the cross-product estimator of fitness with the frequency of the polymorphism across years using a regression analysis. Both studied species showed patterns of negative frequency-dependent selection. In addition, in <i>L. fabalis</i>, contributions from genetic drift could explain some of the changes in banding frequency. Overdominance and heterogeneous selection did not fit well to our data. The possible biological explanations resulting on the maintenance of the banding polymorphism are discussed.</p>
Data from: Do the same genes underlie parallel phenotypic divergence in different Littorina saxatilis populations?
Parallel patterns of adaptive divergence and speciation are cited as powerful evidence for the role of selection driving these processes. However, it is often not clear whether parallel phenotypic divergence is underlain by parallel genetic changes. Here, we asked about the genetic basis of parallel divergence in the marine snail Littorina saxatilis, which has repeatedly evolved coexisting ecotypes adapted to either crab predation or wave action. We sequenced the transcriptome of snails of both ecotypes from three distant geographical locations (Spain, Sweden and United Kingdom) and mapped the reads to the L. saxatilis reference genome. We identified genomic regions potentially under divergent selection between ecotypes within each country, using an outlier approach based on FST values calculated per locus. In line with previous studies indicating that gene reuse is generally common, we expected to find extensive sharing of outlier loci due to recent shared ancestry and gene flow between at least two of the locations in our study system. Contrary to our expectations, we found that most outliers were country specific, suggesting that much of the genetic basis of divergence is not shared among locations. However, we did find that more outliers were shared than expected by chance and that differentiation of shared outliers is often generated by the same SNPs. We discuss two mechanisms potentially explaining the limited amount of sharing we observed. First, a polygenic basis of divergent traits might allow for multiple distinct molecular mechanisms generating the same phenotypic patterns. Second, additional, location-specific axes of selection that we did not focus on in this study may produce distinct patterns of genetic divergence within each site.
Data from: Do the same genes underlie parallel phenotypic divergence in different Littorina saxatilis populations?
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Negative frequency dependent selection maintains shell banding polymorphisms in two marine snails (Littorina fabalis and L. saxatilis)
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Data from: Multiple chromosomal rearrangements in a hybrid zone between Littorina saxatilis ecotypes
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Data from: Shared and non-shared genomic divergence in parallel ecotypes of Littorina saxatilis at a local scale
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Data from: Genome-wide association analyses reveal polygenic genomic architecture underlying divergent shell morphology in Spanish Littorina saxatilis ecotypes
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Data from: Selection on outlier loci and their association with adaptive phenotypes in Littorina saxatilis contact zones
A fundamental issue in speciation research is to evaluate phenotypic variation and the genomics driving the evolution of reproductive isolation between sister taxa. Above all, hybrid zones are excellent study systems for researchers to examine the association of genetic differentiation, phenotypic variation and the strength of selection. We investigated two contact zones in the marine gastropod Littorina saxatilis and utilized landmark-based geometric morphometric analysis together with amplified fragment length polymorphism (AFLP) markers to assess phenotypic and genomic divergence between ecotypes under divergent selection. From genetic markers, we calculated the cline width, linkage disequilibrium and the average effective selection on a locus. Additionally, we conducted an association analysis linking the outlier loci and phenotypic variation between ecotypes and show that a proportion of outlier loci are associated with key adaptive phenotypic traits.
Data from: Selection on outlier loci and their association with adaptive phenotypes in Littorina saxatilis contact zones
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Data from: Genetic variation for adaptive traits is associated with polymorphic inversions in Littorina saxatilis
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Data from: Genomic divergence between Spanish Littorina saxatilis ecotypes unravels limited admixture and extensive parallelism associated with population history
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Littorina saxatilis Comparative Genomics Hybridization (CGH) study with NimbleGen 090824_L_saxatilis_expr_HX12
GEO Series GSE120698. Littorina saxatilis. 69 samples. Type: Genome variation profiling by array.
Littorina saxatilis Expression study with NimbleGen 090824_L_saxatilis_expr_HX12
GEO Series GSE120697. Littorina saxatilis. 22 samples. Type: Expression profiling by array.
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