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

<p>Abstract</p> <p>Background</p> <p>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 <it>Littorina saxatilis</it>. 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.</p> <p>Results</p> <p>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.</p> <p>Conclusions</p> <p>We conclude that the partial plastic response shown in <it>Littorina saxatilis </it>has a significant impact on fitness, although this remains small compared to the overall adaptive difference between ecotypes.</p

The attractiveness fragment—AFLP analysis of local adaptation and sexual selection in a caeliferan grasshopper, Chorthippus biguttulus

Genetic variability among males is a necessary precondition for the evolution of female choice based on indirect genetic benefits. In addition to mutations and host-parasite cycles, migration of locally adapted individuals offers an explanation for the maintenance of genetic variability. In a previous study, conducting a reciprocal transplant experiment on a grasshopper, Chorthippus biguttulus, we found that environmental conditions significantly influenced not only body condition but also an important trait of male calling song, the amplitude of song. Although not significant, all other analysed physical and courtship song traits and attractiveness were superior in native than in transferred males. Thus, we concluded that local adaptation has a slight but consistent influence on a range of traits in our study populations, including male acoustic attractiveness. In our present study, we scanned male grasshoppers from the same two populations for amplification fragment length polymorphism (AFLP) loci connected with acoustic attractiveness to conspecific females. We found greater differences in allele frequencies between the two populations, for some loci, than are expected from a balance between drift and gene flow. These loci are potentially connected with locally adapted traits. We examined whether these alleles show the proposed genotype environment interaction by having different associations with attractiveness in the two populations. One locus was significantly related to sexual attractiveness; however, this was independent of the males' population affiliation. Future research on the evolution of female choice will benefit from knowledge of the underlying genetic architecture of male traits under intraspecific sexual selection, and the ‘population genomics' approach can be a powerful tool for revealing this structur

The maintenance of an inversion polymorphism in Coelopa frigida

The seaweed fly, Coelopa frigida, lives in piles of rotting seaweed deposited on beaches by tides and winds. In all populations studied it is polymorphic for two gene arrangements on Chromosome I. A polymorphism at the alcohol dehydrogenase locus is strongly associated with this inversion and can be used to estimate karyotype frequencies. An extensive series of samples from natural populations has revealed a seasonal cycle in inversion frequencies but otherwise frequencies are remarkable constant both geographically and temporally. There is a consistent excess of heterokaryotypes in these samples. Three selective forces influencing inversion frequencies have been investigated. 1) An association between karyotype and development time, previously observed in the laboratory, has been demonstrated in conditions case to those in natural populations. 2) Viability differences between karyotypes have been examined. In natural populations there is some evidence that the excess of heterokaryotypes increases with larval density. In the laboratory heterokaryotypes are shown to have higher viability than either homokaryotype but the strong density dependence reported previously has not been observed. Viability differences are concentrated in the first two days of larval life and are probably related to the rate of supply of nutrients. 3) An association is demonstrated between karyotype and adult size – especially in males. Adult size is shown to correlate with longevity and fecundity of both sexes. Several experiments indicate that large males enjoy greater mating success than small males. The relationship between larval density, development time and adult size is described. The possibility that the effect of the inversion varies between populations or between alcohol dehydrogenase genotypes has also been investigated. A simulation has been used to study how these selective forces interact with one another, and with the changeable environment in which the flies live, and to examine the extent to which they can account for the observed karyotype frequencies

The maintenance of an inversion polymorphism in Coelopa frigida

The seaweed fly, Coelopa frigida, lives in piles of rotting seaweed deposited on beaches by tides and winds. In all populations studied it is polymorphic for two gene arrangements on Chromosome I. A polymorphism at the alcohol dehydrogenase locus is strongly associated with this inversion and can be used to estimate karyotype frequencies. An extensive series of samples from natural populations has revealed a seasonal cycle in inversion frequencies but otherwise frequencies are remarkable constant both geographically and temporally. There is a consistent excess of heterokaryotypes in these samples. Three selective forces influencing inversion frequencies have been investigated. 1) An association between karyotype and development time, previously observed in the laboratory, has been demonstrated in conditions case to those in natural populations. 2) Viability differences between karyotypes have been examined. In natural populations there is some evidence that the excess of heterokaryotypes increases with larval density. In the laboratory heterokaryotypes are shown to have higher viability than either homokaryotype but the strong density dependence reported previously has not been observed. Viability differences are concentrated in the first two days of larval life and are probably related to the rate of supply of nutrients. 3) An association is demonstrated between karyotype and adult size – especially in males. Adult size is shown to correlate with longevity and fecundity of both sexes. Several experiments indicate that large males enjoy greater mating success than small males. The relationship between larval density, development time and adult size is described. The possibility that the effect of the inversion varies between populations or between alcohol dehydrogenase genotypes has also been investigated. A simulation has been used to study how these selective forces interact with one another, and with the changeable environment in which the flies live, and to examine the extent to which they can account for the observed karyotype frequencies

Signals of demographic expansion in Drosophila virilis

Background. The pattern of genetic variation within and among populations of a species is strongly affected by its phylogeographic history. Analyses based on putatively neutral markers provide data from which past events, such as population expansions and colonizations, can be inferred. Drosophila virilis is a cosmopolitan species belonging to the virilis group, where divergence times between different phylads go back to the early Miocene. We analysed mitochondrial DNA sequence variation among 35 Drosophila virilis strains covering the species' range in order to detect demographic events that could be used to understand the present characteristics of the species, as well as its differences from other members of the group. Results. Drosophila virilis showed very low nucleotide diversity with haplotypes distributed in a star-like network, consistent with a recent world-wide exponential expansion possibly associated either with domestication or post-glacial colonization. All analyses point towards a rapid population expansion. Coalescence models support this interpretation. The central haplotype in the network, which could be interpreted as ancestral, is widely distributed and gives no information about the geographical origin of the population expansion. The species showed no geographic structure in the distribution of mitochondrial haplotypes, in contrast to results of a recent microsatellite-based analysis. Conclusion. The lack of geographic structure and the star-like topology depicted by the D. virilis haplotypes indicate a pattern of global demographic expansion, probably related to human movements, although this interpretation cannot be distinguished from a selective sweep in the mitochondrial DNA until nuclear sequence data become available. The particular behavioural traits of this species, including weak species-discrimination and intraspecific mate choice exercised by the females, can be understood from this perspective.peerReviewe

Using replicate hybrid zones to understand the genomic basis of adaptive divergence

Combining hybrid zone analysis with genomic data is a promising approach to understanding the genomic basis of adaptive divergence. It allows for the identification of genomic regions underlying barriers to gene flow. It also provides insights into spatial patterns of allele frequency change, informing about the interplay between environmental factors, dispersal and selection. However, when only a single hybrid zone is analysed, it is difficult to separate patterns generated by selection from those resulting from chance. Therefore, it is beneficial to look for repeatable patterns across replicate hybrid zones in the same system. We applied this approach to the marine snail Littorina saxatilis, which contains two ecotypes, adapted to wave-exposed rocks vs. high-predation boulder fields. The existence of numerous hybrid zones between ecotypes offered the opportunity to test for the repeatability of genomic architectures and spatial patterns of divergence. We sampled and phenotyped snails from seven replicate hybrid zones on the Swedish west coast and genotyped them for thousands of single nucleotide polymorphisms. Shell shape and size showed parallel clines across all zones. Many genomic regions showing steep clines and/or high differentiation were shared among hybrid zones, consistent with a common evolutionary history and extensive gene flow between zones, and supporting the importance of these regions for divergence. In particular, we found that several large putative inversions contribute to divergence in all locations. Additionally, we found evidence for consistent displacement of clines from the boulder–rock transition. Our results demonstrate patterns of spatial variation that would not be accessible without continuous spatial sampling, a large genomic data set and replicate hybrid zones.publishedVersio

Human-facilitated metapopulation dynamics in an emerging pest species, Cimex lectularius

Peer reviewe

Towards the completion of speciation : the evolution of reproductive isolation beyond the first barriers

y Speciation, that is, the evolution of reproductive barriers eventually leading to complete isolation, is a crucial process generating biodiversity. Recent work has contributed much to our understanding of how reproductive barriers begin to evolve, and how they are maintained in the face of gene flow. However, little is known about the transition from partial to strong reproductive isolation (RI) and the completion of speciation. We argue that the evolution of strong RI is likely to involve different processes, or new interactions among processes, compared with the evolution of the first reproductive barriers. Transition to strong RI may be brought about by changing external conditions, for example, following secondary contact. However, the increasing levels of RI themselves create opportunities for new barriers to evolve and, and interaction or coupling among barriers. These changing processes may depend on genomic architecture and leave detectable signals in the genome. We outline outstanding questions and suggest more theoretical and empirical work, considering both patterns and processes associated with strong RI, is needed to understand how speciation is completed. This article is part of the theme issue 'Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.Peer reviewe

Evolution of a Complex Locus: Exon Gain, Loss and Divergence at the Gr39a Locus in Drosophila

Background. Gene families typically evolve by gene duplication followed by the adoption of new or altered gene functions. A different way to evolve new but related functions is alternative splicing of existing exons of a complex gene. The chemosensory gene families of animals are characterised by numerous loci of related function. Alternative splicing has only rarely been reported in chemosensory loci, for example in 5 out of around 120 loci in Drosophila melanogaster. The gustatory receptor gene Gr39a has four large exons that are alternatively spliced with three small conserved exons. Recently the genome sequences of eleven additional species of Drosophila have become available allowing us to examine variation in the structure of the Gr39a locus across a wide phylogenetic range of fly species. Methodology/Principal Findings. We describe a fifth exon and show that the locus has a complex evolutionary history with several duplications, pseudogenisations and losses of exons. PAML analyses suggested that the whole gene has a history of purifying selection, although this was less strong in exons which underwent duplication. Conclusions/Significance. Estimates of functional divergence between exons were similar in magnitude to functional divergence between duplicated genes, suggesting that exon divergence is broadly equivalent to gene duplication.Publisher PDFPeer reviewe