Genetic hitchhiking in a subdivided population of Mytilus edulis

<p>Abstract</p> <p>Background</p> <p>Few models of genetic hitchhiking in subdivided populations have been developed and the rarity of empirical examples is even more striking. We here provide evidences of genetic hitchhiking in a subdivided population of the marine mussel <it>Mytilus edulis</it>. In the Bay of Biscay (France), a patch of <it>M. edulis </it>populations happens to be separated from its North Sea conspecifics by a wide region occupied only by the sister species <it>M. galloprovincialis</it>. Although genetic differentiation between the two <it>M. edulis </it>regions is largely non-significant at ten marker loci (average F_ST~0.007), a strong genetic differentiation is observed at a single locus (F_ST= 0.25). We validated the outlier status of this locus, and analysed DNA sequence polymorphism in order to identify the nature of the selection responsible for the unusual differentiation.</p> <p>Results</p> <p>We first showed that introgression of <it>M. galloprovincialis </it>alleles was very weak in both populations and did not significantly affect their differentiation. Secondly, we observed the genetic signature of a selective sweep within both <it>M. edulis </it>populations in the form of a star-shaped clade of alleles. This clade was nearly fixed in the North Sea and was segregating at a moderate frequency in the Bay of Biscay, explaining their genetic differentiation. Incomplete fixation reveals that selection was not direct on the locus but that the studied sequence recombined with a positively selected allele at a linked locus while it was on its way to fixation. Finally, using a deterministic model we showed that the wave of advance of a favourable allele at a linked locus, when crossing a strong enough barrier to gene flow, generates a step in neutral allele frequencies comparable to the step observed between the two <it>M. edulis </it>populations at the outlier locus. In our case, the position of the barrier is now materialised by a large patch of heterospecific <it>M. galloprovincialis </it>populations.</p> <p>Conclusion</p> <p>High F_SToutlier loci are usually interpreted as being the consequence of ongoing divergent local adaptation. Combining models and data we show that among-population differentiation can also dramatically increase following a selective sweep in a structured population. Our study illustrates how a striking geographical pattern of neutral diversity can emerge from past indirect hitchhiking selection in a structured population.</p> <p>Note</p> <p>Nucleotide sequences reported in this paper are available in the GenBank™ database under the accession numbers <ext-link ext-link-type="gen" ext-link-id="EU684165">EU684165</ext-link> – <ext-link ext-link-type="gen" ext-link-id="EU684228">EU684228</ext-link>.</p

Differential expression of the heat shock protein Hsp70 in natural populations of the tilapia, Sarotherodon melanotheron, acclimatised to a range of environmental salinities

<p>Abstract</p> <p>Background</p> <p>The relationship between environmental variation and induction of heat shock proteins <it>(Hsps) </it>has been much documented under experimental conditions. However, very little is known about such induction in natural populations acclimatised to prevailing environmental conditions. Furthermore, while induction of stress proteins has been well documented in response to environmental contaminants and thermal stressors, little is known about whether factors, such as extreme salinity, are also potential inductors. The black-chinned tilapia <it>Sarotherodon melanotheron </it>is unusual for its ability to colonise estuarine environments in West Africa that are characterised by extremely high salinities. The relationships between mRNA levels of the 70 kDa heat shock protein (<it>Hsp70</it>) and Na⁺, K⁺-ATPase1α (<it>Naka</it>) in the gills, environmental salinity, and a life-history trait (condition factor) were investigated in wild populations of this species sampled from three locations in the Saloum estuary, at salinities ranging from 40 to 100 psu.</p> <p>Results</p> <p>The highest <it>Hsp70 </it>and <it>Naka </it>mRNA levels, and the poorest condition factors were recorded in the most saline sampling site (100 psu). The <it>Hsp70 </it>and <it>Naka </it>mRNA were correlated amongst themselves and showed a direct positive correlation with environmental salinity, and a negative correlation with fish condition factor. Thus, the <it>Hsp70 </it>is constitutively overexpressed by <it>S. melanotheron </it>acclimatised to extreme hypersalinity.</p> <p>Conclusions</p> <p>These results indicate that, although <it>S. melanotheron </it>can colonise extremely saline environments, the overexpression of <it>Hsp70 </it>combined with the higher <it>Naka </it>mRNA expression reveals that this represents a chronic stress. The induction of <it>Hsp70 </it>was, therefore, a biomarker of chronic hyper-osmotic stress which presumably can be linked to the impaired growth performance and precocious reproduction that have been demonstrated in the populations at the extremely saline sites.</p

Diferencia de eficacia biológica entre fenotipos crípticos relacionados con salinidad en la lubina (Dicentrarchus labrax)

The existence of cryptic salinity-related phenotypes has been hypothesized in the “euryhaline” sea bass (Dicentrarchus labrax). How differential osmoregulation costs between freshwater and saltwater environments affect fitness and phenotypic variation is misunderstood in this species. During an experiment lasting around five months, we investigated changes in the whole body mass and in the expression of growth-related genes (insulin-like growth factor 1 [IGF-1]; growth hormone receptor [GHR]) in the intestine and the liver of sea bass thriving in sea water (SSW), successfully acclimated to freshwater (SFW), and unsuccessfully acclimated to freshwater (UFW). Albeit non-significant, a trend toward change in body mass was demonstrated among SSW, UFW and SFW fish, suggesting that SSW fish were a mixture of the other phenotypes. Several mortality peaks were observed during the experiment, with batches of UFW fish showing higher expression in the osmoregulatory intestine due to down-regulation of genes in the liver and significant up-regulation of GHR in the intestine compared with SFW fish. Energy investment toward growth or ion homeostasis hence partly mediates the fitness difference between cryptic SFW and UFW phenotypes. The use of a genetic marker located within the IGF-1 gene showed no genotype-phenotype relationship with levels of gene expression.En la especie “eurihalina” de la lubina (Dicentrarchus labrax) se ha planteado la existencia de fenotipos crípticos relacionados con la salinidad. En esta especie los costes diferenciales de osmoregulación a la adaptación en de agua dulce y salada son aún desconocidos. Durante un experimento de aproximadamente 5 meses, se investigó los cambios en la masa corporal y en la expresión de genes relacionados con el crecimiento (factor de crecimiento similar a la insulina 1 [IGF-1]; receptor de la hormona del crecimiento [GHR]) en el intestino y el hígado de lubina en individuos que prosperan en agua de mar (SSW), individuos aclimatados con éxito con el agua dulce (SFW), e individuos no aclimatados al agua dulce (UFW). Aunque no es significativa, se observa una tendencia de cambio en la masa corporal entre individuos SSW, UFW y SFW. Estos resultados sugieren que los individuos SSW son una mezcla de los otros fenotipos. Se observaron varios picos de mortalidad durante el experimento, con lotes de peces UFW que presentan una expresión génica más elevada en el intestino osmoregulador, debido a la regulación a la baja de genes en el hígado y regulación hacia arriba en la GHR del intestino cunado se compara con los peces SFW. Por lo tanto, la inversión de energía hacia el crecimiento o la homeostasis iónica explica en parte la diferencia de adaptación entre los crípticos fenotipos SFW y UFW. El uso de un marcador genético localizado dentro del gen de IGF-1 no demuestra relación genotipo-fenotipo con los niveles de expresión génica

Species-wide distribution of highly polymorphic minisatellite markers suggests past and present genetic exchanges among house mouse subspecies

Global analysis of four minisatellite loci in House Mouse reveals unexpected long-range gene flow between populations and subspecies

Effect of aliphatic chain length on the chemical structures of low molecular weight hyperbranched polyesters

Low molecular weight hyperbranched (HB) polyesters were synthesized via melt polymerization from trimethylolpropane and three aliphatic dicarboxylic acids, namely, succinic acid (SA), adipic acid (AA) and dodecanedioic acid (DA). The degrees of branching (DBs) ranged between 30% and 75% depending on the monomer ratio. Their DB, cyclic index and terminal index were all determined, indicating that the shorter chain HB polyesters PE-SA and PE-AA showed a greater degree of intramolecular cyclization, compared to the longer aliphatic chains within PE-DA. The HB polyesters form stable colloidal suspensions in buffered aqueous media and were found to be pH responsive. The stability of the colloidal suspensions is enhanced by two factors: (1) increasing the aliphatic chain length and (2) increasing the pH of the solution for the same HB polyester

Fitness difference between cryptic salinity-related phenotypes of sea bass (<em>Dicentrarchus labrax</em>)

The existence of cryptic salinity-related phenotypes has been hypothesized in the “euryhaline” sea bass (Dicentrarchus labrax). How differential osmoregulation costs between freshwater and saltwater environments affect fitness and phenotypic variation is misunderstood in this species. During an experiment lasting around five months, we investigated changes in the whole body mass and in the expression of growth-related genes (insulin-like growth factor 1 [IGF-1]; growth hormone receptor [GHR]) in the intestine and the liver of sea bass thriving in sea water (SSW), successfully acclimated to freshwater (SFW), and unsuccessfully acclimated to freshwater (UFW). Albeit non-significant, a trend toward change in body mass was demonstrated among SSW, UFW and SFW fish, suggesting that SSW fish were a mixture of the other phenotypes. Several mortality peaks were observed during the experiment, with batches of UFW fish showing higher expression in the osmoregulatory intestine due to down-regulation of genes in the liver and significant up-regulation of GHR in the intestine compared with SFW fish. Energy investment toward growth or ion homeostasis hence partly mediates the fitness difference between cryptic SFW and UFW phenotypes. The use of a genetic marker located within the IGF-1 gene showed no genotype-phenotype relationship with levels of gene expression