RESUMEN
Divergent selection at ecologically important traits is thought to be a major factor driving phenotypic differentiation between populations. To elucidate the role of different evolutionary processes shaping the variation in gill raker number of European whitefish (Coregonus lavaretus sensu lato) in the Baltic Sea basin, we assessed the relationships between genetic and phenotypic variation among and within three whitefish ecotypes (sea spawners, river spawners and lake spawners). To generate expected neutral distribution of FST and to evaluate whether highly variable microsatellite loci resulted in deflated FST estimates compared to less variable markers, we performed population genetic simulations under finite island and hierarchical island models. The genetic divergence observed among (FCT = 0.010) and within (FST = 0.014-0.041) ecotypes was rather low. The divergence in gill raker number, however, was substantially higher between sea and river spawners compared to observed microsatellite data and simulated neutral baseline (PCT > FCT ). This suggests that the differences in gill raker number between sea and river spawners are likely driven by divergent natural selection. We also found strong support for divergent selection on gill raker number among different populations of sea spawners (PST > FST ), most likely caused by highly variable habitat use and diverse diet. The putative role of divergent selection within lake spawners initially inferred from empirical microsatellite data was not supported by simulated FST distributions. This work provides a first formal test of divergent selection on gill raker number in Baltic whitefish, and demonstrates the usefulness of population genetic simulations to generate informative neutral baselines for PST -FST analyses helping to disentangle the effects of stochastic evolutionary processes from natural selection.
Asunto(s)
Adaptación Fisiológica , Peces/fisiología , Branquias , Animales , Países Bálticos , Peces/genética , Variación Genética , Genotipo , Repeticiones de Microsatélite/genéticaRESUMEN
The SmaI family of repeats is present only in the chum salmon and the pink salmon, and it is not present in five other species in the same genus or in other species in closely related genera. In the present study, we showed that another short interspersed repetitive elements (SINEs) family, which is almost identical to the SmaI family, is present in all fishes in the subfamily Coregoninae, being regarded as the most primitive salmonids. This new family of SINEs was designated the SmaI-cor family (SmaI family of repeats in coregonids). The consensus sequence of the SmaI-cor family was found to be 98.6% homologous to that of the SmaI family. Accordingly, it is difficult to explain the high degree of homology between these two families of SINEs by any mechanism other than the horizontal transfer of SINEs. The estimates of the rate of neutral mutation of nuclear genes, comparing chum salmon and European whitefish, confirmed this possibility. Our results strongly suggest that a member(s) of the SmaI-cor family might have been transferred horizontally from one coregonid species to a common ancestor of chum and pink salmon or to these two species independently, to allow subsequent amplification of the SmaI family in their respective genomes.
Asunto(s)
Oncorhynchus keta/genética , Secuencias Repetitivas de Ácidos Nucleicos , Animales , Secuencia de Bases , Clonación Molecular , ADN , Desoxirribonucleasas de Localización Especificada Tipo II , Variación Genética , Datos de Secuencia Molecular , Especificidad de la EspecieRESUMEN
There are at least three different kinds of short interspersed repetitive elements in salmonid genomes. Of these, members of the HpaI family are found to be most widely distributed in salmonid species. The HpaI family is present with high copy numbers in all members of the subfamily Salmoninae, such as salmon, trout and charr. In order to determine whether the amplification of the Hpa sequence occurred rapidly within a short evolutionary period or gradually, over the long term, a search was made for Hpa sequences in primitive salmonids; namely, grayling and whitefish. A grayling species has fivefold fewer copies of these sequences than the Salmoninae species, whereas several whitefish species have 200-fold to 20-fold fewer copies than the Salmoniae species. Characterization of the Hpa sequences in these species allowed us to recognize two distinct Hpa subfamilies on the basis of diagnostic substitutions as well as a new short interspersed element with an Hpa-related sequence. The distribution of these sequences revealed that distinct members of the HpaI or Hpa-related family were amplified during establishment of each subfamily lineage in a manner very similar to the amplification of the human Alu family. We provide evidence for the validity of a model that involves "multiple source genes" to explain diagnostic substitutions of the Hpa subfamilies and the timing of their appearance during evolution.