Recombination plays an important role in species adaptation since it acts as an evolutionary force that can influence genome pattern organization. However, recombination can be detrimental in some situations, causing the breakdown of some adaptive gene combinations such as coadapted gene complexes. Genetic and cytological chromosome maps allow recombination throughout the genome to be analyzed. In this study we compare the recombination rate of two types of homokaryotypic lines of D. subobscura (O(ST) and O( 3+4 )) using a set of at least 13 microsatellite loci. The genetic maps obtained present similar lengths: 184 and 196 cM for O(ST) and O( 3+4 ) chromosomes, respectively. For most pairs of markers analyzed, a sample size of about 150 individuals appeared sufficient to obtain appropriate recombination values, with the exception of markers located in the same cytological band. Recombination rates seemed to be fairly uniform along the O chromosome, but some regional differences were observed. Several recombination hot and coldspots were detected, and their numbers were different in the homokaryotypic line types (O(ST) and O( 3+4 )). This variability could be attributed to differences between the genetic content of the two arrangements or to differences between the lines.
Several pieces of evidence indicate a Mediterranean origin of the colonization of America by Drosophila subobscura. To ascertain whether the origin was from the Eastern or the Western Mediterranean region, samples from Barcelona (Spain) and Mt Parnes (Greece) were collected and O chromosomal inversion polymorphism and lethal genes were analysed. The frequencies of lethal chromosomes were 0.244 ± 0.039 in Barcelona and 0.336 ± 0.043 in Mt Parnes, consistent with the expectations for large populations located in the central area of the species distribution. Lethal genes seem to be distributed at random along the O chromosome in both populations. The intra-populational allelism frequencies of Barcelona and Mt Parnes were 0.016 ± 0.007 and 0.012 ± 0.005 respectively. Thus, the estimates of the effective population size were high in both populations (between 6964 and 13 004 in Barcelona and 11 874 to 26 828 in Mt Parnes). The cases of allelism in Mt Parnes were observed only between individual lethal genes, but in Barcelona some concatenated clusters of allelism were detected. This pattern of allelism can be explained by synthetic lethality, hybrid dysgenesis, the induction of recurrent lethal mutations by different factors or an effect of microdifferentiation in subpopulations. In both populations, a reduction in fitness in the heterozygotes for lethal genes has been detected. Furthermore, the estimates of the migration coefficient (between 0.0085 and 0.0120 in Barcelona, and 0.0057 and 0.0087 in Mt Parnes) confirm the existence of gene flow between Palearctic populations of D. subobscura. Our lethal genes and chromosomal inversion results are consistent with a Mediterranean origin of the colonization of America by D. subobscura, but are inconclusive with regard to the identification of the population from which these colonizers came.
SUMMARY:The genetic structure of populations/species was established during the Quaternary glaciations. Over the last 250 ka (Pleistocene), the South American marine biogeographic history recorded three main glaciations: the most extensive one between 140 and 180 ka, a minor one between 60 and 70 ka, and the last glaciation approximately between 15 and 35 ka. With the aim of assessing the pattern of molecular diversity and historical demography of weakfish (Cynoscion guatucupa), a 365 bp sequence of the mitochondrial control region was amplified at four coastal sites located in the southwestern Atlantic. Haplotype diversity was high, whereas nucleotide diversity was low and similar at each sample site. AMOVA failed to detect population structure. This lack of differentiation was subsequently observed in the distribution of samples sites in the haplotype network. Fu's Fs was negative and highly significant while the mismatch analysis yielded a unimodal distribution, indicating a global population expansion. The Bayesian skyline plot revealed a coalescence time of weakfish population at approximately 210 ka, and a very rapid expansion from 180-190 ka, probably caused by a habitat expansion, as these two events coincide in time.Keywords: bayesian skyline plots, glacial cycles, mitochondrial DNA, control region, population genetics, marine fish, Atlantic coast, Pleistocene. RESUMEN: Filogeografía de la pescadilla de red CynosCion guatuCupa (Perciformes: Sciaenidae) del Atlántico suroccidental. -La estructura genética de las poblaciones/especies se ha establecido principalmente durante las glaciaciones del Cuaternario. La historia biogeográfica marina en Sudamérica en los últimos 250 ka registra durante el Pleistoceno tres glaciaciones importantes: la más extensa entre 140-180 ka, una menor entre 60-70 ka y la última glaciación aproximadamente entre 15-35 ka. Para evaluar el patrón de diversidad molecular y de demografía histórica en la pescadilla de red (Cynoscion guatucupa) se analizó una secuencia de 365 pb de la región control del ADN mitocondrial en cuatro zonas costeras del Atlán-tico suroccidental. La diversidad haplotípica fue alta mientras que la diversidad nucleotídica fue baja y ambos índices fueron similares en cada sitio de muestreo. El análisis de varianza molecular (AMOVA) no detectó estructura poblacional. Esta falta de diferenciación posteriormente se pudo observar en la distribución de los sitios de muestreo en la red de haplotipos. El test de Fu fue negativo y altamente significativo y el análisis de falta de coincidencia produjo una distribución unimodal indicando expansión poblacional. El skyline plot indicó un tiempo a la coalescencia de 210 ka y una expansión poblacional que comenzó hace 180-190 ka que probablemente se produjo por la expansión del hábitat ya que ambos eventos coinciden en el tiempo.
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