Padrões de distribuição genotipica em litorinideos (Mollusca : Gastropoda) da costa brasileira / Genotypic distribution patterns in littorinids (Mollusca : Gastropoda) from Brazilian coast

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

One of the main questions in biology is the link between a species ecology and its evolution. Evaluating the relationship between the geographical scale over which populations differ genetically and the species dispersal ability is a way to understand how this link is established. Spatial variation in allellic frequencies of natural populations may be explained by isolation by distance, population history or diversifying selection. Populational analyses at different scales are appropriate to evaluate how gene frequencies vary in time and space. The main goal of this study was to analyse the distribution of the genetic variability in littorinids using allozymes as molecular marker. In the first chapter, the pattern of heterozygote deficiency was evaluated in three littorinid species (Echinolittorina lineolata, Littoraria flava and L. angulifera) at a macrogeographic scale along the Brazilian coast (4,000 Km). A homogeneity test among loci showed heterogeneous FIS values in most populations. This result ruled out inbreeding and Wahlund effect as the main causes of departure of Hardy- Weinberg expectations. In the three littorinid species, at least one Pgm locus had homogeneous FIS values along all sampled populations, which suggests that this enzyme may have an important role in the fitness or may be linked to a locus under selection. In the second chapter, local-scale subdivision in Littoraria flava was investigated in order to test if Hardy- Weinberg deviations could be explained by micro-structuring, despite the planktotrophic larval phase. Samples were collected along horizontal transects in rocky shores of three different beaches, three times over a year. With this sampling design, and using 15 polymorphic allozymic loci, we searched for indications of any micro-scale or short-temporal subdivision in contrast with macrogeographic (200 Km) structuring. Littoraria flava samples presented significantly more structure within transects and along the temporal scale than at large-scale. Eighteen percent of the Ewens- Watterson neutrality test showed significant deviation of neutrality expectation. This suggested that there could be a balance among several recurrent colonizations by cohorts with different allelic frequencies, followed by a directional selection on different loci at different times and localities. In the last chapter, we assessed if environmental heterogeneity could affect radular form in L. flava and L. angulifera. We also made a reciprocal transfer experiment in natural conditions between mangrove and rocky shore locations, apart nearby 100 m. Individuals of both species from mangrove showed less variation in the shape of radula than those from rocky shores, implying in a different environmental effect in each species. In the natural transfer experiment, radulae morphology of L. flava individuals changed within 40 days, but the length of the radulae were strongly infIuenced by the original substrate (F6,22= 17 .13, p<0.00l). Changes in the shape had different intensities, suggesting that this trait could be subject to phenotypic plasticity

ASSUNTO(S)

population genetics genetica de populações phenotype molusco fenotipo isoenzimas isoenzymes mollusk

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