Population genetic structure of a planktonic diatom in the Gulf of naples : Pseudo-nitzschia multistriata

Tesson, Sylvie Vaïana Marie (2010). Population genetic structure of a planktonic diatom in the Gulf of naples : Pseudo-nitzschia multistriata. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d654


The planktonic diatom Pseudo-nitzschia multistriata appeared in plankton samples taken at the MareChiara station in the Gulf of Naples in 1995 and has appeared ever since in autumn, and since 2004 also in summer. This thesis focuses on population genetic structure of the species using the Internal Transcribed Spacers in the rDNA as well as seven polymorphic microsatellites.

Cells isolated from plankton samples taken in 2008 and 2009 were grown into strains from which ITS-sequences and microsatellite fingerprints were gathered. Microsatellite screening uncovered two populations in sympatry in 2008, only one of which reappeared in 2009. The ITS sequences of the strains were of distinct types,called A-type, B-type and a mixed type (NB). The two populations also showed distinct ITS-type proportions, supporting their distinctness. Intra-population proportions of ITS-types among strains were similar in summer and autumn 2009,indicating stability over the growth season. But proportions differed between 2008 and 2009, suggesting substantial immigration the second year. These results suggest that, P. multistriata in the Mediterranean consists of a dynamic patchwork of populations.

Microsatellite inheritance was studied over the vegetative and sexual parts of the biennial life cycle of P. multistriata. Substitutions were recorded in strains rescreened after several months of maintenance. Longer microsatellites changed more frequently than shorter ones and di-nucleotide ones more than tri-nucleotide ones.Strains interbred independently from population assignment. Comparisons of parental and offspring genotypes showed microsatellite inheritance patterns generally following Mendelian rules. Genetic identity of sister cells emerging from zygotes formed on the same parental gametangia indicate that one of the two nuclei resulting from the first meiotic division is eliminated. Each maternal gamete fuses with a paternal gamete, resulting in two zygotes, which, in this case, were genotypically identical. Thus, mutations boost allele diversity and biennial sexual reproduction boosts ITS-genotype variability and genotypic diversity.

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