Local adaptation and spatio-temporal dynamics in Pyura chilensis Molina 1782 tunicate (Urochordata: Ascidiacea) along the Humboldt Current system.
AuthorSegovia-Cortés, Nicolás Ignacio
Local adaptation corresponds to the processes that arise because natural selection in heterogeneous environments is opposed to the homogenizing effects of neutral processes such as gene flow. In coastal marine habitats, many animals have dispersive larval stages that allow connectivity between local populations, yet connectivity is not necessarily correlated with dispersal capacity. On the other hand, the physical transport of individuals among l... Ver más
Local adaptation corresponds to the processes that arise because natural selection in heterogeneous environments is opposed to the homogenizing effects of neutral processes such as gene flow. In coastal marine habitats, many animals have dispersive larval stages that allow connectivity between local populations, yet connectivity is not necessarily correlated with dispersal capacity. On the other hand, the physical transport of individuals among local populations does not ensure effective gene flow, since this requires that the migrants survive and reproduce, implying that the selective mortality of migrants’ due to environmental pressures could reduce gene flow between local populations. Few phylogeographic studies have been approached using molecular markers to detect the effects of natural selection and contrast them with the effects of neutral evolutionary forces. For a better understanding of the contributions of different evolutionary forces in phylogeographic structuring, the tunicate Pyura chilensis, a sessile organism endemic to the highly heterogeneous Humboldt Current System (HCS), was used as the study model. This organism has a low capacity for larval dispersion, but a high dispersion capacity due to anthropogenic transport on deck of boats. Previous studies showed moderate genetic structure in the species and absence of genetic differentiation associated with a biogeographic break widely reported at 30°S, which has been reported for other marine taxa with low dispersal capacity. In the first chapter, we compared the genetic diversity and differentiation of six local populations between 24º and 42ºS using data from 2251 neutral SNPs and 81 putative under positive selection loci obtained through Genotyping-by-Sequencing. The results founded with neutral and putatively under selection loci were contrasting. Neutral loci, on the one hand, recognize a significantly differentiated evolutionary lineage in the local population of Los Molinos (39ºS), with FST values of an order of magnitude higher than the rest of the pairwise comparisons. These results are consistent with those reported in a previous study using sequences of the mitochondrial gene Cytochrome Oxidase I (COI). Neutral SNP loci did not detect a genetic discontinuity associated with the biogeographic break at 30°S as main signal. On the other hand, SNPs putatively under selection showed that most of the divergence is explained by the presence of two groups of populations, one to the north and one to the south of the 30s (which includes Los Molinos). Therefore, the phylogeographic structure of loci under selection would be more influenced by dissimilar environmental conditions in the biogeographic break at 30°S than by the history of divergence detected with neutral markers. The second chapter of this thesis, we evaluated the spatial-temporal genetic structure in Pyura chilensis using 592 partial sequences of the mitochondrial COI gene in six localities (between 24º and 42ºS) and three annual sampling (2012, 2014 and 2015). The COI sequences corresponded to three lineages previously described in the literature, one of which is highly divergent and restricted to the 39ºS zone. The data indicate a general pattern of temporal stability, although significant differences was observed from year to year in certain geographical sites and when analyzing these three lineages separately. The detected temporal stability could be due to restrictions on the genetic flow, as expected for species with low larval dispersion and susceptible to environmental changes. In the third chapter, seascape genomics tools were used to associate environmental variables with the adaptive genetic structure associated with the biogeographic break of the 30ºS using SNPs scattered throughout the genome in Pyura chilensis along the HCS. In this study, we analyzed the relationship between nine oceanographic variables and the adaptive genetic structure using 2902 SNPs in 149 individuals from 5 sites between 24º and 42ºS. Hundreds of candidate loci (between 390-585) were detected, which were significantly associated with sea surface temperature (SST) and variables associated with upwelling and productivity such as phosphate, nitrate, fluorescence and pH. The spatial genetic structure of candidate markers for each environmental variable consistently shows the break at 30°S beyond what is expected by the distance between study sites, suggesting that these variables could model the adaptive divergence between the north and south of 30°S in species such as P. chilensis. In addition, 5 genes with known annotated functions were identified, which are associated with processes such as filtration and ventilation (biogenesis of cilia), growth and transcription factors. The susceptibility of the model species to environmental changes suggested in this study is in contrast to previous reports, where the species was considered as a single fishing stock along its geographical distribution. This is critical for both the conservation of the resource and future prospects for studies of the resilience of marine organisms to climate change. In this thesis, it has been shown how the heterogeneity of the seascape can counteract the effect of gene flow along the Humboldt Current System, which has repercussions on the adaptive genetic structure of P. chilensis through associated local adaptation processes to a biogeographic break associated with the 30ºS. Ver menos
InstitutionUNIVERSIDAD DE CHILE
Date de publicación2018
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