Investigating scales of physiological and genetic adaptation to climate change along environmental gradients in keystone intertidal organisms from Macaronesia

Objectivo

Until now, only a few studies have successfully decoupled the confounding roles of environmentally-induced phenotypic plasticity an local genetic adaptation in generating differences among marine populations. The rocky intertidal communities of continental Europe and Macaronesia represent ideal systems for a comprehensive analysis of the contributions of plasticity versus genetic variation to phenotypic variation due to the fact that environmental gradients across the intertidal zone within shores, as well as among localities across the European coast and Macaronesian islands impose predictably varying selection with the potential to drive adaptive differentiation across a variety of scales.

This PhD proposal will be integrated in a funded research project scheduled to start in May 2012, aiming to compare the scale of adaptive genetic differentiation along wide environmental gradients among intertidal limpet populations under different gene flow scenarios. This innovative project will combine field experiments and state-of-the-art molecular techniques to test the hypothesis that populations which are more connected by gene flow will exhibit less genetic differentiation attributable to local adaptation, compared to more isolated populations. The study here proposed will provide novel insights into the processes by which a species with broad geographical ranges can adapt to local environmental conditions, despite the homogenizing effects of gene flow. Moreover, it will also contribute to understanding whether Macaronesian limpet populations are particularly vulnerable to extinction for having developed locally adaptive traits to a subset of the environmental variability found throughout the species range, or instead, if the amount of phenotypic plasticity within each population represents the entire range of tolerances found in the species. Understanding how adaptive genetic variation is distributed geographically will enable prioritizing for conservation those populations possessing greater genetic potential for adaptation to environmental change.

This is a very important point to consider in order to accurately predict species’ range shifts and extinction probability, because environmental tolerance is not likely to be homogeneous among populations throughout the whole species’ range. Extinction risk will be underestimated if local adaptation reduces the range of environmental tolerance of individual populations compared to the species as a whole. A good example of this seems to come from a previous study conducted by the research team that the PhD candidate will join. In this study, a climate envelope model was employed to accurately predict the recent bridging of a gap in northern Portugal by the lusitanian limpet Patella rustica. However the model also wrongly predicted a poleward range extension under present climatic conditions . Although a possible barrier to dispersal could explain this result, population genetic studies conducted afterwards suggested that limpet populations, including P. rustica, are broadly connected through dispersal . The disagreement between modelled and real distributions could thus be explained by unaccounted local adaptation to warmer micro-climate conditions of populations near the northern range edge of the species.
The proposed doctoral plan will focus on the limpet species P. candei, endemic to the Macaronesian archipelagos of Azores, Madeira, Selvagens and Canaries. The plan has four critical elements, including (1) measuring the range of intertidal temperatures experienced by P. candei throughout its range, (2) characterizing the transcriptome of P. candei, (3) obtaining gene
expression profiles throughout Macaronesia, (4) quantifying plastic and genetic components of gene expression and correlating those patterns with levels of environmental stress along the species range and among micro-habitats.

Specifically, this proposal will:

1. Estimate approximate body temperatures of limpets in situ by deploying a network of bio-mimetic thermal loggers at several shores in the Azores, Madeira and Canaries. These are cost-effective sensors designed by a member of our research team [27] that match the temperature trajectories of live limpets and collect temperature data autonomously for several months. These data will serve as proxy for environmental variation experienced by limpets throughout Macaronesian islands.
2. Produce a comprehensive catalogue of annotated genes and genetic markers through state-of-the-art massive parallel sequencing of the transcriptome of P. candei. This species was selected because it lives in the midshore, thus being more susceptible to environmental stress. Transcriptomic information will provide the foundation for the remaining objectives. Screening of the markers will enable evaluating physiological responses of limpets to environmentally-induced stress by means of simultaneous measurements of gene expression at a large number of loci, under different environmental conditions.
3. Characterize temporal patterns of gene expression profiles during tidal cycles. This will be done using custom-made cDNA microarray chips, which allow surveying the expression level of thousands of genes simultaneously and in a cost-effective manner. Expression profiles of P. candei will be monitored throughout Macaronesia (Azores, Madeira, Selvagens and Canaries), in summer and winter. This analysis will enable identifying the different physiological states of limpets, how they vary in space, and also the genes, or families of genes, involved in responses to environmental stress.
4. Isolate the genetic basis of geographical variation in physiological responses to environmental stress, measured previously by gene expression analysis, using a common-garden experiment. Groups of limpets will be collected from five locations in Macaronesia and kept them under the same conditions in the laboratory. With this approach it will be possible, after a period of acclimation, to monitor expression profiles of limpets at different experimental temperatures and evaluate, by comparison, how much of the variation among populations (assessed previously) is due to phenotypic plasticity. This approach will also offer an insight onto genes that may be under selective pressure.

This project focuses on developing a multidisciplinary understanding of the effects of climate change on organisms that are critical grazers, shaping the dynamics of intertidal communities throughout the world’s oceans. From an ecological perspective, limpets have become model species. However, virtually nothing is known about the genetic basis of their extraordinary ecological success, and consequently how climate change will affect their numbers and distributions. In this sense, the expected outcomes of this thesis will have extremely important socio-economic implications in Macaronesia, and especially in the Azores, by identifying the mechanisms involved in physiological responses to environmental stress by limpets and thus providing an accurate prediction of how climate warming will impose additional pressure to these species. Identifying how adaptive genetic variation is distributed is important also because locally adapted populations should be prioritized for conservation if they can serve as sources for repopulation of disturbed areas.

Finally, characterizing the transcriptome in limpets will also provide the international research community with a novel toolbox to address a number of fundamental questions in these ecological model organisms that fall outside the scope of this project.
The PhD candidate will join an international team with extensive research experience in intertidal ecosystems, covering experimental ecology, biogeography, larval biology, dispersal and connectivity, local adaptation and evolution of marine organisms, mainly invertebrates. The candidate will thus have the opportunity to develop multidisciplinary skills and mature as a researcher in a stimulating research environment.