Why study just one reef : spatial patterns of environmental heterogeneity and genetic relatedness for the coral, Pocillopora damicornis

Gorospe, Kelvin Dolauta
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[Honolulu] : [University of Hawaii at Manoa], [May 2013]
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In molecular ecology, researchers often test for a pattern of isolation-by-distance, whereby genetic variants sampled closer together in space are, on average, found to be more genetically related than variants sampled at further distances. Such a pattern can be explained by dispersal processes, whereby the genetic homogenizing effects of migration are distance-limited. Several examples from the marine environment, however, do not fit this expectation (Selkoe et al. 2010), either because the patterns are anisotropic (i.e., geographically asymmetric) or stochastic, leading researchers to give ad hoc explanations for the paradoxical pattern. For corals, in particular, geographic patterns of genetic variation have often proven difficult to interpret (Adjeroud & Tsuchiya 1999; Ayre & Hughes 2000; Magalon et al. 2005; Baums et al. 2006; Severance & Karl 2006; Souter et al. 2009). The interest of this dissertation is to gain insight into this coral population genetic paradox. The original paradigm of dispersal in the sea was one of demographically open populations connected by planktonic larvae capable of dispersing long-distances across open ocean. Subsequent genetic studies, however, uncovered genetic differentiation on much smaller than expected scales. In fact, for corals, several studies demonstrate the potential for adaptive genetic variation (e.g., D'Croz & Maté 2004; Vermeij et al. 2007). Barshis et al. 2010). Corals also share several terrestrial plant-like characteristics (e.g., dispersing propagule stage, sessile adult stage, and the ability to reproduce clonally) that are also believed to enhance the adaptive capacities of plants to small-scale environmental heterogeneity (Vekemans & Hardy 2004). Taken together, these observations warrant the study of coral genetic and environmental variation on an intrareef scale, a scale for which there has been little interest. In ecology, it is widely recognized that patterns are scale-dependent. Population genetic sampling designs, however, rarely bridge across scales and have only recently been subject to explicitly spatial analyses whereby the spatial coordinates of individual sampling units are examined alongside genetic data (Storfer et al. 2007). Thus, taking a landscape-genetics approach, combining spatial analysis with landscape ecology and population genetics, will be important for making inferences on the processes driving patterns of genetic variation on an intra-reef scale. The dataset for this dissertation is a near-exhaustive assessment of individual-level spatial genetic patterns for the widely-studied, cosmopolitan, pan-Pacific coral, Pocillopora damicornis within a single coral reef as well as a reef-wide characterization of environmental heterogeneity. The focal study site, Reef 19, is a single patch reef (diameter of ~40 m) in Kāne'ohe Bay (21.45767°N, 157.80677°W) with a depth of between 1 and 5 m. Using a two year temperature dataset taken across a 4 m grid, Chapter 2 describes the environmental heterogeneity of Reef 19 in terms of temperature, depth, and habitat cover (Gorospe & Karl 2011). Chapter 3 (Gorospe & Karl, accepted) introduces the genetic and spatial data for P. damicornis, with an interest in dispersal and colonization on this scale. These data included a near-exhaustive (n=2352) genetic sampling and spatial mapping of P. damicornis throughout Reef 19, as well as a much smaller, stratified random sampling effort on three neighboring reefs. Rarely, however, is it practical to sample so intensively. Thus, the consequences of sampling effort and design on the characterization of reef genetic diversity are explored in Chapter 4. Finally, in Chapter 5, environmental, genetic, and spatial data are combined in a landscape genetics approach to tease apart the influence of spatially-versus environmentally mediated processes on intra-reef patterns of genetic variation. Throughout, results are discussed from the perspective of P. damicornis' breadth of literature, within the framework of the coral population genetics paradox described above, as well as within the context of global climate change and coral reef conservation.
Ph.D. University of Hawaii at Manoa 2013.
Includes bibliographical references.
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Zoology.
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