Species boundaries in scleractinian corals: a case study of the Acropora humilis species group

Wolstenholme, Jackie (2003) Species boundaries in scleractinian corals: a case study of the Acropora humilis species group. PhD thesis, James Cook University.

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Abstract

Accurate identification of species is critical for studies of biological systems, including biodiversity analyses, understanding evolutionary processes and ecological dynamics, and for effective conservation and management of the environment. However, defining species boundaries in scleractinian corals is impeded by the difficulties of distinguishing between ecological and evolutionary influences on the appearance of colony morphology. In this study, I used three criteria, i.e. reproductive, morphometric and molecular evidence to determine the extent to which intraspecific and interspecific morphological variation is indicative of evolutionary relationships in species of the Acropora humilis species group. Reproductive criteria, including relative timing of spawning and potential to interbreed in fertilization experiments, provided the greatest level of taxonomic resolution. Discriminant analysis of morphometric data provided a moderate level of resolution. Molecular phylogenetic analysis of two markers, the 28S rDNA unit (domains 1 and 2) and the mtDNA intergenic region, provided the lowest level of resolution of the three criteria. Twenty-one morphs were recognized in field surveys, conducted in seven regions in the western and central Pacific, and these were used as sampling units throughout this study. The morphs were defined using morphological characters that are traditionally used to identify species of the genus Acropora and included the eight currently recognized species of the A. humilis group, seven intermediate morphs and six sub-morphs. The intermediate morphs were characterized by intermediate morphologies that prevented colonies from being confidently assigned to a single species, and the sub-morphs formed recognizable units within the range of morphological variation of one species. Differentiation between species and morphs greatly enhanced the interpretation of evolutionary relationships in this species group, with consistent patterns being found throughout the geographic scale of this project. Colonies identified as A. humilis, A. samoensis, A. gemmifera, A. monticulosa and A. digitifera were shown to be valid species on the basis of reproductive data. Although these species showed no potential to interbreed, it is possible that indirect introgression, through interbreeding between some members of these species and morphs may be retarding divergence of these species. Reproductive data were not obtained for A. globiceps, A. retusa and A. multiacuta. The taxonomic status of A. globiceps is therefore unresolved, due to its lack of morphological and genetic differentiation from A. humilis. Acropora retusa and A. multiacuta appear to be valid species, on the basis of morphological and molecular differentiation. Acropora humilis, A. samoensis, A. globiceps and morphs of these species share the greatest evolutionary affinity, on the basis of morphological overlap and lack of genetic differentiation. The most closely related to these three species appears to be A. gemmifera, with this species and morphs common to these four species also being genetically undifferentiated. Acropora digitifera was morphologically and genetically distinct from all other species of the A. humilis group, although an intermediate morph between this species and A. gemmifera was genetically undifferentiated but reproductively isolated from A. digitifera. On the basis of morphological affinity, this morph is proposed as a possible hybrid between these species. Acropora monticulosa was morphologically distinct from all other species, although it appears to share evolutionary connections on two fronts. Firstly, low levels of genetic differentiation for the mitochondrial marker, between this species and A. humilis, A. samoensis, A. globiceps and A. gemmifera, suggest recent divergence from these species. Secondly, A. monticulosa also appears to share evolutionary affinities with A. digitifera on the basis of morphological similarities between morphs of each of these species, with one of these morphs grouping with A. monticulosa for the 28S marker and with A. digitifera for the mitochondrial marker. This study demonstrates that examining intraspecific and interspecific patterns of polymorphism are valuable for interpreting evolutionary relationships in corals. Evidence derived from these criteria suggest that the morphs are at various stages of divergence from the species with which they share morphological characters and that the morphs may indicate possible zones of speciation and hybridization. Recognition of morphs also avoided the possibility of taxonomic error as a result of ‘forcing’ colonies into incorrect or inappropriate species categories and was therefore essential for accurate interpretation of evolutionary boundaries. Using multiple criteria and samples collected across a broad biogeographic scale facilitated the clarification of relationships within and between species.

Item ID: 58
Item Type: Thesis (PhD)
Keywords: Scleractinian corals, Colony morphology, Biodiversity, Evolutionary processes, Ecological dynamics, Conservation, Environment
Date Deposited: 18 Dec 2006
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060301 Animal Systematics and Taxonomy @ 70%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060302 Biogeography and Phylogeography @ 30%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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