The Platygyra species complex: implications for coral taxonomy and evolution

Miller, Karen Joy (1994) The Platygyra species complex: implications for coral taxonomy and evolution. PhD thesis, James Cook University of North Queensland.

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Abstract

The majority of species boundaries are based on morphology, under the assumption that morphological discontinuities equate with reproductive incompatibility. In scleractinian corals, morphological boundaries between some species are indistinct, particularly where skeletal characters overlap to form a morphologic continuum between species. It has recently been discovered that at least some morphological species of coral are capable of interbreeding in vitro and it may be that events such as hybridisation could explain the high degrees of morphological variation recorded in many coral species and genera. In order to better understand the nature of species relationships and evolutionary processes within the Scleractinia, a detailed analysis of species boundaries within a single, representative genus - Platygyra - was carried out.

A multivariate morphometric analysis based on nine skeletal characters indicated seven morphologically distinct species (including two undescribed species) were present within the genus Platygyra on the Great Barrier Reef. However, no diagnostic species characters were found. The range of skeletal variation within a morphological species was also examined and was found to be high, with discrete morphotypes occurring within the species Platygyra daedalea. Intra-specific morphological variation in corals is often attributed to environmental influences whereby colonies of the same species will display different morphologies in different environments. A transect survey of all species and morphs of Platygyra on Davies Reef, central Great Barrier Reef, indicated that all morphs and species occurred sympatrically in six different reef habitats. Similarly, a broad scale survey of Platygyra colonies on reefs around Australia showed all species and morphs were widespread and common across their range. The sympatric distribution of Platygyra species suggest morphological variation in these corals is not a result of environmental influences at either a between-reef or within-reef scale.

Fertilisation trials between morphological species of Platygyra showed all seven species are capable of hybridisation in vitro. Hybridisation rates were equivalent to rates of within-species fertilisation (50-60%) and hybrid corals developed normally, surviving at least 31/2 years. Fertilisation data suggest that morphological species of Platygyra are not reproductively isolated. This is further supported by the absence of temporal barriers to hybridisation between species and preliminary evidence which suggests there is no species-specific sperm attraction acting to reproductively isolate species.

Studies using allozyme electrophoresis at nine polymorphic loci confirmed the findings from fertilisation trials and suggested hybridisation occurs naturally between species of Platygyra on the reef. There were no fixed gene differences between morphological species and values of Nei's D between species were low (< 0.2), which is within the range usually associated with intra-specific variation. Deviations from Hardy-Weinberg equilibria show there is some degree of non-random mating within species of Platygyra, however this is most likely to be associated with aspects of their reproductive biology rather than species-specific differences. Histocompatibility tests between morphological species of Platygyra suggested there was no relationship between morphology and interaction outcome, nor was there any association between genetic difference between colonies and interactive response.

By combining results from morphometric, ecological, reproductive and genetic studies of Platygyra, I conclude that hybridisation occurs naturally between the seven morphological species of Platygyra. Natural hybridisation will be an important consideration when defining species boundaries and evolutionary processes in corals. By drawing analogies between corals and plants, I propose that processes such as reticulate evolution and introgression, as have been described in plants, will also be taking place in the Scleractinia. In light of these findings, traditional species concepts which assume reproductive isolation between species will not be appropriate to define coral species. In the future it will be important to realise that coral taxonomy, which defines morphologically distinct species, may not necessarily reflect breeding relationships or have any bearing on evolutionary processes.

Item ID: 33784
Item Type: Thesis (PhD)
Keywords: corals; Platygyra; morphology; hybridization; Davies Reef; taxonomy; species concept
Date Deposited: 30 Jul 2015 01:27
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060301 Animal Systematics and Taxonomy @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
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