The molecular basis of fertilisation in coral Acropora and its role in speciation

Iguchi, Akira (2007) The molecular basis of fertilisation in coral Acropora and its role in speciation. PhD thesis, James Cook University.

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Abstract

To shed light on diversification of the genus Acropora (Scleractinia, Cnidaria), one of the most widespread, abundant, and species-rich genera of hard corals (113–180 species), I searched for fertilisation-related genes in a model coral, Acropora millepora, and examined variations in these genes among Acropora species. First, by focusing on the ADAM–integrin interaction, which is involved in sperm–egg binding and membrane fusion in mammals, 15 ADAM–integrin interactions related to candidate genes obtained from an expressed sequence tag (EST) database of A. millepora were screened using hierarchical strategies (gene structures, gene expression patterns, fertilisation inhibition experiments). However, no evidence was found that these genes, other than integrin betacn1, are involved in Acropora fertilisation. To identify fast-evolving genes from Acropora species as fertilisation candidates, I then performed direct comparative sequence analysis with EST datasets from two acroporid species: A. millepora and A. palmata from the Caribbean Sea. Comparison of selected 849 independent genes from the A. palmata EST database (4,017 ESTs) to 10,232 ESTs from A. millepora resulted in the identification of 513 putative homologues. Within 163 homologous pairs in which dN and dS were examined, 93 homologous pairs had dN/dS ratios significantly <1, which suggests that these genes are under selection pressure associated with functional constraints. Six independent genes showed dN/dS ratios >1, and two of these had a significant deviation from one, suggesting that they are fast-evolving genes. It was unclear whether these fast-evolving genes are involved in fertilisation. Finally, variations of integrin betacn1, which is involved in coral fertilisation, were compared among some Acropora species. Interestingly, comparison of integrin betacn1 sequences demonstrated that there are some mutations around the DxSxS motif, and two combinations of eight different clones showed significant possibilities of positive selection. However, it is unclear whether these variations are related to Acropora speciation. The next step is to characterise proteins making molecular complexes with integrin betacn1 or fast-evolving genes, and to compare these amino acid sequences among Acropora species.

Item ID: 5568
Item Type: Thesis (PhD)
Keywords: Acropora millepora, hard corals, fertilisation, reproduction, speciation, diversification, molecular basis, fast-evolving genes, gene structures, gene expression, proteins, integrins, tetraspanins, DNA
Date Deposited: 17 Dec 2009 23:43
FoR Codes: 06 BIOLOGICAL SCIENCES > 0601 Biochemistry and Cell Biology > 060110 Receptors and Membrane Biology @ 30%
06 BIOLOGICAL SCIENCES > 0604 Genetics > 060409 Molecular Evolution @ 30%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060311 Speciation and Extinction @ 40%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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