Interspecific hybridization in Acropora (Cnidaria: Scleractinia): mechanisms and evolutionary consequences

This thesis addresses several aspects of the genetics and reproductive biology of cross-fertile, mass-spawning scleractinian corals, specifically in the genus Acropora, and the results presented contribute to our understanding of the evolutionary consequences of hybridization in this animal group. The rDNA ITS1-5.8S-ITS2 region has been used for phylogenetic analysis of cross-fertile coral species in the genus Acropora, and has shown patterns of variation consistent with reticulate evolution. However, results from a number of analyses in this thesis, including the occurrence of deamination-like substitutions at methylation sites; differences in evolutionary rates among clades of a 5.8S phylogeny; and ocurrence of non-compensatory mutations that may affect the rRNA secondary structure, suggest that at least part of rDNA diversity in Acropora is due to pseudogenes. Natural hybridization in coral genera may cause taxa to merge through homogenization of gene pools or may create new hybrid species. Here I demonstrate that high cross-fertilization in vitro does not guarantee the merging of species. Data from eight polymorphic allozyme loci indicate small but significant differentiation between sympatric populations of A. cytherea and A. hyacinthus, a pair of acroporid corals with very high interspecific fertilization rates in vitro. The biological significance of differences between the species in sympatry is highlighted by the absence of genetic differentiation between widely allopatric populations within each species. Moreover, a Nested Clade Analysis using sequence data from a nuclear intron indicates that these two species constitute distinct evolutionary lineages. I conclude that A. cytherea and A. hyacinthus are neither merging nor constitute morphs within a single species, but rather conform distinct cohesion species. Cross-fertilization trials may overestimate the rate of hybridization that occurs under natural conditions, because they are non-competitive, involving the exclusive combination of sperm from one species with eggs from another. I designed breeding trials using acroporid corals to test whether the mixture of conspecific and heterospecific sperm inhibits interspecific fertilization, promoting conspecific sperm precendence. However, spawning failure and low cross-fertilization rate between the study species did not allow evaluating this hypothesis properly. Integrins are proteins involved in cell adhesion that play major roles in gamete binding and fusion in mammals. A cDNA sequence encoding for a b1- class integrin has been identified in the scleractinian coral Acropora millepora. Given that the integrin mRNA is present in unfertilized eggs, the corresponding protein may have a potential role in coral fertilization. As a first attempt to elucidate the molecular basis of gamete specificity in corals, I studied the role of the Acropora millepora bCn1 integrin in fertilization. I examined the effect of polyclonal antiserum raised against a substantial part of the bCn1 integrin on fertilization rates of A. millepora eggs. The results indicate that Acropora bCn1 integrin is involved in sperm-egg binding but does not confer reproductive specificity. The implication of a disintegrin-integrin binding in the fertilization process in Acropora suggests that some functions of these molecules may have been conserved in corals and humans