Genetic systems and hereditary structures of reef corals

Processes involved in the generation of new coral colonies were investigated with a view to elucidating their consequences for population genetic structure. Electrophoretic and histocompatibility methods of estimating clonal structure in populations were evaluated. Spatial patterns of graft acceptance and rejection were different for two species of Montipora on a patch reef in Kaneohe Bay, Hawaii. Electrophoretically distinct tissues were capable of fusing in both species, indicating that clonal identity is not nescessarily inferred by acceptance of grafts. Tissue fusion between graft pairs of dissimilar genotypes occurred in 18 of 40 cases for M. dilatata and in 3 of 7 cases for M. verrucosa. In the majority of these allogeneic fusions, the donor and recipient colonies had one allele in common at the single polymorphic locus assayed. In one case, fusion occurred between genotypes which shared no alleles. The specificity of the self-recognition response, although different for the two species, was constant through time. It is suggested that self-recognition systems have species-specific levels of genotypic discrimination. A greater understanding of the genetics of invertebrate immunology is required before histocompatibility criteria can be used independently to assess population genetic structure with confidence. The mode and timing of sexual reproduction was investigated in the genus Montipora. During late spring and summer M. verrucosa, M. dilatata, M. verrili, M. studeri and AL flabellata were studied in Kaneohe Bay, Hawaii. Two populations of M. digitata were studied in the central region of the Great Barrier Reef, Australia. All species were simultaneous hermaphrodites, bearing male and female gonads in the same polyp. In each case, there were four male and four female mesenteries arranged as alternating pairs. An annual gametogenic cycle was apparent for M. digitata and inferred for the Hawaiian species. Oocytes mature over approximately six months, exhibiting a steady increase in mean diameter. The most rapid increase in oocyte size occurs in the last month prior to spawning. Testes may also be present at the onset of oocyte development, although almost all development takes place in the final two months prior to spawning. Spawning, which followed a lunar rhythm, was observed for the first time in M. digitata, M. verrucosa and M. dilatata. These species packaged eggs and sperm into a single cluster in each polyp. The gamete clusters, which were shed through the mouth, were highly buoyant and broke up on the surface of the sea. Fertilization and development were external. The mating patterns of four species which broadcast gametes were compared. All species were simultaneous hermaphrodites, from the central region of the Great Barrier Reef, which shed eggs and sperm into the water during brief annual spawning events. Gametes were collected from colonies which had been isolated in separate containers and combined in controlled fertilization trials. Montipora digitata cross-fertilized exclusively. Acropora tenuis, Goniastrea aspera and G. favulus-were capable of self-fertilization, but to varying degrees. In all species, cross-fertilization was the dominant mating pattern. Studies of gamete viability indicated that cross-fertilizations were possible at least 6 hours after spawning, although fertilization rates suggest that most fertilization will occur within 3 hours at high gamete concentrations. Embryogenesis and larval development were studied in detail in the broadcast spawners Montipora digitata, Acropora pulchra, Lobophyllia hemprichii and Favites abdita. Similar developmental patterns and rates were observed in all species. Cleavage was initiated 1.5 to 2.5 hours after spawning. Cell division proceeded by progressive furrow formation. Hollow blastulae were formed within 5 to 6 hours. Endoderm formation appeared to be by delamination after the blastulae had flattened into a disk shape. All species were mobile after 24 hours. Highly mobile planulae were formed after 48 hours. Earliest settlement was after 3 days by Acropora pulchra. The remaining 3 species first settled on day 4. Approximately 45 % of L. hemprichii and F. abdita larvae settled on day 4. In all species the majority of larvae had settled within 7 days. These results suggest that broadcast spawning species have less probability of settling on the parental reef than brooders. Rapidly dividing embryonic tissue was used to investigated coral cytogenetics. A comparison of chromosome number and morphology was made for 4 species of broadcast spawning corals. All species were diploid, with 28 (n = 14) mostly metacentric chromosomes. Differences between the species were largely restricted to alterations in the position of the centromeres on particular chromosomes. Fusion and fission type chromosomal rearrangements, or large quantitatative changes in DNA do not appear to have occurred in the evolution of these broadcast spawning species