EVIDENCE FOR RESTRICTED GENE FLOW IN THE VIVIPAROUS CORALSERIATOPORA HYSTRIX ONAUSTRALIA'S GREAT BARRIER REEF

Abstract

Abstract. -Viviparous, branching corals such as Seriatopora hystrix are expected to generate most recruits through asexual reproduction (fission or fragmentation) but are expected to use sexual reproduction to produce widely dispersed colonists. In this study, allozyme electrophoresis was used to test for variation in the relative contributions of sexual and asexual reproduction to recruitment and to assess the apparent scale of larval dispersal (gene flow) in the central Great Barrier Reef. Fifty-seven collections (within ::s 25 m-) of fragments from sets of approximately 40 colonies were made (where possible) within each of five habitats on each of 12 reefs. These reefs, within the central region of the Great Barrier Reef, were separated by up to 90 km and included one inner-shelf continental island and groups of seven midshelf reefs and four outer-shelf reefs. Most collections contained a high level of multilocus genotypic diversity and hence showed little evidence of recruitment through fragmentation, although the majority of collectionsdisplayed large and consistent deficits ofheterozygotes. Allele frequencies varied greatly among collections (Fon: = 0.43),and this variation wassufficientto explain two-thirds of observed deficiencies ofheterozygotes via a Wahlund effect. A hierarchical assessment of F ST values revealed that 45% of allelic variation occurred among reefs(Fon: = 0.20),and only 16%of variation within reefswasexplained by variation among five major habitat types (FST = 0.05). A relatively small component of the total variation among samples was attributable to across-shelf variation among the groups of middle-and outershelf reefs (Fon: = 0.03); however, the outer-shelf reefs form a single UPGMA cluster separate from all but 4 of the other 43 collections. These data imply that widespread dispersal does occur but that the direction or magnitude of gene flow may be influenced by the along-shelf movement of major ocean currents and weather-dependent currents on or near reefs. Each reef, therefore, forms a partially isolated and highly subdivided population.

Mating system variation in the hermaphroditic brooding coral, Seriatopora hystrix

“…Moreover, the levels of selfing detected in this study are consistent with previous reports of large heterozygote deficiencies in populations of S. hystrix from two other studies along the Great Barrier Reef (Ayre and Dufty, 1994; Ayre and Hughes, 2000). Heterozygote deficits are a common feature of many marine populations and are often presented as evidence in support of mating systems characterized by high levels of inbreeding and/or selfing (Ayre and Dufty, 1994;Edmands and Potts, 1997;Viard et al, 1997;Hughes, 2000, 2004;Reusch, 2001;Goffredo et al, 2004). Nevertheless, other potential factors may contribute to heterozygote deficits, for example, selection against heterozygotes (Borsa et al, 1992), the presence of null alleles (Foltz, 1986;Ayre et al, 1997), and the inadvertent sampling of multiple genetic subpopulations with differing allele frequencies (that is, the Wahlund effect, Hedrick, 2000).…”

“…Previous population genetic studies of S. hystrix on the Great Barrier Reef have shown that certain allozyme loci are highly variable and provide a useful marker system to assess population structure and mating system in this species (for example, Ayre and Dufty, 1994;Ayre and Hughes, 2000). Tissue extracts and electrophoresis methods were the same as those described by Ayre and Dufty (1994).…”

“…Tissue extracts and electrophoresis methods were the same as those described by Ayre and Dufty (1994). Electrophoresis was carried out on horizontal starch gels (12% w/v) using a tris citrate (TC8), tris-EDTA-borate (TEB) or tris-maleate (TM) buffer modified from Selander et al (1971).…”

“…These reef-building corals display a diverse range of reproductive strategies that include sexual and asexual modes, hermaphroditic and gonochoristic species and broadcast spawning and internal brooding modes of fertilization (Harrison and Wallace, 1990). Surprisingly, however, little is known about selfing rates within coral species, though studies of the population structure of a number of species suggest that inbreeding, including the extreme of selfing may be important (for example, Ayre and Dufty, 1994;Brazeau et al, 1998;Ayre and Hughes, 2000;Goffredo et al, 2004). In vitro fertilization trials have been carried out to determine the extent of self-compatibility for a number of broadcast spawning species (Heyward and Babcock, 1986;Willis et al, 1997;Baums et al, 2005) and have been informative about the potential for selfing within some species.…”

Genetic diversity and connectivity in a brooding reef coral at the limit of its distribution

“…Recent work on S. hystrix populations using highly variable microsatellite markers showed dispersal mostly occurred over small spatial scales (less than 100 m), although this was supplemented by less frequent longer-distance dispersal (Underwood et al 2007). Localized dispersal generally leads to high population differentiation, and earlier research on GBR S. hystrix populations (using allozymes) confirms this (F ST ¼ 0.43, Ayre & Dufty 1994; F ST ¼ 0.15, Ayre & Hughes 2000). Despite the usually rapid settlement of larvae, some brooding corals produce a small proportion of larvae that have extended competency periods of over 100 days in the water column (e.g.…”