David J. Ayre

GENOTYPIC DIVERSITY AND GENE FLOW IN BROODING AND SPAWNING CORALS ALONG THE GREAT BARRIER REEF, AUSTRALIA

Abstract Marine organisms exhibit great variation in reproductive modes, larval types, and other life‐history traits that may have major evolutionary consequences. We measured local and regional patterns of genetic variation in corals along Australias Great Barrier Reef to determine the relative contributions of sexual and asexual reproduction to recruitment and to infer levels of gene flow both locally (among adjacent sites, < 5 km apart) and regionally (among reefs separated by 500–1200 km). We selected five common brooding species (Acropora cuneata, A. palifera, Pocillopora damicornis, Seriatopora hystrix, and Stylophora pistillata) and four broadcast spawners (Acropora hyacinthus, A. cytherea, A. millepora, and A. valida), which encompassed a wide range of larval types and potential dispersal capabilities. We found substantial genotypic diversity at local scales in six of the nine species (four brooders, two spawners). For these six, each local population displayed approximately the levels of multilocus genotypic diversity (Go) expected for outcrossed sexual reproduction (mean values of Go:Ge ranged from 0.85 to 1.02), although consistent single‐locus heterozygous deficits indicate that inbreeding occurs at the scale of whole reefs. The remaining three species, the brooder S. hystrix and the spawners A. valida and A. millepora displayed significantly less multilocus genotypic diversity (Go) than was expected for outcrossed sexual reproduction (Ge) within each of several sites. Acropora valida and A. millepora showed evidence of extensive localized asexual replication: (1) a small number of multilocus (clonal) genotypes were numerically dominant within some sites (Go:Ge values were as low as 0.17 and 0.20): (2) single‐locus genotype frequencies were characterized by both excesses and deficits of heterozygotes (cf. Hardy‐Weinberg expectations), and (3) significant linkage disequilibria occurred. For the brooding S. hystrix Go:Ge values were also low within each of four sites (x̄= 0.48). However, this result most likely reflects the highly restricted dispersal of gametes or larvae, because levels of genetic variation among sites within reefs were extremely high (FSR= 0.28).

The evolutionary ecology of corals.

Corals display a wide range of complex life histories. The evolutionary consequences of factors such as clonality, indeterminate growth, asexual reproduction coupled with various (sexual) breeding systems, different levels of gene flow, and strongly overlapping generations have only just begun to be explored. We identify a series of problems and areas for new research that may be resolved b y the application of novel theoretical approaches (including nonequilibrium population genetic models and demographic models incorporating modular processes such as colony fission and polyp mortality), greater in situ experimentation, long-term monitoring of population dynamics and the use of new genetic techniques.

Does life history predict past and current connectivity for rocky intertidal invertebrates across a marine biogeographic barrier

The southeast Australian coast potentially includes a complex biogeographic barrier, largely lacking exposed rocky shore that may limit the dispersal of rocky intertidal taxa and contribute to the maintenance of two biogeographic regions. Surprisingly, within the 300‐km barrier region, several species considered exposed rocky shore specialists occurred within sheltered sites. We analysed COI sequence variation for 10 rocky intertidal invertebrate species, with a range of life histories, to test the hypotheses that larval type and habitat specificity are strong predictors of gene flow between biogeographic regions. Our data revealed that the southeast corner of Australia includes a strong barrier to gene flow for six of eight species with planktonic larvae, and a coalescence analysis of sequence differentiation (IM model) suggests that a barrier has existed since the Pleistocene. In contrast, two direct developers were not affected by the barrier. Our comparative approach and data from earlier studies (reviewed here) do not support the hypothesis that larval type predicts gene flow across this barrier, instead we found that the ability to utilize sheltered habitat provides a clearer explanation of the phylogeographic break. Indeed, the species that displayed little or no evidence of a phylogeographic break across the barrier each displayed unexpectedly relaxed habitat specificity.

Evidence for restricted gene flow in the viviparous coral Seriatopora hystrix on Australia's Great Barrier Reef

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 ≤ 25 m2) 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 collections displayed large and consistent deficits of heterozygotes. Allele frequencies varied greatly among collections (FST = 0.43), and this variation was sufficient to explain two‐thirds of observed deficiencies of heterozygotes via a Wahlund effect. A hierarchical assessment of FST values revealed that 45% of allelic variation occurred among reefs (FST = 0.20), and only 16% of variation within reefs was explained 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 outer‐shelf reefs (FST = 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.

Factors controlling fruit set in hermaphroditic plants: Studies with the Australian proteaceae

The number of fruits produced by many hermaphroditic plants is usually far fewer than the number of flowers available for fertilization. There are various possible explanations for the low fruit:flower ratio, some proximate and others ultimate. Recent studies, especially in northern hemisphere systems, have used field experiments to test some of them, but there are potential difficulties with the methodology of some experiments and with the testing of ultimate hypotheses. It is important to examine the possible explanations for low fruit: flower ratios with a range of different systems. This article reviews studies on Australian species of woody, perennial shrubs in the family Proteaceae; this evolutionarily distinct group of plants and pollinators has several unusual and interesting characteristics, and provides a valuable addition to the better-known northern hemisphere studies.

Microsatellite diversity and genetic structure of fragmented populations of the rare, fire‐dependent shrub Grevillea macleayana

Recent habitat loss and fragmentation superimposed upon ancient patterns of population subdivision are likely to have produced low levels of neutral genetic diversity and marked genetic structure in many plant species. The genetic effects of habitat fragmentation may be most pronounced in species that form small populations, are fully self‐compatible and have limited seed dispersal. However, long‐lived seed banks, mobile pollinators and long adult lifespans may prevent or delay the accumulation of genetic effects. We studied a rare Australian shrub species, Grevillea macleayana (Proteaceae), that occurs in many small populations, is self‐compatible and has restricted seed dispersal. However, it has a relatively long adult lifespan (c. 30 years), a long‐lived seed bank that germinates after fire and is pollinated by birds that are numerous and highly mobile. These latter characteristics raise the possibility that populations in the past may have been effectively large and genetically homogeneous. Using six microsatellites, we found that G. macleayana may have relatively low within‐population diversity (3.2–4.2 alleles/locus; Hexp= 0.420–0.530), significant population differentiation and moderate genetic structure (FST = 0.218) showing isolation by distance, consistent with historically low gene flow. The frequency distribution of allele sizes suggest that this geographical differentiation is being driven by mutation. We found a lack mutation‐drift equilibrium in some populations that is indicative of population bottlenecks. Combined with evidence for large spatiotemporal variation of selfing rates, this suggests that fluctuating population sizes characterize the demography in this species, promoting genetic drift. We argue that natural patterns of pollen and seed dispersal, coupled with the patchy, fire‐shaped distribution, may have restricted long‐distance gene flow in the past.

Population structure is not a simple function of reproductive mode and larval type: insights from tropical corals

1. For a wide range of organisms, heritable variation in life-history characteristics has been shown to be strongly subject to selection, reflecting the impact that variation in characters such as genotypic diversity, duration of larval development and adaptations for dispersal can have on the fitness of offspring and the make-up of populations. Indeed, variation in life-history characteristics, especially reproduction and larval type, have often been used to predict patterns of dispersal and resultant population structures in marine invertebrates. 2. Scleractinian corals are excellent models with which to test this relationship, as they exhibit almost every possible combination of reproductive mode and larval type. Some general patterns are emerging but, contrary to expectations, genetic data suggest that while populations of broadcast spawning species may be genotypically diverse they may be heavily reliant on localized recruitment rather than widespread dispersal of larvae. 3. Here we use microsatellites to test the importance of localized recruitment by comparing the genetic structure of populations of two broadcast spawning corals with contrasting modes of reproduction and larval development; Goniastrea favulus is self-compatible, has sticky, negatively buoyant eggs and larvae and is expected to have restricted dispersal of gametes and larvae. In contrast, Platygyra daedalea is self-incompatibile, spawns positively buoyant egg-sperm bundles and has planktonic development. 4. Surprisingly, spatial-autocorrelation revealed no fine-scale clustering of similar genotypes within sites for G. favulus, but showed a non-random distribution of genotypes in P. daedalea. Both species showed similar levels of genetic subdivision among sites separated by 50-100 m (F(ST) = 0.03), suggesting that larval dispersal may be equivalent in both species. 5. Interestingly, as fragmentation has been considered rare in massive corals, our sample of 284 P. daedalea colonies included 28 replicated genotypes that were each unlikely (P < 0.05) to have been derived independently from sexual reproduction. 6. We conclude that the extreme life history of G. favulus does not produce unusually fine-scale genetic structure and subsequently, that reproductive mode and larval type may not be not good predictors of population structure or dispersal ability.

Population structure in the coral Pavona cactus: clonal genotypes show little phenotypic plasticity

Electrophoretic data were used to examine the relationship between genotype and growth form, and to assess the contribution of asexual reproduction to recruitment within six local populations of the agaricid coral Pavona cactus from the central and northern Great Barrier Reef. The data revealed the presence of highly replicated clonal genotypes in the five densest populations. In three cases, samples of 50 to 60 colonies collected from 25 m2 areas within the Eclipse Island and Pandora Reef populations each consisted of colonies with only two distinct 4-locus genotypes. More intensive sampling of the population at Eclipse Island showed that colonies with the same 4-locus genotypes were separated by distances of up to 93 m. In contrast, the population at Watsons Bay (Lizard Island) consisted of a few widely scattered and genetically distinct colonies. The samples collected from each population contained a range of growth forms including, in some cases, the entire morphological range described for this species. A strong association of genotype and growth form was detected in samples from all populations, with the exception of Watsons Bay where no such comparison was possible. Nevertheless, some genotypes were represented by more than one growth form and this could reflect the effects of limited phenotypic plasticity. The effects of asexual reproduction reduced the value of these data as a test of the genetic connectedness of the six populations studied. The genetic distance between samples was not simply correlated with geographic distance. This may reflect either the true structure of the breeding population(s) or the effects of asexual reproduction on estimates of allelic frequencies.

Localized adaptation of clones of the sea anemone Actinia tenebrosa

Adult Actinia tenebrosa were reciprocally transplanted within and between colonies separated by 2 or 4 km. These experiments revealed powerful genetic and environmental effects on adult size and asexual fecundity and demonstrate that clones may be highly locally adapted. Local adults displayed significantly greater asexual fecundity than do adults transplanted from other colonies, although all groups showed similar survivorships. This provides the first demonstration of fine scale adaptation, in either terrestrial or marine environments, for a clonal species with large, panmictic sexually breeding populations. These findings, together with the results of earlier studies of this species, strongly support the assumptions and predictions of the Strawberry‐Coral Model (Williams, 1975).

Protection of genetic diversity and maintenance of connectivity among reef corals within marine protected areas

High-latitude coral reefs (HLRs) are potentially vulnerable marine ecosystems facing well-documented threats to tropical reefs and exposure to suboptimal temperatures and insolation. In addition, because of their geographic isolation, HLRs may have poor or erratic larval connections to tropical reefs and a reduced genetic diversity and capacity to respond to environmental change. On Australias east coast, a system of marine protected areas (MPAs) has been established with the aim of conserving HLRs in part by providing sources of colonizing larvae. To examine the effectiveness of existing MPAs as networks for dispersal, we compared genetic diversity within and among the HLRs in MPAs and between these HLRs and tropical reefs on the southern Great Barrier Reef (GBR). The 2 coral species best represented on Australian HLRs (the brooding Pocillopora damicornis and the broadcast-spawning Goniastrea australensis) exhibited sharply contrasting patterns of diversity and connectedness. For P. damicornis, the 8-locus genetic and genotypic diversity declined dramatically with increasing latitude (N(a)= 3.6-1.2, H(e)= 0.3-0.03, N(g):N = 0.87-0.06), although population structure was consistent with recruitment derived largely from sexual reproduction (G(o):G(e)= 1.28-0.55). Genetic differentiation was high among the HLRs (F(ST)[SD]= 0.32 [0.08], p < 0.05) and between the GBR and the HLRs (F(ST)= 0.24 [0.06], p < 0.05), which indicates these temperate populations are effectively closed. In contrast for G. australensis, 9-locus genetic diversity was more consistent across reefs (N(a)= 4.2-3.9, H(e)= 0.3-0.26, N(g):N = 1-0.61), and there was no differentiation among regions (F(ST)= 0.00 [0.004], p > 0.05), which implies the HLRs and the southern GBR are strongly interconnected. Our results demonstrate that although the current MPAs appear to capture most of the genetic diversity present within the HLR systems for these 2 species, their sharply contrasting patterns of connectivity indicate some taxa, such as P. damicornis, will be more vulnerable than others, and this disparity will provide challenges for future management.