Line K Bay^1,3, M Julian Caley² and Ross H Crozier¹

¹School of Marine and Tropical Biology, James Cook University, Townsville, Qld 4811, Australia

²Australian Institute of Marine Science, PMB #3, Townsville MC, QLD 4810, Australia

³ARC Centre of Excellence for Coral Reef Studies, Townsville, Qld 4811, Australia and Australian Institute of Marine Science, PMB #3, Townsville MC, QLD 4810, Australia

This is an Open Access article distributed under the terms of the Creative Commons Attribution License. BMC Evolutionary Biology 2008, 8:248.

Abstract

Background

Management strategies for coral reefs are dependant on information about the spatial population structure and connectivity of reef organisms. Genetic tools can reveal important information about population structure, however, this information is lacking for many reef species. We used a mitochondrial molecular marker to examine the population genetic structure and the potential for meta-population dynamics in a direct developing coral reef fish using 283 individuals from 15 reefs on the Great Barrier Reef, Australia. We employed a hierarchical sampling design to test genetic models of population structure at multiple geographical scales including among regions, among shelf position and reefs within regions. Predictions from island, isolation-by-distance and meta-population models, including the potential for asymmetric migration, local extinction and patterns of re-colonisation were examined.

Results

Acanthochromis polyacanthus displayed strong genetic structure among regions (Φ_ST = 0.81, P < 0.0001) that supported an equilibrium isolation-by-distance model (r = 0.77, P = 0.001). Significant structuring across the continental shelf was only evident in the northern region (Φ_ST = 0.31, P < 0.001) and no evidence of isolation-by-distance was found within any region. Pairwise Φ_ST values indicated overall strong but variable genetic structure (mean Φ_ST among reefs within regions = 0.28, 0.38, 0.41), and asymmetric migration rates among reefs with low genetic structure. Genetic differentiation among younger reefs was greater than among older reefs supporting a meta-population propagule-pool colonisation model. Variation in genetic diversities, demographic expansion and population growth estimates indicated more frequent genetic bottlenecks/founder effects and subsequent population expansion in the central and southern regions compared to the northern one.

Conclusion

Our findings provide genetic evidence for meta-population dynamics in a direct developing coral reef fish and we reject the equilibrium island and isolation-by distance models at local spatial scales. Instead, strong non-equilibrium genetic structure appears to be generated by genetic bottlenecks/founder effects associated with population reductions/extinctions and asymmetric migration/(re)-colonisation of such populations. These meta-population dynamics varied across the geographical range examined with edge populations exhibiting lower genetic diversities and higher rates of population expansion than more central populations. Therefore, coral reef species may experience local population reductions/extinctions that promote overall meta-population genetic differentiation.