Genetic and Color Interactions at a Contact Zone of Acanthochromis polyacanthus: A Marine Fish Lacking Pelagic Larvae

Abstract

- Acanthochromis polyacanthus is an unusual tropical marine damselfish that uniquely lacks pelagic larvae and has lost the capacity for broad-scale dispersal among coral reefs. Different color morphs exist in different regions of the Great Barrier Reef, and morphs from northern and southern regions are genetically distinct. In the Hydrographers Passage area, which is a large break through the reef matrix in the central Great Barrier Reef that may have acted as a bottleneck on the migration of these animals during sea level rise, three morphs recognized from other regions were found on neighboring reefs. The transition between them is abrupt with three loci (AAT-2*, GPI-1*, and PGM*) showing allelic frequency patterns close to fixation between opposite alleles within a few kilometers. On two reefs (Hyde, Bebe), a pair of morphs was found to coexist and exhibited a habitat partitioning pattern with each morph restricted to one side on the reef and steep transitions in between. Outside these transition zones, phenotypes and genotypes matched those on surrounding reefs without coexistence and were little changed from reefs several hundred kilometers away. An electrophoretic survey across one transition zone on Hyde Reef showed steep genetic gradients along one kilometer of reef slope. Significant linkage disequilibria in samples collected in Hyde Reef as a result of dispersal of parental combinations of alleles into the center or because parental combinations of alleles confer greater fitness, allowed us to estimate the dispersal rate (189 m/generation) and the selection pressure on the marker loci (0.411). Finally, we investigated models that could lead to such a steep transition in genotypic and phenotypic combinations. Both contact zones on each side of Hyde Reef were associated with geomorphological discontinuities in the reef structure. We suggest that assortative mating may be a proximal mechanism for maintaining isolated each color morph, which could be reinforced by selective predation against hybrids outside the zone of their formation (i.e., the frequency-dependent selection model of Mallet and Barton (1989). Acanthochromis is a midwater planktivore and, when in coexistence, the two morphs forage in different habitats amid multispecific flocks of other damselfishes of matching colors.

Genetic structure among populations of Potamonautes perlatus (Decapoda: Potamonautidae) from the Olifants River system in the Western Cape, South Africa

“…In both marine and freshwater systems, speciation has been linked to eustatic changes that have resulted in periods of allopatric divergence followed by secondary contact (Planes & Doherty, 1997). Although the processes of speciation remain unclear, limited evidence exists to suggest an allopatric mode of speciation.…”

Reef fish hybridization: lessons learnt from butterflyfishes (genusChaetodon)

“…Similarly, the hybridizing color morphs of Acanthochromis polyacanthus in the northern hybrid zone on the Great Barrier Reef (GBR), not differentiated by allozyme data (Planes and Doherty 1997b), showed high levels of mtDNA Hypervariable Region 1 (HVR1) introgression (van Herwerden and Doherty 2006). In contrast, the hybridizing southern GBR A. polyacanthus color morph, divergent from all other populations by 2.8% mtDNA cyt b (Planes et al 2001), showed a distinct allozyme structure (Planes and Doherty 1997a) and negligible introgression in the HVR1 (van Herwerden and Doherty 2006).…”

The Use of Genetic Clines to Estimate Dispersal Distances of Marine Larvae

“…Finally, the genetic information needed to detect levels of LD is not formidably large. A survey of the recent animal literature on linkage disequilibrium suggests that, on average, four polymorphic loci (range 2-9 loci) genotyped for ;50 animals per location have been used to detect an LD of ;0.16 (range of significant results 0.02-0.35: see Szymura and Barton 1986, Mallet et al 1990, Bert and Arnold 1995, Duggins et al 1995, Hare and Avise 1996, Planes and Doherty 1997, Lenormand et al 1998, Rawson et al 1999, Bierne et al 2002, 2003, Dasmahapatra et al 2002, Bronson et al 2003, Morgan-Richards and Wallis 2003, Nielsen et al 2003, Rawson et al 2003, Vines et al 2003. In general, it is more difficult to quantify gametic disequilibrium among systems that have low levels of population differentiation.…”

“…However, we know of only one marine study that utilized cline theory to explicitly estimate dispersal distances. Planes and Doherty (1997) describe a steep cline between two color morphs in a single species of tropical damselfish, Acanthochromis polyacanthus, on the Great Barrier Reef (GBR) ( Fig. 2A).…”