Diverse associations among coral host haplotypes and algal endosymbionts may drive adaptation at geographically peripheral and ecologically marginal locations

Aim

To understand the evolutionary history and local adaptation of marginal, subtropical populations of a common reef-building coral (genus Pocillopora) and their Symbiodinium endosymbionts.

Location

Subtropical coastal (Solitary Islands, Flinders Reef) and offshore (Lord Howe Island, Middleton Reef, Elizabeth Reef) eastern Australia and the southern Great Barrier Reef (GBR).

Methods

Coral genetic diversity and identity were assessed by sequence analysis of a mitochondrial (mtDNA) region (ORF), a PCR-based mtDNA screening assay, and microsatellite loci. The identity of the Symbiodinium was determined using denaturing gradient gel electrophoresis (DGGE) analysis of nuclear rDNA ITS2.

Results

The mitochondrial haplotype Pocillopora damicornis Type α was present at all locations: 100% on the GBR and Flinders Reef, 85% and 87% respectively at Middleton Reef and Elizabeth Reef, 25% at Lord Howe Island and 5% at the Solitary Islands. An additional haplotype was observed at the Solitary Islands (corresponding to the recently described Pocillopora aliciae), and a third haplotype at the offshore high-latitude reefs (termed the Lord Howe Island haplotype). Congruent with the mtDNA results, microsatellite markers showed the same three population clusters. One Symbiodinium profile occurred in 100% of the samples at two of the three GBR reefs and Flinders Reef, and a different profile occurred in all colonies analysed from Lord Howe Island. These two profiles were observed at Middleton and Elizabeth reefs in association with both P. damicornis Type α and the Lord Howe Island coral haplotype. Two unique Symbiodinium profiles were detected at the Solitary Islands.

Main conclusions

We detected evidence for local adaptation of subtropical coral and Symbiodinium, flexible coral–Symbiodinium associations, and the presence of transition zones that may contain evolutionarily significant novelty.