Abstract
BackgroundThe symbiosis between corals and the dinoflagellate alga Symbiodinium is essential for the development and survival of coral reefs. Yet this fragile association is highly vulnerable to environmental disturbance. A coral’s ability to tolerate temperature stress depends on the fitness of its resident symbionts, whose thermal optima vary extensively between lineages. However, the in hospite population genetic structure of Symbiodinium is poorly understood and mostly based on analysis of bulk DNA extracted from thousands to millions of cells. Using quantitative single-cell PCR, we enumerated DNA polymorphisms in the symbionts of the reef-building coral Pocillopora damicornis, and applied a model selection approach to explore the potential for recombination between coexisting Symbiodinium populations.ResultsTwo distinct Symbiodinium ITS2 sequences (denoted C100 and C109) were retrieved from all P. damicornis colonies analysed. However, the symbiont assemblage consisted of three distinct Symbiodinium populations: cells featuring pure arrays of ITS2 type C109, near-homogeneous cells of type C100 (with trace ITS2 copies of type C109), and those with co-dominant C100 and C109 ITS2 repeats. The symbiont consortia of some colonies consisted almost entirely of these putative C100 × C109 recombinants.ConclusionsOur results are consistent with the occurrence of sexual recombination between Symbiodinium types C100 and C109. While the multiple-copy nature of the ITS2 dictates that the observed pattern of intra-genomic co-dominance may be a result of incomplete concerted evolution of intra-genomic polymorphisms, this is a less likely explanation given the occurrence of homogeneous cells of the C109 type. Conclusive evidence for inter-lineage recombination and introgression in this genus will require either direct observational evidence or a single-cell genotyping approach targeting multiple, single-copy loci.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0325-1) contains supplementary material, which is available to authorized users.
Highlights
The symbiosis between corals and the dinoflagellate alga Symbiodinium is essential for the development and survival of coral reefs
DGGE and DNA sequencing The excision and sequencing of DGGE bands revealed that all six P. damicornis colonies hosted Symbiodinium internal transcribed spacer 2 (ITS2) types C100 (GenBank accession number HM222433; [53]) and C109 (GenBank accession number KJ530690; novel sequence)
No other Symbiodinium sequences were detected, including the rare types C103 and C118 previously identified from P. damicornis at Lord Howe Island (LHI) [53]
Summary
The symbiosis between corals and the dinoflagellate alga Symbiodinium is essential for the development and survival of coral reefs. Using quantitative single-cell PCR, we enumerated DNA polymorphisms in the symbionts of the reef-building coral Pocillopora damicornis, and applied a model selection approach to explore the potential for recombination between coexisting Symbiodinium populations. Heterogeneous Symbiodinium cells outnumbered ‘pure’ genotypes in more than half of the colonies sampled, suggesting that rare sexual reproduction events between C100 and C109 may facilitate asexual proliferation of the F1 generation, with potentially important functional implications for the coral colony. A small number of genetically heterogeneous symbionts featured CC100:CTOTAL ratios near 0.75, and potentially represent F1 × C100 backcross genotypes. This pattern could have arisen from ICEAP, differential rDNA inheritance in the F1 generation This study was not sufficiently resourced to carry out such a comprehensive task; it does serve to highlight the perils of dismissing symbionts that persist in low abundance as biologically-irrelevant or representing surface contamination
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