Comparative transcriptomic analyses of Chromera and Symbiodiniaceae.

Reef-building corals live in a mutualistic relationship with photosynthetic algae (family Symbiodiniaceae) that usually provide most of the energy required by the coral host. This relationship is sensitive to temperature stress; as little as a 1 degrees C increase often leads to the collapse of the association. This sensitivity has led to an interest in the potential of more stress-tolerant algae to supplement or substitute for the normal Symbiodiniaceae mutualists. In this respect, the apicomplexan-like microalgaChromerais of particular interest due to its greater temperature tolerance. We generated ade novotranscriptome for aChromerastrain isolated from a GBR coral ('GBR Chromera') and compared with those of the reference strain ofChromera('SydneyChromera'), and to those of Symbiodiniaceae (Fugacium kawagutii,Cladocopium goreauiandBreviolum minutum), as well as the apicomplexan parasite,Plasmodium falciparum. In contrast to the high sequence divergence amongst representatives of different genera within the family Symbiodiniaceae, the twoChromerastrains featured low sequence divergence at orthologous genes, implying that they are likely to be conspecifics. Although KEGG categories provide few criteria by which true coral mutualists might be identified, they do supply a molecular rationalization that explains the ecological dominance ofCladocopiumspp. amongst Indo-Pacific reef corals. The presence of HSP20 genes may contribute to the high thermal tolerance ofChromera.