Chromera velia de novo transcriptome assembly, annotation and comparative transcriptomics

Reef-building corals live in a mutualistic relationship with photosynthetic algae (family Symbiodiniaceae) that usually provide the bulk of the energy required by the coral host. This relationship is very sensitive to temperature stress, with as little as 1°C increase above mean in sea surface temperatures (SSTs) often leading to the collapse of the association. The meta-stability of these associations has led to interest in the potential of more stress tolerant algae to supplement or substitute for the normal Symbiodiniaceae mutualists. In this respect, the apicomplexan-like microalga Chromera is of particular interest as it is considerably more temperature tolerant than are most members of the Symbiodiniaceae. Here we generated a de novo transcriptome for a Chromera strain isolated from a GBR coral (“GBR Chromera”) and compared to those of the reference strain of Chromera (“Sydney Chromera”), and to those of Symbiodiniaceae algae (Fugacium, Cladocopium and Breviolum), as well as the apicomplexan parasite, Plasmodium falciparum. By contrast with the Symbiodiniaceae, the two Chromera strains had a high level of sequence similarity evident by very low levels of divergence in orthologous genes. Although surveys of specific KEGG categories provided few general criteria by which true coral mutualists might be identified, they provide a molecular rationalization for the near ubiquitous association of Cladocopium strains with Indo-Pacific reef corals in general and with Acropora spp. in particular. In addition, HSP20 genes may underlie the higher thermal tolerance shown by Chromera compared to Symbiodiniaceae We assembled a de novo transcriptome for a new strain of Chromera velia isolated from Montipora digitata at Magnetic Island on the Great Barrier Reef (GBR) "GBR Chromera". This transcripome was compared to the reference Chromera strain "Sydney Chromera", members of the Symbiodiniaceae and the apicomplexan parasite Plasmodium Falciparum. We sought to address two specific objectives here, in both cases use of a denovo-assembled transcriptome generated for a Chromear strain isolated from a GBR coral to estimate the degree of divergence between this strain and the Sydney strain. Second is to investigate the repretoires of genes that play roles in symbioses and stress tolerance between the two Chromera and three members of the Symbiodiniaceae including mtualists.