Abstract
To clarify the establishment process of coral-algal symbiotic relationships, coral transcriptome changes during increasing algal symbiont densities were examined in juvenile corals following inoculation with the algae Symbiodinium goreaui (clade C) and S. trenchii (clade D), and comparison of their transcriptomes with aposymbiotic corals by RNA-sequencing. Since Symbiodinium clades C and D showed very different rates of density increase, comparisons were made of early onsets of both symbionts, revealing that the host behaved differently for each. RNA-sequencing showed that the number of differentially-expressed genes in corals colonized by clade D increased ca. two-fold from 10 to 20 days, whereas corals with clade C showed unremarkable changes consistent with a slow rate of density increase. The data revealed dynamic metabolic changes in symbiotic corals. In addition, the endocytosis pathway was also upregulated, while lysosomal digestive enzymes and the immune system tended to be downregulated as the density of clade D algae increased. The present dataset provides an enormous number of candidate symbiosis-related molecules that exhibit the detailed process by which coral-algal endosymbiosis is established.
Highlights
The association between scleractinian corals and algal symbionts (Symbiodinium spp.), is essential to primary production and reef building in tropical and subtropical oceans
Previous studies focused on the very early symbiotic state of planura larvae[31] or on the corals colonized with symbiont at high density[34,35], but few studies have investigated the process of increasing algal density in the corals. Because the latter process may involves key mechanisms for establishing endosymbiosis being expressed inside corals, it is possible that the molecular process underlying such will be further clarified by studying the genes whose expression changes with increasing Symbiodinium density
Clade C symbionts hardly increased in corals during the first 2 months after inoculation, whereas clade D symbionts increased quickly[36], implying that the process of symbiont establishment differs considerably depending on the genetic type of Symbiodinium
Summary
The association between scleractinian corals and algal symbionts (Symbiodinium spp.), is essential to primary production and reef building in tropical and subtropical oceans. We generated RNA-seq data using the scleractinian coral Acropora tenuis in the early endosymbiosis stage (approximately 10 days and 20 days after the start of co-incubation following inoculation) with clade C and clade D Symbiodinium. We investigated the algal symbiont-dependent changes in gene expression by comparing transcriptomes between the aposymbiotic and symbiotic to clarify the establishment of symbiosis.
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