Abstract
Although numerous dinoflagellate species (Family Symbiodiniaceae) are present in coral reef environments, Acropora corals tend to select a single species, Symbiodinium microadriaticum, in early life stages, even though this species is rarely found in mature colonies. In order to identify molecular mechanisms involved in initial contact with native symbionts, we analyzed transcriptomic responses of Acropora tenuis larvae at 1, 3, 6, 12, and 24 h after their first contact with S. microadriaticum, as well as with non-native symbionts, including the non-symbiotic S. natans and the occasional symbiont, S. tridacnidorum. Some gene expression changes were detected in larvae inoculated with non-native symbionts at 1 h post-inoculation, but those returned to baseline levels afterward. In contrast, when larvae were exposed to native symbionts, we found that the number of differentially expressed genes gradually increased in relation to inoculation time. As a specific response to native symbionts, upregulation of pattern recognition receptor-like and transporter genes, and suppression of cellular function genes related to immunity and apoptosis, were exclusively observed. These findings indicate that coral larvae recognize differences between symbionts, and when the appropriate symbionts infect, they coordinate gene expression to establish stable mutualism.
Highlights
Symbioses are ubiquitous in nature and are intricately involved in adaptation, ecology, and evolution of most life forms[1, 2]
We performed 3’ mRNA sequencing of Acropora tenuis larvae inoculated with S. microadriaticum (Smic), Snat, and Stri and with no Symbiodinium exposure (Supplementary Table S2)
Larvae inoculated with Snat and Stri showed completely different transcriptomic responses (Table 1): 19 genes were differentially expressed in the Snat-inoculated samples and 49 genes in Stri-inoculated samples at 1 hpi (Supplementary Table S3)
Summary
Symbioses are ubiquitous in nature and are intricately involved in adaptation, ecology, and evolution of most life forms[1, 2] Cnidarians, such as reef-building corals, are associated with endosymbiotic dinoflagellates of the family Symbiodiniaceae[3, 4]. Several studies have examined transcriptomic responses of coral larvae to symbiotic dinoflagellates during initial contact[25,26,27,28], no studies have used their native algal symbionts in early coral life stages. We recently developed an Acropora larval system as a model to study symbiont selection and recognition by host corals[29] Using this system, we previously documented transcriptomic responses of A. tenuis during symbiosis establishment with its native symbiont, S. microadriaticum (Smic)[30]. In order to highlight gene expression changes exclusive to native symbionts, we investigated transcriptomic responses of A. tenuis larvae exposed to a closely related, non-symbiotic Symbiodinium taxon S. natans, ( Snat), and an occasionally symbiotic Symbiodinium, S. tridacnidorum ( Stri)
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