Exploring symbiont gene expression in two echinoid-associated shrimp species under host separation

Symbiotic relationships are omnipresent and particularly diverse in the marine world. In the Western Indian Ocean, the sea urchin Echinometra mathaei associates with two obligate ectosymbiotic shrimp species, Tuleariocaris holthuisi and Arete indicus. These shrimps are known for their host-dependent nature. T. holthuisi, for example, exhibits severe host separation syndrome, showing signs of stress and rapid mortality when isolated. Specific host pigments called spinochromes seem essential for T. holthuisi survival. Our study employs a transcriptomic approach to assess the stress induced by host separation on these shrimps. Using paired-end Illumina HiSeq technology, we analyzed transcriptomes of both species under three conditions: (i) symbionts on their host (CC), (ii) isolated symbionts in seawater (IC), and (iii) isolated symbionts in spinochrome-enriched seawater (IC + S). Sequencing revealed a total of 217,832 assembled unigenes, with an N50 value of 2061 bp. Isolated T. holthuisi showed 16.5 % DEGs (IC/CC), reduced to 8.5 % with spinochromes (IC + S/CC), both compared to the control condition (CC). Further analyses of stress-related genes show that T. holthuisi expressed stress-related genes when isolated in comparison to the control (IC/CC). Notably, heat shock proteins (HSPs) were significantly up-regulated in isolated T. holthuisi, especially without spinochromes. In contrast, A. indicus displayed differential expression of diverse genes, suggesting an adaptive micro-regulation mechanism to cope with isolation stress. This study pioneers the use of NGS in exploring the transcriptomic responses of symbiotic shrimp species, shedding some light on the molecular impact of the host-separation syndrome and chemical dependencies.