Titre de la dissertation: « Host separation syndrome and chemical dependency of ectosymbiotic decapods associated with echinoderms ».

Promoteurs de thèse: Monsieur Igor Eeckhaut et Monsieur Jérôme Mallefet (UCLouvain)

Résumé de la dissertation: The existence of symbiosis is facilitated by the secretion of specific secondary metabolites known as kairomones, secreted by the hosts. These molecules are crucial for the survival of symbionts as they allow the recognition of the hosts. In echinoderms, the nature of kairomones attracting symbionts is only known at the level of associations involving sea cucumbers or sea urchins. Also, when separated from their hosts, the environment (both chemical and nutritional) of the symbionts become unsuitable leading to a « host separation syndrome ». This syndrome, currently known in one association involving a sea urchin species, is characterized by an alteration of the behavior and the state of health, all leading to the death of individuals. The general aim of this thesis is to contribute to the knowledge of these kairomones playing a crucial role in symbiotic associations. More specifically, we have been interested in symbioses associating decapods with echinoderms which are frequent symbioses in the marine environment. We first wanted to know the extent of this host recognition mediated by kairomones and we analyzed this phenomenon in crinoid and starfish hosts. We then looked at the host separation on various symbiotic decapods to characterize the syndrome in several species and to understand its impact at the behavioral, anatomical and physiological levels.

The thesis begins with a review of the diversity of macro-ectosymbiotic decapods associated with echinoderms (i.e., the “Introduction” chapter). This includes an exhaustive list of 220 decapod species associated with all five classes of echinoderms, and an analysis of all these associations, described since 1849. The review also investigated association characteristics, such as the benefits for symbionts and their dependency on hosts, as well as co-evolution involving morphological, physiological, and behavioral adaptations. The introduction is followed by four chapters that provide new insight on the host separation syndrome and the host chemical dependency of decapod symbionts.

Chapter 1 characterizes the chemical environment produced by crinoids and explores host recognition behavior in the symbiotic pistol shrimps, Synalpheus stimpsonii. The chemical attractiveness of two crinoid hosts and one non-host species, as well as commercial anthraquinone, were tested in olfactometry. Mass spectrometry revealed on the crinoid Phanogenia distincta, the presence of three different anthraquinones (rhodoptilometrin, comantherin, and a new crinoid anthraquinone) that trigger host recognition.

Chapter 2 investigates host recognition in the adult and larvae of Zenopontonia soror, an obligate symbiont of sea stars. We examine the semiochemicals that influence host selection using chemical extractions, behavioral experiments in olfactometers, and mass spectrometry analyses. Our results demonstrate that asterosaponins are species-specific and play a role in host recognition. While adult shrimps were attracted only by their original host species Culcita novaeguineae, larvae were attracted by different species of seastars. This study provides, for the first time, the chemical identification of an olfactory cue used by larvae of symbiotic organisms to locate their hosts for recruitment.

Chapter 3 focus on the mimetic pigmentation. This study focus on four symbiotic pairs: Z. soror – C. novaeguineae, S. stimpsonii – P. distincta, T. holthuisi –E. mathaei and A. indicus – E. mathaei. The results suggest a strong host dependency of all four symbionts, but only three of them exhibit a host chemical dependency. The host isolation impacts the pigmentation of the symbionts, with a nuanced and dynamic pigmentation variation depending on the species. The chemical extraction highlights similar carotenoids in Z. soror and C. novaeguineae,. At the opposite, the crinoid P. distincta did not possess the carotenoids found in its associated shrimps. Histological sections show the presence of chromatophores in all the species and when shrimps discolored, their chromatophores agglomerated.

Chapter 4 explored the host separation syndrome effects by a transcriptomic approach. Paired-end Illumina HiSeq technology has been used to analyze transcriptomes from A. indicus and T. holthuisi confronted with three different conditions used previously to investigated host separation syndrome. The three conditions analyzed were: individuals associated with their host, individuals separated from their host and individuals separated from their host but conditioned by naphtoquinone that is a kairomone involved in the host recognition by decapods. A total of 217,832 assembled unigenes were obtained, with an N50 of 2,061 bp. Our results show a proportion of 16.5% of differentially expressed genes in isolated T. holthuisi shrimp compared to the control treatment and a proportion of 8.5% of DEGs in isolated T. holthuisi shrimps in presence of spinochromes compared to the control.  For A. indicus, there were fewer variations in DEG proportions among the same comparisons. Yet, analyses highlight a complex transcriptomic effect on both symbiotic species, with notably a differential expression of heat shock protein in isolated T. holthuisi.

In conclusion, the results of this thesis will be linked together to provide new insights into symbiotic associations between decapod symbionts and their echinoderm hosts. It sheds light on the symbiont response to specific chemicals produced by the host and to the host separation. This research will contribute significantly to future studies on co-evolution, host recognition, and chemical ecology within symbiotic systems.