RESUMEN
The symbiotic hydra Hydra viridissima has a stable symbiotic relationship with the green alga Chlorella. This hydra appears to cospeciate with the symbiotic alga, and some strains are known to have strain-specific host/symbiont combinations. To investigate the mechanism of the specificity between host and symbiont, we explored the effect of the removal or exchange of symbionts in two distantly related H. viridissima strains (K10 and M9). In the K10 strain, severe morphological and behavioural changes were found in symbiont-removed and symbiont-exchanged polyps. Interestingly, both polyps showed a similar gene expression pattern. The gene ontology (GO) enrichment analysis revealed that the removal or exchange of symbionts caused the downregulation of genes involved in the electron transport chain and the upregulation of genes involved in translation in the K10 strain. On the other hand, symbiont-removed and symbiont-exchanged M9 polyps showed modest changes in their morphology and behaviour compared with the K10 strain. Furthermore, the patterns of the gene expression changes in the M9 strain were quite different between the symbiont-removed and symbiont-exchanged polyps. Our results suggested that the regulation of energy balance is one of the crucial mechanisms for maintaining symbiotic relationships in green hydra, and this mechanism differs between the strains.
RESUMEN
Cladobranchian sea slugs are characterized by a number of dorsal projections, called "cerata," which are presumably involved in such biological functions as kleptocnidal defense, gas exchange, and symbiotic photosynthesis. Here, we investigated the developmental pattern of ceras formation in a cladobranchian, Pteraeolidia semperi, using field-collected individuals at various postembryonic developmental stages. As the body length increased, the total number of cerata increased in a logistic manner, up to 280 per individual. On the dorsal side of the body, the cerata exhibited a conspicuous formation of repeated, laterally-paired clusters, or rows, along the antero-posterior axis of the animals. As the body length increased, the number of ceras rows increased in a logistic manner, reaching a plateau at around 15 rows per individual. Two types of ceras clusters were observed: well-developed ceras clusters forming a glove-like structure with a basal bulge, which tended to be found in larger animals and at the anterior body region, and less-developed ceras clusters without the bulge, which tended to be found in smaller animals and at the posterior body region. Statistical and simulation analyses suggested that bulge formation underlies increased ceras number, even after the plateaued formation of new ceras rows. These results indicate that, in the postembryonic development of P. semperi, the increase of dorsal cerata entails the following processes: (i) increase of the number of ceras rows, (ii) formation of the basal bulge in each ceras cluster, and (iii) increase of the number of cerata per ceras cluster.
