Where the endosymbiont of tube worm, Lamellibrachia satsuma, in gas seep area comes from?

Abstract

There are two possible pathways of endosymbiont acquisition by the gutless vestimentiferan tubeworms; (1) vertical transmission from parent to progeny or (2) horizontal transmission in which the symbionts are acquired de novo by each generation from the ambient environment. The endosymbionts are seemed to be host-specific, however, the pathway of symbiont acquisition is still unclear. Recently, it was demonstrated that the bacterial symbionts are acquired by entering into the tubeworm larvae through the skin after settlement and proliferate to form the trophosome. In the present study bacterial clones were compared between endosymbiont of vestimentiferan tubeworm and ambient sediment environment in order to elucidate the source of endosymbiont acquisition. Bacterial and archaeal rRNA was PCR-amplified using DNA which extracted from the trophosome of the matured vestimentifera, Lamellibrachia satsuma and the surrounding sediment of Kagoshima Bay (depth 112 m). Only one symbiotic bacterial clone, γ-Proteobacteria, was detected from the trophosome of L. satsuma and the phylogenic distance is quite remote from the clones which were detected from the sediment. The 16S rRNA gene sequence identical to that of the endosymbiont also could not be amplified from the sediments using by specific primer designed based on the 16S rRNA gene sequence of endosymbiont. These results support that the seep vestimentifera, Lamellibrachia satsuma, harbors a monospecific bacterial endosymbiont, γ-Proteobacteria, and endosymbiont clone is not detected from the surrounding sediment. However, this study demonstrated no supportive evidence for the exclusion of horizontal transmission. Extensive study on the clones from the sediment depending on the season and those from water column is necessary. [Supported by MEGRC]