From Sporulation To Intracellular Offspring Production: Evolution Of The Developmental Program Of Epulopiscium

Abstract

Epulopiscium sp. type B is an unusually large intestinal symbiont of the surgeonfish Naso tonganus. Unlike most other bacteria, Epulopiscium sp. type B has never been observed to undergo binary fission. Instead, to reproduce, it forms multiple intracellular offspring. We believe this process is related to endospore formation, an ancient and complex developmental process performed by certain members of the Firmicutes. Endospore formation has been studied for over 50 years and is best characterized in Bacillus subtilis. To study the evolution of endospore formation in the Firmicutes and the relatedness of this process to intracellular offspring formation in Epulopiscium, we have searched for sporulation genes from the B. subtilis model in all of the completed genomes of members of the Firmicutes, in addition to Epulopiscium sp. type B and its closest relative, the spore-forming Cellulosilyticum lentocellum. By determining the presence or absence of spore genes, we see the evolution of endospore formation in closely related bacteria within the Firmicutes and begin to predict if 19 previously characterized non-spore-formers have the genetic capacity to form a spore. We can also map out sporulation-specific mechanisms likely being used by Epulopiscium for offspring formation. Lastly we focus on the relevance of one sporulation homolog found in Epulopiscium, spoIIE, by using reverse transcription quantitative PCR to determine that its expression profile during offspring formation is highly similar to that seen in B. subtilis.