Cryptosporulation inKurthiaspp. forces a rethinking of asporogenesis in Firmicutes

Abstract

38Sporulation is a complex morphophysiological process resulting in a cellular structure that is more 39 resistant than the vegetative form. In Firmicutes, this structure is produced within the mother cell, 40 and is called an endospore. Endospore formation is thought to have evolved in the common 41 ancestor of Firmicutes. However, sporulation has apparently been lost in some extant lineages that 42 are defined as asporogenic. We isolated strain 11kri321, a representative of the genus Kurthia, 43 from an oligotrophic geothermal reservoir. While Kurthia spp. is considered to comprise only 44 asporogenic species, strain 11kri321 produced spores. Genomic reconstruction of the sporulation 45 pathway shows elements typical of sporulation in Bacilli, including the signaling for sporulation 46 onset. However, key genes were missing, including those involved in engulfment and dipicolinic 47 Importance 58Endospore-forming Firmicutes include many environmental and medical relevant bacterial clades. 59In these microorganisms, the ability to produce endospores is essential for survival in the 60 environment and even for pathogenesis. The minimum core of genes required to produce a viable 61 and resistant spore, the distinction between endospore-forming and asporogenic groups, as well as 62 the evolution of sporulation have been a subject of investigation and debate for decades. Here, we 63 demonstrate endosporulation in the genus Kurthia, considered as asporogenic. Morphological, 64 physiological and genomic analyses were undertaken to demonstrate that sporulation is not lost 65 within this lineage. Based on our results we propose a re-examination of the minimal genetic 66 requirements of sporulation and the use of the term cryptosporulant to describe lineages of 67