[B-CG05] Decoding the history of Earth: From Hadean to the present

[BCG05-01] Evolutionary implications of chitin-based cell wall biosynthesis for the Proterozoic eukaryotic diversification

★Invited Papers

*Yosuke Hoshino¹, Takuto Ando² (1.GFZ - German Research Center for Geosciences, Potsdam, Germany, 2.Estuary Research Center, Shimane University, Shimane, Japan)

Keywords:Chitin, Eukaryotes, Cell wall, FTIR, Cellulose, Heterotrophy

Chitin is the second most abundant polysaccharides in nature after cellulose. Chitin is a major component of cell walls and exoskeletons in various eukaryotes, including fungi, metazoans, diatoms and ciliates. Chitin-associated biomaterials have extensively been investigated for their biotechnological and medical applications. In contrast, the evolutionary implication of chitin biosynthesis for early eukaryotes is still largely unknown. The last eukaryotic common ancestor is unlikely to have had cell walls and thus the evolution of a cellulose- or chitin-based cell wall that is commonly observed in modern eukaryotes is a later adaptation in the course of evolution. In particular, chitin is most likely a eukaryotic invention, unlike cellulose that is inferred to have evolved well before eukaryotes, and thus the emergence of chitin potentially reflects selective pressures that ancient eukaryotes experienced. Understanding the origin and the evolutionary trajectory of chitin biosynthesis would provide beneficial information to elucidate the molecular adaptation of ancestral eukaryotes to cope with environmental stress and/or interact with other organisms (e.g. autotrophs vs. heterotrophs).

In the current study, comprehensive genetic analyses were performed for biosynthesis pathways of chitin and other cell wall components, in order to trace the compositional evolution of chitinous biopolymers (e.g. cell wall, cyst) in ancestral eukaryotes. While chitin has traditionally been associated with Opisthokonta (fungi and metazoa), our up-to-date molecular analyses suggest that species from the SAR Supergroup played an important role in the early diversification of chitin biosynthesis, contrary to previous inferences that those protists only obtained chitin biosynthesis genes via horizontal gene transfer from Opisthokonta. Our genomic data are being further integrated with FT-IR/Raman spectroscopic data of chitinous biopolymers from extant eukaryotic species to explore the potential of chitinous organic matter in the geological record as a taxonomical marker for particular eukaryotic lineages in the Paleozoic and older. Our preliminary results suggest that chitin generally form composite polymers with a variety of proteins and other organic materials and those polymers display complex spectral profiles, depending on individual lineages. The diversity as well as the function of observed chitinous biopolymers will be discussed in the evolutionary context.