Mechanisms and evolutionary origins of germ-soma specification in a multicellular green alga, Volvox carteri

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The majority of the biosphere is populated by microscopic, single-celled bacteria and protists. Yet, in a few rare instances single-celled organisms acquired the ability to aggregate or to remain associated after cell division and form collectives, thus setting the stage for the remarkable evolution of large complex multicellular life including animals, plants and a few other multicellular groups like fungi and macroalgae. This project aims to understand an important but largely mysterious step that has repeatedly occurred in conjunction with the transition to complex multicellularity: the evolution of distinct reproductive and non-reproductive cell types. In most cases this key transition is so ancient that its origins are difficult or impossible to reconstruct. By investigating cell-type specialization in a much more recently evolved multicellular species, the green alga Volvox, and comparing it to a closely-related single-celled relative, Chlamydomonas, the research team aims to understand in unprecedented genetic and genomic detail the evolutionary pathway and genetic mechanisms that enabled separate reproductive and non-reproductive cell types to arise. This project will not only address fundamental questions underlying one of evolution's most enigmatic transitions, but also has practical implications for understanding how coordinated metabolic reprogramming and specialization might be engineered into economically valuable green algae in order to tune the balance between resources allocated to cell growth versus allocation towards increased production of biofuels or high-value products that are enhanced when cell growth is minimized. The project includes graduate student and postdoctoral associate training in interdisciplinary scientific research. Outreach to local middle school students will enable students to participate in scientific discovery.The long-term goal of this research is to understand the origins and genetic mechanisms associated with multicellular organization and germ-soma division of labor. Volvox carteri 'Volvox' exhibits a streamlined and experimentally tractable form of germ-soma differentiation. Each spheroidal individual contains just two cell types: ~2000 sterile somatic cells that furnish motility and produce extracellular matrix, but which are destined to senesce and die; and ~16 large reproductive cells called gonidia, each of which undergoes a stereotyped pattern of cell divisions and morphogenesis to produce a new spheroid. It has been hypothesized that the germ-soma dichotomy in Volvox evolved by cooption of temporally-regulated and transiently-expressed differential gene expression programs from a unicellular ancestor; but this idea has not been tested on a genome-wide scale. Under this project new molecular-genetic and genomics resources will be leveraged to elucidate gene expression networks that control cell Volvox differentiation and to decipher their origins. The approaches involve: 1) Comparative analyses of cell-type transcriptomes obtained from synchronized wild-type Volvox and from cell differentiation mutants regA- and lagA- to elucidate cell-type specification networks with high spatio-temporal resolution; 2) Identification of direct targets of RegA, a nuclear-localized transcription factor and master regulator of somatic differentiation, using chromatin immunoprecipitation and deep sequencing or similar approaches; 3) Characterization and cloning of lag- mutants whose gene products suppress somatic differentiation of germ cells. Together these approaches will define the regulatory networks and control mechanisms involved in Volvox cell-type specification and critically test their origins by comparison with temporal expression programs in close relatives such as Chlamydomonas.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.