Cell-Type Transcriptomes of the Multicellular Green Alga Volvox carteri Yield Insights into the Evolutionary Origins of Germ and Somatic Differentiation Programs

• Published in: G3 : genes - genomes - genetics Vol. 8; no. 2; pp. 531 - 550
• Main Authors: Matt, Gavriel Y, Umen, James G
• Format: Journal Article
• Language: English
• Published: United States Genetics Society of America 01-02-2018; Oxford University Press
• Subjects: basal bodies; carbon metabolism; cell differentiation; cooption; evolution; extracellular matrix; flagella; green algae; Investigation; multicellularity; photosynthesis; secretory pathway; senescence; stem cells; basal bodies; stem cells; cell differentiation; multicellularity; green algae; secretory pathway; evolution; senescence; flagella; cooption; photosynthesis; carbon metabolism; extracellular matrix
• ISSN: 2160-1836; 2160-1836
• DOI: 10.1534/g3.117.300253

Germ-soma differentiation is a hallmark of complex multicellular organisms, yet its origins are not well understood.
is a simple multicellular green alga that has recently evolved a simple germ-soma dichotomy with only two cell-types: large germ cells called gonidia and small terminally differentiated somatic cells. Here, we provide a comprehensive characterization of the gonidial and somatic transcriptomes of
to uncover fundamental differences between the molecular and metabolic programming of these cell-types. We found extensive transcriptome differentiation between cell-types, with somatic cells expressing a more specialized program overrepresented in younger, lineage-specific genes, and gonidial cells expressing a more generalist program overrepresented in more ancient genes that shared striking overlap with stem cell-specific genes from animals and land plants. Directed analyses of different pathways revealed a strong dichotomy between cell-types with gonidial cells expressing growth-related genes and somatic cells expressing an altruistic metabolic program geared toward the assembly of flagella, which support organismal motility, and the conversion of storage carbon to sugars, which act as donors for production of extracellular matrix (ECM) glycoproteins whose secretion enables massive organismal expansion.
orthologs of diurnally controlled genes from
a single-celled relative, were analyzed for cell-type distribution and found to be strongly partitioned, with expression of dark-phase genes overrepresented in somatic cells and light-phase genes overrepresented in gonidial cells- a result that is consistent with cell-type programs in
arising by cooption of temporal regulons in a unicellular ancestor. Together, our findings reveal fundamental molecular, metabolic, and evolutionary mechanisms that underlie the origins of germ-soma differentiation in
and provide a template for understanding the acquisition of germ-soma differentiation in other multicellular lineages.