| Abstrakt: |
SUMMARY: Volvox carteri is an excellent system for investigating the origins of cell differentiation because it possesses just two cell types, reproductive gonidia and motile somatic cells, which evolved relatively recently. The somatic phenotype depends on the regA gene, which represses cell growth and reproduction, preventing cells expressing it from growing large enough to become gonidia. regA encodes a putative transcription factor and was generated in an undifferentiated ancestor of V. carteri through duplication of a progenitor gene whose ortholog in V. carteri is named rlsD. Here we analyze the function of rlsD through knockdown, overexpression, and RNA‐seq experiments, to gain clues into the function of a member of an understudied putative transcription factor family and to obtain insight into the origins of cell differentiation in the volvocine algae. rlsD knockdown was lethal, while rlsD overexpression dramatically reduced gonidial growth. rlsD overexpression led to differential expression of approximately one‐fourth of the genome, with repressed genes biased for those typically overexpressed in gonidia relative to somatic cells, and upregulated genes biased toward expression in soma, where regA expression is high. Notably, rlsD overexpression affects accumulation of transcripts for genes/Pfam domains involved in ribosome biogenesis, photosynthetic light harvesting, and sulfate generation, functions related to organismal growth, and responses to resource availability. We also found that in the wild type, rlsD expression is induced by light deprivation. These findings are consistent with the idea that cell differentiation in V. carteri evolved when a resource‐responsive, growth‐regulating gene was amplified, and a resulting gene duplicate was co‐opted to repress growth in a constitutive, spatial context. Significance Statement: Little is known about the function of VARL (Volvocine Algal RegA‐Like) family transcription factors beyond RegA. Moreover, the relationship (if any) between function across VARL gene products and how cell differentiation arose in the green alga Volvox carteri has only begun to be examined. Here we provide additional evidence that the evolution of cell differentiation in V. carteri involved rewiring of a pathway that regulates cell and organismal growth in response to resource limitation. [ABSTRACT FROM AUTHOR] |
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