| 1Department of Biological Sciences, UMBC, 1000 Hilltop Circle, Baltimore, MD 21250, USA; 2Frontier Research System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan |
| The types of molecular genetic events that have facilitated the evolution of multicellularity are not well known for any lineage. We are using comparative genomics and molecular genetic approaches to learn how two key functions (reproduction and motility) carried out by an ancestral unicellular volvocine alga have been segregated into two cell types in Volvox carteri. Our efforts focus on V. carteri regA and related rls/RLS (regA-like sequence) genes, all of which encode proteins with a conserved putative DNA binding domain, the VARL domain. regA is expressed exclusively in somatic cells and is required for the maintenance of their terminally differentiated state as non-reproductive, motile cells. Interestingly, the closest Chlamydomonas reinhardtii homolog of regA, RLS1, is orthologous to another V. carteri VARL gene, rlsD. While regA does not have a C. reinhardtii ortholog, it is part of a tandem array of four closely related paralogs that includes rlsA-C (none of which have orthologs in C. reinhardtii). We have begun expression analyses of V. carteri rls genes to learn more about their functions. Current expression analysis indicates that rlsB and rlsC transcript levels peak in young juveniles at the very beginning of somatic cell differentiation, but do not accumulate in older somatic cells or in mature gonidia or embryos. rlsD, however, is expressed at varying levels throughout the life cycle, with transcripts accumulating in gonidia, embryos, and juveniles. Interestingly, under nutrient starvation conditions, rlsD expression appears to be highly elevated compared to that of regA and other rls genes analyzed under the same nutrient limitations. Based on this data, it appears that rlsB and rlsC may play a role in establishing (but not maintaining) the somatic cell fate, while rlsD likely functions to prevent reproduction under sub-optimal growth conditions. This is consistent with the idea that the progenitor of rlsD/RLS1 in the unicellular ancestor of V. carteri and C. reinhardtii was responsible for repressing reproduction under conditions of nutrient limitation, and that this function was co-opted by regA and certain rls genes to constitutively prevent reproduction in somatic cells, thereby establishing and maintaining their fate.
|