The transition between unicelluar and multicellular organisms presents one of the major evolutionary events of life on Earth, but the factors favoring this increase in morphological complexity remain contentious. Exploration of gene expression in model lineages that present a continuum of unicellular to multicellular members, such as the Volvocales, can provide important insights into transitions leading to permanently multicellular organization. Comparative genomic and transcriptomic analyses of unicellular species that can form multicellular phenotypes with permanently multicellular species can show whether genes active in producing inducible multicellular phenotypes in typically unicellular taxa are similar to genes active in permanently multicellular forms. This work evaluates transcriptomic (RNA-seq) data from the defensive multicellular phenotype of Chlamydomonas reinhardtii to link the genes expressed in the production of simple multicellular phenotypes to analogous genes in the permanently multicellular Volvox carteri.
Results/Conclusions
Of the 829 genes differentially expressed in the transition from unicellular to multicellular Chlamydomonas phenotypes, approximately 94% show a one-to-one relationship for their homologs in V. carteri, a finding consistent with the previously noted strong genomic similarity in the lineage. For genes with expanded or constricted numbers of orthologs in V. carteri, a fascinating relationship emerges with genes upregulated or down regulated in the production of multicellular Chlamydomonas showing a respectively increased or decreased number of orthologs in permanently multicellular Volvox. This finding highlights how conscription of genes active in the temporary induction of multicellularity might provide a pathway to permanent multicellularity, while providing insight into the selective pressures and genetic constraints influencing the transition to multicellularity.