KUO, Dian-Han; SHANKLAND, Marty; University of Texas at Austin: Evolutionary of cell fate specification mechanisms in the O/P equivalence group of the leech genus Helobdella

Body segments of clitellate annelids are formed by posterior addition of blast cells arising from five bilateral pairs of embryonic stem cells, the teloblasts. Each of the teloblasts gives rise to a stereotypic set of descendants. Although fate maps of the teloblast lineages are highly conserved among clitellate annelids, previous studies reveal an unexpected diversity of the cell fate specification mechanisms of the O lineage (which gives rise to ventrolateral ectoderm) and the P lineage (which gives rise to dorsolateral ectoderm) among distantly related clitellate species. To explore the evolutionary dynamics of the cell fate specification mechanisms in the O and P lineages of clitellate annelids, we experimentally examine cell-cell interactions that are involved in the specification of O and P fates in three different laboratory populations belonging to closely related species of the leech genus Helobdella. In leech, the O and P lineages arise from a developmental equivalence group, the O/P equivalence group. In this study, we find that the cell-cell interactions involved in cell fate specification of the O/P equivalence group differ among these populations. In two populations, the dorsomost ectodermal teloblast lineage, the Q lineage, is absolutely required for the P fate in the more dorsal O/P lineage. In the third population, the pathway that involves the Q lineage and an additional pathway that involves the mesoderm lineage redundantly specify the P fate. Our data suggest that the pattern of cell-cell interactions during the development of the O/P equivalence group has undergone evolutionary diversification in closely related species despite maintaining a conserved morphology. Our finding also supports the notion that redundancy plays an important role in generating the diversity of developmental pathways.