CYTOSKELETAL ACTIN GENES FUNCTION DOWNSTREAM OF HNF-3-BETA IN ASCIDIAN NOTOCHORD DEVELOPMENT

We have examined the expression and regulation of cytoskeletal actin g enes in ascidians with tailed (Molgula ocolata) and tailless larvae (M olgula occulta). Four cDNA clones were isolated representing two pairs of orthologous cytoskeletal actin genes (CA1 and CA2), which encode p roteins differing by five amino acids in the tailed and tailless speci es. The CA1 and CA2 genes are present in one or two copies, although s everal related genes may also be present in both species. Maternal CA1 and CA2 mRNA is present in small oocytes but transcript levels later decline, suggesting a role in early oogenesis. In the tailed species, embryonic CA1 and CA2 mRNAs first appear in the presumptive mesenchyme and muscle cells during gastrulation, subsequently accumulate in the presumptive notochord cells, and can be detected in these tissues thro ugh the tadpole stage. CA1 mRNAs accumulate initially in the same tiss ues in the tailless species but subsequently disappear, in concert wit h the arrest of notochord and tail development. In contrast, CA2 mRNAs were not detected in embryos of the tailless species. Fertilization o f eggs of the tailless species with sperm of the tailed species, which restores the notochord and the tail, also results in the upregulation of CA land CA2 gene expression in hybrid embryos. Antisense oligodeox ynucleotide experiments suggest that CA1 and CA2 expression in the not ochord, but not in the muscle cells, is dependent on prior expression of Mocc FHl, an ascidian HNF-3 beta-like gene. The expression of the C A1 and CA2 genes in the notochord in the tailed species, downregulatio n in the tailless species, upregulation in interspecific hybrids, and dependence on HNF-3 beta activity is consistent with a role of these g enes in development of the ascidian notochord.