California Sea Grant College Program

Two modes of development have been observed in ascidians: tailed species produce a larva featuring a tail with notochord, spinal cord, and rows of muscle cells, as well as a pigmented otolith sensory organ in the brain, while tailless species produce a larva lacking the tail and otolith. Investigation of the closely related ascidian species Molgula oculata (tailed) and Molgula occulta (tailless), as well as hybrids produced by their cross-fertilization and exhibiting a tail and otolith, has proven useful in identifying genes whose differential expression is responsible for a switch in developmental mode during evolution. The genesMocuF'H1(tailed) andMoccFH1 (tailless), members of the HNF-3/forkhead gene family, have been identified as key components in the modification of the chordate body plan which has occurred in the tailless species.

In the tailed species, MocuFH1 was expressed early in notochord, endoderm, and mesenchyme precursors and later in the trunk endoderm, in notochord, and in ventral cells of the brain and spinal cord. Similar MoccFH1 expression patterns were observed in the tailless species through the neurula stage, but subsequently transcript levels decrease relative toMocuF'H1levels in the tailed species. Disruption oftheMocuFH1 mRNA using antisense oligodeoxynucleotides (ODNs) prevents normal gastrulation and leads to axially deficient embryos lacking a tail and otolith. Expression of FH1 in hybrids is very similar to that in the tailed species, and levels remain high after neurulation as in the tailed embryos. When hybrid embryos are treated with antisense ODNs, they fail to develop an archenteron and are unable to form a tail and otolith as they normally would, indicating that FH1 is required for the restoration of tailed features to hybrid embryos. However, when tailless embryos are treated with antisense ODNs, they are able to form morphologically normal larvae featuring an archenteron, suggesting that they do not require MoccFHJ to pattern their larvae. In summary, the data implicate the MocuFHJ and MoccFH1 genes in the processes of gastrulation, anterior movements of endoderm to form the archenteron, and posterior movements of notochord cells to form the tail.