The forkhead gene FH1 is involved in evolutionary modification of the ascidian tadpole larva

The forkhead gene FH1 encodes a HNF-3 beta protein required for gastrulatio n and development of chordate features in the ascidian tadpole larva. Altho ugh most ascidian species develop via a tadpole larva, the conventional lar va has regressed into an anural (tailless) larva in some species. Molgula o culata (the tailed species) exhibits a tadpole larva with chordate features (a dorsal neural sensory organ or otolith, a notochord, striated muscle ce lls, and a tail), whereas its sister species Molgula occulta (the tailless species) has evolved an anural larva, which has lost these features. Here w e examine the role of FH1 in modifying the larval body plan in the tailless species. We also examine FH1 function in tailless species x tailed species hybrids, in which the otolith, notochord, and tail are restored. The FH1 g ene is expressed primarily in the presumptive endoderm and notochord cells during gastrulation, neurulation, and larval axis formation in both species and hybrids. In the tailless species, FH1 expression is down-regulated aft er neurulation in concert with arrested otolith, notochord, and tail develo pment. The FH1 expression pattern characteristic of the tailed species is r estored in hybrid embryos prior to the development of chordate larval featu res. Antisense oligodeoxynucleotides (ODNs) shown previously to disrupt FH1 function were used to compare the developmental roles of this gene in both species and hybrids. As described previously, antisense FH1 ODNs inhibited endoderm invagination during gastrulation, notochord extension, and larval tail formation in the tailed species. Antisense FH1 ODNs also affected gas trulation in the tailless species, although the effects were less severe th an in the tailed species. and an anural larva was formed. In hybrid embryos , antisense FH1 ODNs blocked restoration of the otolith, notochord, and tai l, reverting the larva back to the anural state. The results suggest that c hanges in FH1 expression are involved in re-organizing the tadpole larva du ring the evolution of anural development. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.