Ascidian larvae exhibit the simplest chordate body plan, with a head and a tail containing a notochord, three rows of muscle cells, and a hollow dorsal nerve. I have identified four novel maternal genes expressed in ascidian eggs that may be integral to the larval chordate phenotype. These genes are expressed in the eggs of Molgula oculata (tailed larva) but are not expressed in eggs of a sister-species (Molgula occulta) that lacks a tadpole tail. One of these genes is a tyrosine kinase, and may be involved in signal transduction during oogenesis and/or development. The other two, Uro-2 and manx, are likely to be nuclear proteins (one contains a leucine zipper, and the other a zinc "finger" DNA binding domain and a nuclear localization signal) and may be transcription factors. The fourth gene, p58, is a unique protein found in the myoplasm of ascidian eggs, but which is missing in those species which have eliminated tail muscle cells and the tadpole larval stage.

Molgula occulta (C) and Molgula oculata (A) are closely enough related that they can be cross-hybridized artificially. These hybrids (B) have an intermediate phenotype between both of the parental larval henotypes. The zinc-finger gene manx is expressed during gastrulation in these hybrid embryos. When we inhibit gene function by treating embryos with antisense deoxyoligonucleotides to manx, the embryos develop without a tail or an otolith (brain sensory organ). This suggests that manx is required for tail and otolith formation during ascidian embryogenesis, and is a case of a single gene disruption resulting in a completely altered larval phenotype that is not lethal in nature. Further experiments will focus on the mode of action of manx at the cellular level in the embryo, and investigate the mechanism(s) involved in the sympatric speciation of these two Molgula species that can cross-hybridize in the laboratory.

Selected Publications:

Swalla, B.J., Makabe, K.W., Satoh, N., and Jeffery, W.R. (1993) "Novel genes expressed differentially in ascidians with alternate modes of development." Development 119:307-318.

Swalla, B.J., Pederson, E.L., Just M.L. and Jeffery, W.R. (1999) "Manx/Bobcat Gene Complex and the Evolution of Tailless Ascidians: A multigene locus containing the manx and bobcat genes is required for development of chordate features in the ascidian tadpole larva." Development 126:1643-1653.

Swalla, B.J. and Jeffery, W.R. (1996) "Requirement of the manx gene for expression of chordate features in a tailless ascidian larva." Science 274:1205-1209.

Seufert, D. W., Kos, R., Erickson, C.A. and B. J. Swalla (2000) "pDEAD-box RNA helicase, is expressed in chordate embryo neural and mesodermal tissues." JEZ: Mol. and Develop. Evolution 288: 193-204.