JTW's Evolutionary Origins - Author: Swalla, Billie J.
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Ascidean Larvae
Ascidian larvae provide essential clues to the earliest chordate body plan. Ascidian larvae exhibit the simplest anatomical expression of the familiar "vertebrate-like" chordate with a notochord, segmental musclurature, visceral pharyngeal arches, and the definitive hollow dorsal nerve cord . Billie J. Swalla has identified what appear to be four maternal genes expressed in during early development that appear to be integral to the larval chordate phenotype: a Tyrosine Kinase gene, Uro-2, manx, p58. During development, these genes are expressed in the eggs of Molgula oculata but not in Molgula occulta. The larval Molgula occulta lacks a tail, even though it is closely related to Molgula oculata.Tyrosine Kinase is an enzyme involoved in signal transduction during oogenesis and/or development.
The Uro-2 gene appears to produce only maternal transcripts. In contrast, the Uro-11 gene may also produce zygotic transcripts, which accumulate between gastrulation and neurulation in posterior epidermis, neural and tail muscle cells. Zygotic expression of the Uro-11 gene was not detected in embryos of the anural species. The predicted Uro-2 protein contain, a leucine zipper motif, suggesting that it may dimerize with another protein. The predicted Uro-11 protein contains a nuclear localization signal, a region with similarity to part of the DNA-binding motif in the bacterial histone-like HU and IHF proteins, 12 repeats of the proposed DNA-binding motif S(T)PXX, and a potential zinc finger of the C6 or C6H2 class, suggesting that it may be a DNA-binding protein. The Uro-2 and Uro-11 proteins are candidates for regulatory factors involved in the evolutionary transition from urodele to anural development.
The other two genes, 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.
Source: http://evodevo.bsd.uchicago.edu/
Tailed and tailless ascidian larvae. The species with the tailed larva is Molgula oculata. The species with the tailless larva is Molgula occulta.
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