The Interactive Fly
Evolutionarily conserved developmental pathways
Mesoderm has a different embryological origin in Drosophila than it does in vertebrates. In Drosophila, twist and snail, genes whose transcription is directed by Dorsal, define the mid-ventral domain of the blastoderm. Cells from this domain, through the action of twist and snail are fated to become mesoderm, after they invaginate through the ventral furrow at gastrulation. This is in contrast to events in vertebrates, where invagination takes place through the blastopore. The process of vertebrate invagination is driven by Spemann's organizer, a region that directs the fate of all three germ layers. Goosecoid expression marks the dorsal mesoderm; BMP4 (Drosophila homolog DPP) determines ventral mesoderm, and chordin (like DPP, a TGFbeta homolog) counteracts BMP4 and is neuralizing. Thus the roles of twist and snail in Drosophila gastrulation have no homologous counterparts in the process of vertebrate gastrulation.
Nevertheless, both twist and snail in Drosophila and their vertebrate homologs function in mesodermal differentiation. snail is a transcriptional repressor, repressing neural fate in mesoderm. In contrast, twist plays a positive role in the subdivision of the mesoderm, functioning in conjunction with Wingless and Dpp signals from the overlying ectoderm to determine alternate mesodermal fates (Spicer, 1996). twist expression is required to maintain "persistent twist" cells that give rise to the adult musculature (Bate, 1991).
Documentation of the roles of Twist and Snail homologs in vertebrates is incomplete because of the multiplicity of homologs. Nevertheless, different Twist and Snail homologs are known to play distinct roles in patterning of mesoderm. For example, the Xsna gene of Xenopus is expressed zygotically in all early mesoderm. During gastrulation, each cell begins to express as it involutes so that cells newly expressing Xsna are added to the mesoderm mantle as it forms in an anterior-to-posterior progression. Xsna expression is then down-regulated in a tissue-specific fashion that reveals the subdivision of the mesoderm before its derivatives are overtly differentiated (Essex, 1993).
Interestingly, M-twist inhibits myogenesis by blocking the DNA binding of MyoD (Drosophila homolog, Nautilus) by titrating E proteins (the vertebrate homologs of Daughterless), and by inhibiting trans-activation by MEF2 (the vertebrate homolog of Drosophila MEF2). This role sound remarkably like the role played by Drosophila twist in the maintenace of persistent twist cells (Spicer, 1996).
Bate, M., Rushton, E. and Currie, D. A. (1991). Cells with persistent twist expression are the embryonicprecursors of adult muscles in Drosophila. Development 113: 79-89. PubMed Citation: 1765010
Essex, L. J., Mayor, R. and Sargent, M. G. (1993). Expression of Xenopus snail in mesoderm and prospective neural fold ectoderm. Dev. Dyn. 198: 108-22. PubMed Citation: 8305705
Spicer, D. B., et al. (1996). Inhibition of myogenic bHLH and MEF2 transcription factors by the bHLH protein Twist. Science 272: 1476-80. PubMed Citation: 8633239
date revised: 10 July 97
Developmental Pathways conserved in Evolution
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