Evolutionary Development of Marine Larvae

Access to a growing number of marine invertebrates with genetic and genomic tools has broadened our understanding of the diversity of developmental mechanisms, informing our understanding of larval evolution by allowing the identification of shared or divergent programs for the formation of body plan patterning and organ formation. Two such genetic programs are the apical plate patterning network and the hox/parahox trunk and gut patterning network common to larval and adult forms, respectively. While mounting evidence supports an ancient origin at the base of the Bilateria for both adult and larval forms, it is clear that many distinct organs and structures have appeared independently and can be shifted between the larval and adult phase frequently. Future advances in our understanding of larval evolution are likely to emerge from exhaustive studies of marine invertebrate cell types by single-cell sequencing technologies and through the study of the genetic basis of the metamorphic transition.

Anterior-posterior patterning within the Caenorhabditis elegans endoderm

The endoderm of higher organisms is extensively patterned along the anterior/posterior axis. Although the endoderm (gut or E lineage) of the nematode Caenorhabditis elegans appears to be a simple uniform tube, cells in the anterior gut show several molecular and anatomical differences from cells in the posterior gut. In particular, the gut esterase ges-1 gene, which is normally expressed in all cells of the endoderm, is expressed only in the anterior-most gut cells when certain sequences in the ges-1 promoter are deleted. Using such a deleted ges-1 transgene as a biochemical marker of differentiation, we have investigated the basis of anterior-posterior gut patterning in C. elegans. Although homeotic genes are involved in endoderm patterning in other organisms, we show that anterior gut markers are expressed normally in C. elegans embryos lacking genes of the homeotic cluster. Although signalling from the mesoderm is involved in endoderm patterning in other organisms, we show that ablation of all non-gut blastomeres from the C. elegans embryo does not affect anterior gut marker expression; furthermore, ectopic guts produced by genetic transformation express anterior gut markers generally in the expected location and in the expected number of cells. We conclude that anterior gut fate requires no specific cell-cell contact but rather is produced autonomously within the E lineage. Cytochalasin D blocking experiments fully support this conclusion. Finally, the HMG protein POP-1, a downstream component of the Wnt signalling pathway, has recently been shown to be important in many anterior/posterior fate decisions during C. elegans embryogenesis (Lin, R., Hill, R. J. and Priess, J. R. (1998) Cell 92, 229–239). When RNA-mediated interference is used to eliminate pop-1 function from the embryo, gut is still produced but anterior gut marker expression is abolished. We suggest that the C. elegans endoderm is patterned by elements of the Wnt/pop-1 signalling pathway acting autonomously within the E lineage.