Evidence for a Dispersed Hox Gene Cluster in the Platyhelminth Parasite Schistosoma mansoni

In most bilaterian organisms so far studied, Hox genes are organized in genomic clusters and determine development along the anteroposterior axis. It has been suggested that this clustering, together with spatial and temporal colinearity of gene expression, represents the ancestral condition. However, in organisms with derived modes of embryogenesis and lineage-dependent mechanisms for the determination of cell fate, temporal colinearity of expression can be lost and Hox cluster organization disrupted, as is the case for the ecdysozoans Drosophila melanogaster and Caenorhabditis elegans and the urochordates Ciona intestinalis and Oikopleura dioica. We sought to determine whether a lophotrochozoan, the platyhelminth parasite Schistosoma mansoni, possesses a conserved or disrupted Hox cluster. Using a polymerase chain reaction (PCR)-based strategy, we have cloned and characterized three novel S. mansoni genes encoding orthologues of Drosophila labial (SmHox1), deformed (SmHox4), and abdominal A (SmHox8), as well as the full-length coding sequence of the previously described Smox1, which we identify as an orthologue of fushi tarazu. Quantitative reverse transcriptase-PCR showed that the four genes were expressed at all life-cycle stages but that levels of expression were differentially regulated. Phylogenetic analysis and the conservation of "parapeptide" sequences C-terminal to the homeodomains of SmHox8 and Smox1 support the grouping of platyhelminths within the lophotrochozoan clade. However, Bacterial Artificial Chromosome (BAC) library screening followed by genome walking failed to reconstitute a cluster. The BAC clones containing Hox genes were sequenced, and in no case were other Hox genes found on the same clone. Moreover, the SmHox4 and SmHox8 genes contained single very large introns (>40 kbp) further indicating that the schistosome Hox cluster is highly extended. Localization of the Hox genes to chromosomes using fluorescence in situ hybridization showed that SmHox4 and SmHox8 are on the long arm of chromosome 4, whereas SmHox1 and Smox1 are on chromosome 3. In silico screening of the available genome sequences corroborated results of Southern blotting and BAC library screening that indicate that there are no paralogues of SmHox1, SmHox4, or SmHox8. The schistosome Hox cluster is therefore not duplicated, but is both dispersed and disintegrated in the genome.

The last common bilaterian ancestor

Many regulatory genes appear to be utilized in at least superficially similar ways in the development of particular body parts in Drosophila and in chordates. These similarities have been widely interpreted as functional homologies, producing the conventional view of the last common protostome-deuterostome ancestor (PDA) as a complex organism that possessed some of the same body parts as modern bilaterians. Here we discuss an alternative view, in which the last common PDA had a less complex body plan than is frequently conceived. This reconstruction alters expectations for Neoproterozoic fossil remains that could illustrate the pathways of bilaterian evolution.