An Introduction to the Invertebrates, Second Edition - tiera.ru

WHAT DO MOLECULES TELLUS ABOUT RELATIONSHIPS WITHIN PHYLA?279of the prosoma bear the four pairs of walking legs. The unique limbpattern of the prosoma is not due to the absence of any segmentsbut to differences in Hox gene expression: for example no antennaecan be formed in the first segment because as a result of such adifference Dll is not expressed there, and the siting of chelicerateappendages relates to the confinement of Ubx expression to theopisthosoma. In all living chelicerates studied, the expression ofHox genes shows that the prosoma as a whole corresponds to thehead of insects and crustaceans. The appendages used as jaws bymany insects and crustaceans are used as legs by chelicerates.MyriapodsIn myriapods the most anterior segment bears antennae, while thesecond has mandibles, where the expression of Dll promotes a longjointed mandible in contrast to insects and crustaceans, and theremay be maxillae and sometimes a pair of poison claws. All moreposterior segments express both Ubx and Abd-A and have similarwalking legs. The old idea that insects were derived from myriapodlikearthropods is not supported by any kind of molecular evidence.Myriapods, like chelicerates, have the same set of Hox genes asinsects and all other groups of arthropods; that these genes have notbeen used to make specialised segments does not relate them toinsects in particular, and some other evidence suggests that theymay be closer to chelicerates.Onychophora are found to have exactly the same set of Hoxgenes as centipedes, with homologues for each gene expressed inDrosophila. Whether or not there is specialisation of appendages, thegenes are there.In summary, Hox genes, discovered and first studied in relation toontogenetic development, can also assist the assessment of homologyand hence the investigation of phylogeny. Arthropods are seen tobe a monophyletic phylum sharing common Hox genes and havinga common origin of segmentation, with early divergence of downstreamgenes that later led to the separation of the four maingroups. Figure 20.4 indicates a possible phylogeny of these groups.Fig. 20.4 A possible phylogeny forthe arthropod classes and relatedphyla. Alternatively, Insecta mayhave arisen from among Crustacea.