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

274 INVERTEBRATE EVOLUTIONARY HISTORYresponsible for several of the unique features of the phylum.Another general property of genes demonstrated by echinoderms is‘recruitment’, the use of old genes for new roles. Genes regulatingdevelopment have been shown to be extraordinarily flexible; theexpression of echinoderm genes downstream may be unique to thephylum and different in the different classes, and the genes mayeven be expressed at different regulatory levels. For example, the‘downstream’ gene distal-less (Dll ) is well known to be necessaryfor the distal part of the limb to develop in Drosophila: amongechinoderms, Dll is expressed in the tube feet of asteroids, echinoidsand holothuroids but not in those of ophiuroids, where a differentgene (also known from Drosophila) controls development of tubefeet. In asteroid larvae, Dll is expressed in ectodermal cells and inparticular in the brachiolar arms, used for attachment. In echinoidsDll is expressed from the first appearance of the adult rudimentand specifies most of the structures on the oral side, spines as well astube feet. This is a clear example of recruitment of genes to servein quite new ways, differing in different echinoderms. It is alsoan example of unchanging Hox genes acting on different targetsowing to ‘downstream’ changes. A more practical consequence of thisflexibility is that these downstream genes may be quite unreliablefor indicating relationships.20.7.2 HemichordatesMorphological evidence suggests that hemichordates are moreclosely related to echinoderms than to chordates, an idea earlysupported by evidence from 18S rDNA genes and more recentlyconfirmed by DNA data from the whole mitochondrial genomeof Balanoglossus. At once the question foreshadowed in Chapter 18becomes central: which of the apparent ‘chordate characters’ arereally deuterostome characters that have been lost in echinoderms?This may apply to gill slits, as is suggested by the finding of themin several fossil echinoderms and supported by evidence fromcertain pax genes, which are expressed similarly in the pharyngesof hemichordates, urochordates and cephalochordates. This uniformityof development suggests that hemichordate gill slits are nota product of convergence but are the continuation of a primitivedeuterostome character.Morphology suggests homology between the hemichordate andechinoderm nervous systems, with little relation to the chordatehollow double nerve cord. The tornaria larva has a diffuse nerve netvery similar to that of echinoderm larvae which (unlike the echinodermnerve net) continues into the adult with little change. Yet inthe adult enteropneust Saccoglossus kowalevski this diffuse nervenet expresses the same genes, in the same order, as in the highlydeveloped chordate central nervous system. This extends the morphologicalevidence by suggesting that the deuterostome ancestor hada diffuse nervous system, later centralised in the chordate lineage.