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

278 INVERTEBRATE EVOLUTIONARY HISTORYMany biting and chewing insects and crustaceans have mandibleson the third segment: fossil evidence suggests that the mandiblein both classes evolved from the whole limb by truncation of theaxis. Neither insects nor crustaceans express Dll in this limb, so thatonly a short stubby appendage can form.Behind the head, identical Hox genes govern the pattern ofappendages in both Drosophila and Artemia; differences in whether,where or when they are expressed may cause them to have differenteffects. The main Hox genes acting are Antennapedia (Antp),Ultrabithorax (Ubx) and Abdominal A and B (Abd-A and Abd-B). Ubx andAbd-B are ancient genes, shared with Onychophora (the arthropodsister group) and with the nematode Caenorhabditis elegans, but notwith members of the other group of protostomes. These genes arenot concerned with the nature of the limb (e.g. uniramous orbiramous) but only with whether and when it is formed. Dll isalways necessary for limbs to be fully formed (see Chapter 19 foran introduction to all these genes).In Drosophila the thorax has three pairs of limbs and theabdomen has none. Antp is expressed in the thorax, promoting limbformation, and Ubx acts with Abd-A in the abdomen, where Ubxrepresses Dll and limb formation is prevented.In Artemia all three Hox genes are expressed together in allsegments behind the head and all these segments bear (similar)limbs. This is possible because here Ubx can repress Antp withoutrepressing Dll. Other crustaceans are found to share this moreanterior expression of these genes; the flexibility in Ubx action isalso found to be shared by other crustaceans. In Onychophora,Ubx promotes limb formation, suggesting that this is its primitivefunction. The difference has been traced to a particular peptidewhich has been lost in the evolution of insects. This accounts forthe very constant pattern of insect limbs (three pairs in the thorax,none in the abdomen), in contrast to the great variety found incrustaceans. In both Drosophila and Artemia, Abd-B is expressed insegments forming the gonads.Crustacean diversity is associated with many different patterns ofgene expression: for example, in isopods one of the ‘head’ genes isexpressed in the first thoracic segment, where feeding appendagesare formed in place of walking legs; barnacles have lost Abd-A as wellas the abdomen; in decapods alterations in Ubx and Antp expressionhave led to greatly varied limb specialisations.Ubx also governs wing formation in insects: most insects havetwo pairs of wings, on the second and third thoracic segments;Drosophila, like other Diptera, has only one pair, because Ubxtransforms the posterior pair into halteres.CheliceratesThe chelicerate body is uniquely divided into the prosoma and theopisthosoma. Antennae are totally absent, the first segment bearschelicerae, the second bears pedipalps and the last four segments