Arthropod Evolution: Brine shrimp add salt to the stew

The expression patterns of homeotic genes in a crustacean-the brine shrimp Artemia franciscana-provide a new perspective on the evolution of arthropod body plans. The ancestral arthropod is thought to have had a body made up of a series of similar segments, each bearing an unspecialized limb. A major trend in arthropod evolution has been an ever-increasing specialization and regional-ization of segments [1]. Segments, or groups of segments, have repeatedly become specialized for feeding, walking or swimming. The distinct body regions arising from these modifications are called tagmata. All the major groups of extant arthropods-crustaceans, insects, che-licerates and myriapods-have distinct tagmatization patterns. Evidence for other, now lost, forms of arthro-pod tagmatization is abundant in the fossil record [2]. Providing a credible explanation for the evolution of diverse arthropod forms, both past and present, is a challenging task. A molecular genetic explanation for the trend of increasing tagmatization during arthropod evolution was proposed by Ed Lewis [3], based on his discovery that unique identities of Drosophila tagmata are regulated by a small number of homeotic (or Hox) genes, clustered in linear arrays on the chromosome. As mutations in certain Hox genes create flies that, superficially, mimic the morphology of ancestral arthropod species, it was suggested that underlying the increasing complexity in arthropod body plans was a series of Hox gene duplication events, generating an increasingly complex Hox gene cluster [3,4]. The Hox genes are not, however, unique to insects, or even to arthropods-they are highly conserved in other metazoan animals (reviewed in [5]). Even animals with little external regionalization along the body axis, such as annelids, have homologs of nearly all the Drosophila homeotic genes, as well as additional potential Hox genes, with no obvious homologs in Drosophila. At least in terms of sheer numbers, the ancestral arthropod was likely to have had as many Hox genes as a fly, making the simplest gene duplication model for the evolution of morphological complexity in arthropods implausible [6,7]. Similarly, differences in appendage design among insects, such as variations in wing size, seem to have arisen as a result of changes in genes regulated by the Hox genes, and not via the evolution of new Hox genes. In flies, each thoracic and abdominal segment seems to have the potential to make a limb without any input from the Hox genes. Hox genes then act to suppress limb development, or to modify appendage identity (reviewed in …