Developmental genetics and arthropod evolution: part 1, on legs.

Developmental genetic information as it relates to the ontogeny of limbs can help evaluate various scenarios of arthropod evolution proposed in the past, as well as help frame other alternatives. First, the cascade of genetic expressions, which controls the development of the arthropod limb, suggests that a postulated evolution of the crustacean coxa from a proximal endite, a structure seen on certain Cambrian crustaceomorphs, might not be correct. Alternative hypotheses could explain the fossil anatomy, and the genetic patterns of expression demand that we at least be cautious in interpreting the Orsten material. Second, recognition of three distinct models of limb formation in arthropods would appear to preclude Rehbachiella, from the Cambrian Orsten, and Lepidocaris, from the Devonian Rhynie Chert, as members of the crown-group Branchiopoda. The recognition of a distinct Artemia Model of limb induction within living anostracans, notostracans, cladocerans, and conchostracans requires that such a model be part of the ground pattern of the Branchiopoda, a pattern that does not appear to have been possible in the fossil species. Finally, the suggestion that a large number of leg segments must be a plesiomorphic condition in arthropods should be considered cautiously. A sequential occurrence of mutations including, for example, a recessive loss-of-function mutant of a Hox-gene like Antennapedia could have resulted in the apomorphic evolution of long, multisegmented limbs within different groups of arthropods. The need for more comprehensive phylogenetic studies using as many taxa and characters possible is obvious both for the generation of scenarios of evolution, as well as in testing multiple alternative hypotheses of relationships.