ABSTRACT
In the past two centuries, the field of arthropod phylogeny has been the subject of intense discussion. Traditionally, relationships based on morphology and fossil evidence of the four major arthropod lineages have suggested a closer relationship between myriapods and insects to the exclusion of crustaceans. It was also generally recognized that the chelicerates branched off as a basal group. However, recent molecular studies analyzing sequence data strongly contradict these groupings, and instead suggest the following relationships: (i) [insects + crustaceans], and (ii) [chelicerates + myriapods]. As is evident from this lack of congruence, future resolution of arthropod relationships must rely upon a re-evaluation of traditionally assigned morphological homologies. The field of evolutionary developmental biology (evo-devo) has the potential to accomplish this by emphasizing the developmental mechanisms governing formation of particular morphological features. For example, previous studies of gene expression patterns have revealed that all arthropod mandibles are gnathobasic. As a result of these analyses, this feature (mandibular composition) can no longer be used to group myriapods and insects. Future investigations should shift the focus to delineating the genetic mechanisms of structural development down to their most specific events, encompassing regulatory mechanisms to the level of individual target genes. These detailed genetic networks can then be used to establish true homologies of complex morphological traits such as tracheal systems and Malpighian tubules.
