Evolutionary Reduction of the First Thoracic Limb in Butterflies  

One of the factors contributing to the evolutionary success of the arthropods is their segmented body plan.  Most segments bear a pair of appendages, with anterioposterior differentiation of appendage identity (tagmosis) as a feature of their evolution.  The diversity of appendage morphology in Cambrian arthropods is significant in a number of hypotheses about their phylogenetic relationships.  The developmental processes governing the specification of appendage identity are therefore an important field of study in arthropod evolution.  Studies of crustaceans have demonstrated the ability of the Hox genes Ubx and Scr to effect evolutionary transformations of one limb type into another by their presence or absence in a body segment.  As the evolution and differentiation of arthropod limb types in the Cambrian radiation is one of the essential questions in the diversification of the phylum, the investigation of development is crucial to resolving the processes by which this occurred. 

Limb development has received some attention in crustaceans, but the genetic controls leading to differentiated appendage types are not known in insects.  The first thoracic (T1) limb of butterflies in the family Nymphalidae is reduced so much that it cannot be used for walking; it gives the family the common name “brush-footed butterflies”.  The function or development of this limb has never been investigated (it is potentially significant in courtship and feeding).  SEM of the T1 leg in the nymphalid butterfly Bicyclus anynana shows the absence of all five tarsal limb segments and the absence of two claws at the distal terminus.  Proportionally, there is a significant reduction in the length of the distal segments of T1 compared with T2 and T3.  In contrast, in the pierid butterfly Pieris rapae (an outgroup of Nymphalidae) all tarsal segments are present.  Transformation of the regular thoracic walking leg into the brush-foot may be initiated by homologues of the crustacean Hox genes, or changes in expression downstream in the limb developmental network.  One candidate gene is Dll, which is involved in the outgrowth of distal limb structures.  Dll is not expressed in insect mandibles, which have the least distal outgrowth of all appendages.  Thus, the nymphalid brush-foot could lack Dll expression, leading to the fusion or loss of distal limb outgrowths (especially tarsal segments).  Therefore, developing B. anynana limbs were stained with antibodies against Dll to reveal its expression pattern.    Future work will include applying similar methodologies to other extant arthropod taxa, particularly those with anterior specialization of limbs (e.g. crustacean maxillipeds, branchiopod first antennae).  Sampling of phylogenetically diverse arthropods provides a basis for inferring ancestral character states.  Knowledge of character states is crucial for determining polarity of limb character evolution, and therefore homology of limb types.  Fossil arthropods with preserved limbs from Palaeozoic lagerstätten must be included in the phylogenetic dataset, as they provide crucial records of stem-group morphology, filling in some of the gaps created by subsequent extinction.  With this joint approach of developmental gene expression and fossil morphology, the developmental mechanisms underlying the origin of arthropod limb types, such as the Cambrian great appendages or phyllopodous limbs, may be elucidated.