ABDOMINAL LEGS OF MIDDLE PENNSYLVANIAN SROKALARVA: EARLY EXPRESSION OF THE DISTAL-LESS GENE IN HOLOMETABOLOUS INSECTS

A major feature of arthropods is their serially homologous (paralogous) appendages of the thorax and abdomen. The identity of these appendages is under early developmental control and is regulated by homeotic genes linked to genetically similar clusters, termed Hox genes. Under the control of the gene Distal-less (Dll), ventral abdominal appendages paralogous with thoracic walking limbs are expressed in some to most of the 8 to 11 insect abdominal segments typically in basal and extinct clades. Dll is repressed in most insect clades by the Hox genes Ultrabithorax (Ubx), abdominal-A (abd-A), or functional relatives. Mutations in these Hox genes allow expression of Dll as pleuropodia on abdominal segment 1 (Ubx mutation), and prolegs or other appendages variously located on remaining abdominal segments (abd-A mutation).

The repression of Dll development by Ubx and abd-A occurs in holometabolous insects such as Drosophila (fly) and Tribolium (beetle). However, in taxa such as Precis (butterfly) and Manduca (moth), Dll is expressed through the derepression of abd-A, allowing development of prolegs in abdominal segments 3 to 6. Because some extant nonholometabolous hexapodan lineages occasionally express abdominal appendages (springtails, bristletails, grasshoppers) and basal holometabolous insect lineages evidently do not, the occurrence of Srokalarva in the late Middle Pennsylvanian of Mazon Creek, Illinois, is significant. This holometabolan fossil suggests that the basal condition for the clade was larvae that bore clawed abdominal legs. The evolution of holometabolan larval development may have been characterized initially by the expression of Dll and thus the presence of abdominal legs, and succeeded by (1) regulation by Ubx and abd-A and the repression of Dll that resulted in an appendageless abdomen, as well as (2) reversal to a more ancestral condition in other lineages by prolegs occurring on some abdominal segments.