Limb and thoracic segments of crayfish contain the same segmental field

Abstract

The segmented appendages of arthropods are constructed by different types of morphogenetic fields. A long-range field system, called the limb field, promotes continuity for a normally segmented appendage. The control of this continuity also appears to extend onto the nearby body wall. Limb segmental fields act to ensure local positional continuity within a segment, and they are equivalent in the different segments of a leg. As an overview of these principles in crayfish, distal segments of a leg were grafted into the most proximal segment, the coxa. The presence of both field mechanisms was evident during the subsequent intercalary reconstruction of the leg. A jointed intercalate was the hallmark of limb field activity; intrasegmental fusion between matching areas of different segments was the hallmark of limb segmental fields. The principle of limb/body continuity is discussed; its limitation was assessed in crayfish with an operation that provoked extensive intercalary regeneration of legs from ectopic positions in the body.

Several operations were done to test if limb/body continuity includes subcoxa limb segmental fields incorporated into the thoracic body wall proximal to the coxa. Large coxa grafts into a leg site, and patch grafts between coxa and body wall, were used to test the 'subcoxa theory' in crayfish. A distinctive pattern of intrasegmental fusions between the coxa and body wall was found, and so subcoxa limb segmental fields exist in different thoracic body segments proximal to each coxa segmental field. The most unusual feature was that the pattern of fusions indicated that the subcoxa field maps are proximo-distally mirror symmetric to the more distal segmental fields of a leg. The more distal leg segments in crayfish and insects embody tandemly symmetric segmental fields. The finding of subcoxa limb segmental fields in crayfish is discussed in the context of possible mechanisms of arthropod leg development and evolution.