The bagpipe-related homeobox-containing genes are members of the NK family. bagpipe (bap) was first identified in Drosophila and there are three different bagpipe-related genes in vertebrates. Only two of these are found in mammals, the Nkx3.1 and the Bapx1 (Nkx3.2) gene. The targeted mutation in the mouse Bapx1 gene shows a vertebral phenotype in which the ventromedial elements are lacking; these are the centra and the intervertebral discs. In addition, a region of gastric mesenchyme is abnormal. This mesenchyme surrounds the posterior region of the presumptive stomach and duodenum, and in the mutant fails to support normal development of the spleen. In Drosophila, bagpipe has a role in gut mesoderm and the mutant embryos have no midgut musculature. Thus bap related genes in mouse and Drosophila have roles in patterning gut mesoderm; however, neither of the mammalian genes has a discernible role in the gut musculature. In contrast, both mammalian genes have roles in developmental processes that have appeared recently in evolution. The Bapx1 gene found in fish, amphibians, birds and mammals appears to have derived vertebrate specific functions sometime after the split between the jawless fish and gnathostomes.

The pectoral girdle articulates the forelimb with the axial skeleton in all vertebrates with paired anterior appendages. The structure of the pectoral girdle and its position along the axial skeleton has changed significantly during vertebrate evolution. These morphological changes have been well described, but there is little comparative embryology to indicate how these changes may have occurred. It is equally obscure how the muscles that connect the head with the pectoral girdle have maintained appropriate attachments even though these 2 structures have become separated. Here I review the changes in the pectoral girdle across different vertebrate taxa, indicating, where known, the developmental mechanisms underlying these changes. I also suggest how the muscular connections between the head and pectoral girdle have been maintained between these once adjacent bones, displaced during vertebrate evolution.

Emerging developmental studies contribute to our understanding of vertebrate evolution because changes in the developmental process and the genes responsible for such changes provide a unique way for evaluating the evolution of morphology. Endoskeletal limbs, the locomotor organs that are unique to vertebrates, are a popular model system in the fields of palaeontology and phylogeny because their structure is highly visible and their bony pattern is easily preserved in the fossil records. Similarly, limb development has long served as an excellent model system for studying vertebrate pattern formation. In this review, the evolution of vertebrate limb development is examined in the light of the latest knowledge, viewpoints and hypotheses.

Two different modes are utilised by vertebrate species to generate the appendicular muscle present within fins and limbs. Primitive Chondricthyan or cartilaginous fishes use a primitive mode of muscle formation to generate the muscle of the fins. Direct epithelial myotomal extensions invade the fin and generate the fin muscles while remaining in contact with the myotome. Embryos of amniotes such as chick and mouse use a similar mechanism to that deployed in the bony teleost species, zebrafish. Migratory mesenchymal myoblasts delaminate from fin/limb level somites, migrate to the fin/limb field and differentiate entirely within the context of the fin/limb bud. Migratory fin and limb myoblasts express identical genes suggesting that they possess both morphogenetic and molecular identity. We conclude that the mechanisms controlling tetrapod limb muscle formation arose prior to the Sarcopterygian or tetrapod radiation.

The formation of crossveins in Drosophila was an important early case study in understanding the role of the environment in the development and evolution of morphological structures. More recent work has shown that signalling processes play a crucial role in the formation of crossveins in Drosophila and that the interaction of a heat shock factor, Hsp90, with components of signal transduction pathways may account for the sensitivity of these structures to environmental perturbations. A new model for the development of crossveins is presented that divides the formation of crossveins into 3 separate stages. First, the number and placement of the crossveins is determined by signalling along the proximal-distal axis of the wing. This signal may involve the cdc42 gene product and the Jun-N-terminal Kinase signal transduction pathway. Then, during the second stage, an inductive signal from the dorsal wing epithelium is sent to the ventral wing epithelium at locations specified by the first signal. The second signal appears to involve the BMP-like signalling pathway. Finally, in the third stage, a domain of vein competent cells is defined by the signalling from the EGF-receptor-Map Kinase signal transduction pathway, and the exact location of the veins is eventually determined within that domain by Notch-Delta signalling interactions. By altering components of these 3 stages, insects can independently regulate the presence or absence, the number and placement, and the thickness and flexibility of the crossveins. This capacity for the modulation of crossvein structure in many different ways may have contributed to the evolution of different modes of insect flight.