The origin, patterning and evolution of insect appendages

Abstract

SummaryThe appendages of the adult fruit fly and other insects and Arthropods develop from secondary embryonic fields that form after the primary anterior/posterior and dorsallventral axes of the embryo have been determined. In Drosophila, the position and fate of the different fields formed within each segment are determined by genes acting along both embryonic axes, within individual segments, and within specific fields. Since the major architectural differences between most Arthropod classes and orders involve variations in the number, type and morphology of body appendages, the elucidation of the embryology and molecular genetics of the origin and patterning of insect limb fields may help to facilitate an understanding of' both the mechanism of appendage formation and some of the major steps in the morphological evolution of the Arthropods. In this review, we will discuss recent studies that have advanced our understanding of both the origin and patterning of Drosophila leg and wing secondary fields. These results provide fresh insights into potentially general mechanisms of how body parts develop and evolve. IntroductionArthropods and vertebrates have metaineric body plans. created through two main patterning processes. The first, embryonic axis formation, has been extensively studied in Drosophila, and many of the genes and processes involved in determining the anterior/posterior (NP) and dordventral (D/V) polarity of thc embryo havc been defincd. For example, the sequential and combined action of the maternal (eg. bicoid and nanos), gap, pair-rule, segment polarity and homeotic genes ultimately spccify the unique idcntities of cells along the length of the AP axid'). Siinultaneously, the action of the D N polarity genes (cg. dorsal and drcupiituplegir; dpp) assign identities of cells depending on their position along the D/V axis(2) such that, after embryonic axis formation is complete, cvery ectodcrmal cell has the potential knowledge of its respective AP and D/V position within the embryo. The second developmental patterning process is the formation of secondary fields within the primary embryonic fields. In the case of secondary fields giving rise to limbs, this process involves the elaboration of proximal/distal (P/D) positional information essential to the formation of the many types of appendages. While not nearly as much is known about Lhis latter process, the role? of some of the key genes involved in the formation of the appendage primordia and in their growth and patterning during development have been elucidated.This review considers three central aspects of pattern formation in appendages. Firstly. we will discuss recent work in Drosophila that sheds light on the genetic mechanisms that regulate the position, number and identity of the embryonic leg and wing fields. Secondly, we will consider the problem of pattern formation within these discs. Finally, appendage patterning mechanisms will be discussed in a evolulionary context, and related to morphological differences at different taxonomic le...

Phylogeny of Higher Taxa in Insecta: Finding Synapomorphies in the Extant Fauna and Separating Them from Homoplasies

“…Diversification of limbs into all these functions generated a treasure trove of higher groundplan character states, which include many higher synapomorphies. Paleontology and developmental genetics agree that the limb pairs are serially homologous, i.e., that they all evolved from a single, ancestral, polyramous arthropod limb model (Smith 1970(Smith , 1988Kukalová-Peck 1983, 1987Marshall et al 1994, Williams andCarroll 1993;Raff et al 1991;Raff 1996). Such a generalized model is of fundamental importance for higher phylogenetics, because it homologizes the limb-derived appendages in the entire arthropod phylogenetic tree and evaluates them against a monophyletic ancestor.…”

Molecular pathways to parallel evolution: I. Gene nexuses and their morphological correlates

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Structure, development, and evolution of insect auditory systems

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