Box 72 Relationships of the Hexapoda to other Arthropoda

The immense phylum Arthropoda, the joint-legged animals, includes several major lineages: the myriapods (centipedes, millipedes, and their relatives), the chelicerates (horseshoe crabs and arachnids), the crustaceans (crabs, shrimps, and relatives), and the hexapods (the six-legged arthropods: the Insecta and their relatives). The onychophorans (velvet worms, lobopods) traditionally have been included in the Arthropoda, but now are considered either to lie outside as a probable sister group, or may be related to chelicerates. Traditionally, each major arthropod lineage has been considered mono-phyletic, but some investigations have revealed non-monophyly of one or more groups. Estimation of inter-relationships has been contentious. The once-influential view of the late Sidnie Manton proposed three groups of arthropods, namely the Uniramia (lobopods, myriapods, and insects, united by having single-branched legs), Crustacea, and Chelicerata, each derived independently from a different (but unspecified) non-arthropod group. Recent morphological and molecular studies all support mono-phyly of arthropodization, although proposed internal relationships cover a range of possibilities. Part of Manton's Uniramia group comprising myriapods plus hexapods - (Atelocerata or Tracheata) - is supported by the presence (in at least some groups) of a tracheal system, Malpighian tubules, unbranched limbs, eversible coxal vesicles, postantennal organs, and anterior tentorial arms. Furthermore, the diagnostic crustacean second antenna is not seen in hexapods. Proponents of Myriapoda + Hexapoda saw Crustacea grouping either with the chelicerates and the extinct trilobites, distinct from the Atelocerata, or forming its sister group in a clade termed the Mandibulata. In all these schemes, Myriapoda or a subordinate group within Myriapoda was seen as closest relatives of the Hexapoda.

In contrast, certain shared features, including ultrastructure of the nervous system (e.g. brain structure, neuroblast formation, and axon development), the visual system (e.g. fine structure and development of the ommatidia and optic nerves), and development, especially of segmentation including the sequence and function of the regulatory Hox genes, suggested a closer relationship of Hexapoda to Crustacea. Such a grouping, termed the Pancrustacea, excludes myriapods. Molecular sequence data alone, or combined with new morphological studies, increasingly supports Pancrustacea over Atelocerata. Clearly this very ancient divergence requires evidence from very slow evolving genes: faster genes fail to retain phylogenetic signal. This Pancrustacea hypothesis of relationship requires re-consideration of features purported to support monophyly of Atelocerata. Postantennal organs, which occur only in Collembola and Protura in Hexapoda, must be convergent with similar organs in Myriapoda. Malpighian tubules of hexapods must exist convergently in arachnids, and evidence for homology between their structure and development in hexapods and myriapods remains inadequately studied. Coxal vesicles are not always developed and may not be homologous in the Myriapoda and those Hexapoda (apterygotes) possessing these structures. The second antennae of Crustacea appear to be homologous with the labrum of hexapods, derived from the third cephalic segment. Morphological characters supporting Atelocerata appear to be non-homologous and may have been acquired convergently in hexapods and myriapods during adoption of a terrestrial mode of life.

Major findings from molecular embryology include similarities in developmental expression in hexapods and crustaceans, including of homeotic (developmental regulatory) genes such as Dll (Distal-less) in the mandible. This finding refutes Manton's views concerning both arthropod polyphyly and the independent derivation of hexapod mandibles from those of crustaceans. Data derived from the neural, visual, and other aspects of developmental systems seem to reflect more accurately the phylogeny than did many earlier-studied morphological features.

Whether the Crustacea in totality or a component thereof constitute the sister group to the Hexapoda remains unclear. Although morphology can support a monophyletic Crustacea as sister to Hexapoda, molecular data imply a paraphyletic Crustacea. Two possibilities have been suggested: Hexapoda as sister to the Malacostraca (crabs, crayfish, and relatives) with some morphological support, or sister to the Branchiopoda (fairy shrimps, tadpole shrimps, and water fleas), supported by molecular developmental evidence as well as some DNA sequence analyses. This latter suggestion finds support from paleontological data, in which first crustacean fossils are known from marine Upper Cambrian (511 mya) beds, whereas the first hexapods appear some 100 million years later in freshwater deposits from the Devonian (section 8.2.1). An evolutionary scenario can be envisaged in which hexapods originated some 420 mya from a common ancestor shared with branchiopod crustaceans that lived in fresh water, as shown in the figure (after Glenner etal. 2006). Thus the Crustacea colonized land as Hexapoda, whereas the branchiopods remained essentially in fresh water. Such an early Devonian timing for the insects is congruent with the independent transition to land of chelicerates and myriapods, also via a postulated freshwater transition.

shown in Figs 7.2 and 7.3, and the classification presented in the following sections reflect our current synthetic view. Traditionally, Collembola, Protura, and Diplura were grouped as "Entognatha", based primarily on resemblance in mouthpart morphology. Entog-nathan mouthparts are enclosed in folds of the head that form a gnathal pouch, in contrast to mouthparts of the Insecta (Archaeognatha + Zygentoma + Pterygota), which are exposed (ectognathous). However, the Diplura has sometimes been placed as sister to the Insecta, thus rendering the Entognatha paraphyletic. Also the Collembola and Protura have sometimes been grouped as the Ellipura based on certain shared morphological features, including an apparently advanced form of entognathy, Some recent molecular evidence supports the monophyly of the Entognatha, and analyses of data from rRNA genes suggest that Diplura is sister to Protura in a group named the Nonoculata ("no eyes") due to the absence of even simple eyes; in contrast, many Collembola have clustered simple eyes. We have used the latter classification here and treat the Entognatha as a class sister to the insecta, but further gene and taxon sampling is required to confirm or refute the purported relationships.

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