Monophyly of the clade Ecdysozoa was one of the major surprises of the molecular phylogenetics of the Metazoa because it implied that the segmented arthropods and annelids were merely distant relatives, with the former a member of Ecdysozoa, and the latter a member of Lophotrochozoa. Ecdysozoa unites the arthropods (e.g., insects, crustaceans, myriapods, and chelicerates), onychophorans (velvet worms), tardigrades (water bears), nema-todes (roundworms), nematomorphs (horsehair worms), priapulids, kinorhynchs (mud dragons), and loriciferans (in the absence of molecular sequence data, this latter taxon is included solely on the basis of morphological evidence; Figure 7). Molecular support for Ecdysozoa derives from an increasing number of sources, including SSU, LSU, elongation factor-1a, elongation factor-2, myosin heavy-chain type II, Hox genes, and sodium-potassium adenosine triphosphatase (ATPase) a-subunit. Additionally, proposed morphological synapomor-phies of the ecdysozoans include the covering of the body with a cuticle, which is ultrastructurally similar across ecdysozoans, and which is periodically moulted (presumably under hormonal control, although that has so far only been investigated in any detail in the arthropods and nematodes), the loss of motile epidermal cilia, a terminal mouth, and the loss of intestinal cilia. Finally, the neural expression of horseradish peroxidase immunoreactivity has also been proposed as an ecdysozoan apomorphy. The failure of various phy-logenomic studies to confirm the monophyly of Ecdysozoa may be due to insufficient taxon sampling (limited to those taxa whose genomes have been sequenced), and the potentially highly modified nature of the genome of the nematode C. elegans (Copley et al., 2004; Philippe et al., 2005).
With the conspicuous exception of the arthropods and nematodes, not many molecular sequences have so far been generated for the other ecdysozoan phyla. Consequently, on the basis of molecular evidence alone ecdysozoan phylogeny is not yet well resolved. However, on the basis of combined molecular and morphological evidence three likely clades can be hypothesized: Panarthropoda (arthropods, tardigrades, and onychophorans), Scalidophora (priapulids, kinorhynchs, and loriciferans), and Nematoida (nematodes and nematomorphs). The most recent molecular and combined molecular and morphological phylogenetic analyses suggest a sister group relationship between the Nematoida and Panarthropoda, with Scalidophora as a sister group to this clade (Figure 7). However, robust support for this topology remains to be discovered.
1.02.3.4.1 Panarthropoda: Arthropoda, Onycho-phora, and Tardigrada Onychophorans are terrestrial, 15-150mm long, somewhat cigar-shaped animals that are readily identifiable by their 13-43 pairs of walking limbs known as lobopods, and a prominent pair of antennas that adorns the head. This undoubtedly monophyletic group has been at the center of evolutionary speculations ever since the last quarter of the nineteenth century, when onychophorans were considered an important evolutionary link between annelids and arthropods.
Tardigrades, or water bears, are small (mostly less than 1mm) animals. Their generally roundish bodies are carried on four pairs of stubby legs that identify four body segments. They are well known for their ability to survive for extended periods in a state of extreme inactivity while being virtually completely dehydrated, a phenomenon called cryp-tobiosis. This ability is an obvious advantage to the many tardigrades that live in the thin and ephemeral films of water that surround terrestrial lichens and mosses. Unfortunately, their small size makes it very difficult to reconstruct their phylogenetic position with morphological evidence alone because their lack of certain important features uniquely shared between the larger-bodied onychophorans and arthropods may have been secondarily lost, rather than primitively absent. This might be true, for example, of metanephridia that are restricted to a reduced coelom called a sacculus, and a dorsal heart with openings called ostia, two characters shared by onychophorans and arthropods.
The Arthropoda is an amazingly diverse group of animals, with about 1 million described species, and total estimates suggesting up to 10-fold higher diversity. About two-thirds of described invertebrate species are arthropods. They are characterized by the possession of a more or less rigid, articulated chitinous exoskeleton that covers the entire body surface, and the possession of articulated limbs.
There are four extant arthropod groups: (1) Hexapoda or Insecta (note that there is some variation in the use of these terms in the literature, principally by considering the insects as a subgroup of the Hexapoda); (2) Myriapoda (millipedes and centipedes); (3) Crustacea (crabs, lobsters, etc.); and (4) Chelicerata (horseshoe crabs, spiders, scorpions, etc.). The literature on arthropod phylogeny is one of the most voluminous in systematic zoology, and chronicles a rich history that extends back to the very beginning of evolutionary biology. These extensive efforts to reconstruct the phylogenetic relationships of arthropods are classically rooted in morphological (including fossil) information, but the recent application of an increasingly diverse array of molecular data has yielded several important advances in our understanding.
In contrast to traditional ideas that united the myriapods and the hexapods, molecular evidence instead supports a close relationship of the crustaceans and hexapods, either as sister groups, or with hexapods derived from within the paraphyletic crustaceans. In addition, and more controversially, various nuclear and mitochondrial genes support chelicerates and myriapods as closest relatives in a clade that has been named Paradoxopoda or Myriochelata (Mallatt et al., 2004; Pisani et al., 2004). However, whether myriapods and chelice-rates indeed form a clade, or whether chelicerates represent the earliest diverging extant taxon (Wheeler et al., 2004; Giribet et al., 2005), and whether hexapods and crustaceans are really closest relatives awaits further study.
The monophyly of Panarthropoda is widely accepted (Figure 7), but at present its support is mainly morphological. Comprehensively sampled molecular sequences, mainly those of SSU and LSU, provide decidedly less support for panarthro-pod monophyly, or relationships within Panarthropoda. Morphological phylogenies either support the onychophorans or the tardigrades as the sister group to extant arthropods, and a combination of SSU and LSU sequences has recently suggested a clade of tardigrades and onychophor-ans. Before a robust phylogeny of the Panarthropoda and the Arthropoda can be achieved, more sequence data is needed, the choice of morphological characters used in phylogenetic analyses should be better justified, and the crucial role of fossils should be seriously considered. The panarthropods have a rich fossil record, and it has been shown that the incorporation of fossils into a phylogenetic analysis combining both molecular and morphological information may yield dramatically different phylogenies. For example, inclusion of just seven Paleozoic fossils into a monster phylo-genetic analysis of arthropods, including almost 250 taxa, more than 800 morphological characters, and more than 2 kilobases of molecular sequence data, changes support for a clade of hexapods and crustaceans to the traditional Atelocerata hypothesis uniting hexapods and myriapods (Wheeler et al., 2004). Such phylogenetic instability due to the inclusion of different sources of data needs to be more fully explored in future work.
1.02.3.4.2 Scalidophora: Priapulida, Kinorhyncha, and Loricifera The priapulids (penis worms), kinorhynchs (mud dragons), and loriciferans are thought to be a clade called Scalidophora, referring to their most conspicuous synapomorphy: the possession of an anterior end or introvert with spines or scalids, which are epidermal specializations with ciliary receptors. Molecular phylogenetic analyses based on ribosomal sequences support priapulids and kinorhynchs as sister groups, but so far no sequence data has been obtained from loriciferans.
Priapulids are carnivorous worms that possess a large introvert adorned with many spines, and they include both tiny meiobenthic (about 0.5mm) and larger (up to 40cm) macrobenthic species. Although only 18 extant species of priapulids have been described, their fossil record extends back to the Cambrian, showing that priapulids were once a spe-ciose and important component of benthic communities. However, considering that within the last 30 years no less than 10 new species of priapulids have been described (with several collected new species still awaiting description), including the most recent description of a giant Alaskan species (up to 40cm), the group may be more diverse than previously suspected.
All kinorhynchs are tiny (generally less than 1 mm) denizens of the marine meiobenthos. They have relatively narrow and slender bodies that are subdivided into 13 segment-like units called zonites, and their spiny heads covered with scalids make them easy to recognize. Despite superficial similarities to the somites or segments of the arthropod body plan, segmentation of the kinorhynch integumentary, muscular, and nervous systems is generally interpreted to support only the monophyly of the phylum.
The first species of loriciferan was described in 1983. Loriciferans are tiny (between 0.1 and 0.5mm), and exclusively marine and meiobenthic. Loriciferans are encased in a vase-like exoskeleton called a lorica that is composed of six cuticular plates. From the lorica sprouts forth an introvert with a narrow mouth cone surrounded by spiny scalids similar to those that decorate the kinorhynch head.
Although the monophyly of the Scalidophora seems established (Figure 7), the internal phyloge-netic relationships are not yet robustly supported. Most morphological cladistic analyses support kinorhynchs and loriciferans as sister taxa, but some zoologists favor a sister group relationship between priapulids and loriciferans. Sometimes the meiofaunal loriciferans are hypothesized to be the pedomorphic descendants of the larger-bodied priapulids.
1.02.3.4.3 Nematoida: Nematoda and Nemato-morpha The Nematoda is a highly successful phylum. Nematodes or roundworms are small and very slender noncoelomate worms, and they inhabit every part of the world that is even marginally inhabitable. This realization has led to a staggering image: if the entire earth except nema-todes were to become invisible, we would still be able to make out most of the outline of the planet surface, including mountains, and most organisms, both plants and animals, which serve as hosts for a fantastic diversity of roundworms. Interestingly, parasitic nematodes specialized for a range of plant and animal hosts have evolved multiple times convergently.
The nematomorphs or horsehair worms are thin, very long worms (up to 1m, while being only 1-3mm wide). The adults are free-living, but earlier life-cycle stages are all parasitic, mostly on arthropods. The Nematomorpha strongly resemble mermithid nematodes in both morphology and life-cycle characteristics, and some zoologists have regarded these features as homologous. However, recent information strongly suggests that these correspondences have evolved convergently, in the monophyletic Nematoda and Nematomorpha.
Nevertheless, a sister group relationship between the nematodes and nematomorphs in a clade Nematoida is generally accepted on the basis of morphological evidence (Figure 7). Synapomorphies include the presence of longitudinal epidermal cords with nerve cords, a basal cuticle layer with thick, crossing collagenous fibers, and the absence of circular body wall muscles. Molecular phylogenies do not always support Nematoida, but a recent analysis of combined SSU and LSU sequences strongly supported Nematoida (Mallatt et al., 2004).
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