Phylogenetic Analysis of rDNA Sequences from Adenophorean Nematodes and Implications for the Adenophorea-Secernentea Controversy

Phylogenetic relationships of 13 adenophorean and 15 secementean nematodes were analyzed using morphological data and 18S rDNA sequences in parallel. Both data sets strongly supported monophyly of Adenophorea and Secerentea. Monophyly of the subfamily Stilbonematinae which is remarkable for symbiosis with bacteria-was also confirmed. Three taxa without symbionts (Desmodora, Xyzzors, and Acanthopharynx) branched at the base of the adenophoreans. Enoplus meridionalis, representing the Enoplida, was distantly related to the remaining adenophoreans and formed their most basal branch. All analyses also supported monophyly of the genera Eubostrichus, Laxus, Haemonchus, and Caenorhabditis. Within the secernenteans, the genus Rhabditis, however, appeared to be paraphyletic with respect to the genus Rhabditella. Additional key words: Stilbonematinae, phylogeny, ribosomal DNA (rDNA), molecular systematics Nematoda is the third most species-rich phylum (some authors rank it as a class) of the animal kingdom, outnumbered only by arthropods and molluscs (Gerlach & Riemann 1973; Andrassy 1976; Heip et al. 1982). About 20,000 species have been described (Platt & Warwick 1983), their actual number being estimated between 80,000 and 100,000 (Gerlach & Riemann 1973; Andrassy 1976). Nematode systematics is extremely difficult and still unstable. Specialists argue about the diagnostic importance and reliability of characters (Andrassy 1976; Lorenzen 1981; Platt & Warwick 1983; Kastner 1993). Fossils are scarce, because of the lack of shells and skeletons. A few nematode fossils have been found in amber, but are unfortunately of poor quality. A handful of fossils are known from the Tertiary, 30-50 million years ago, some of them showing nematodes extending from the abdomen of insects (Taylor 1935). Embryological data are also difficult to use for phylogenetic inference, either because they are not yet available or because the developmental processes are still incompletely understood (Alberch 1985). Recently, molecular data have been used in evolutionary and phylogenetic studies of some families and genera of the secernenteans (Butler et al. 1981; Brandl et al. 1992; Nadler 1992, 1995; a Author for correspondence. E-mail: jaott@zoo.univie.ac.at Castagnone-Sereno et al. 1993; Baum et al. 1994; Hoberg & Lichtenfels 1994; La Volpe 1994; Okimoto et al. 1994; Vanfleteren et al. 1994; Zarlenga et al. 1994; Fitch et al. 1995; Liu & Berry 1996; Al Banna et al. 1997) but so far not for adenophorean nematodes. The first phylogenetic hypothesis for free-living marine nematodes based on multiple characters, rather than on a single one, was by Filipjev (1918, 1921). Chitwood & Chitwood (1950) suggested subdividing the Nematoda into two classes, Phasmidia and Aphasmidia, according to the presence or absence of small, pore-like or rod-like organs, one occurring on each side of the tail. Phasmids are thought to be sensory organs, although their exact function is not yet known. To avoid confusion with the term "Phasmida," which was already used for an insect order, the class names were altered back to Linstow's (1905) categories Secernentea and Adenophorea. Chitwood's system was the first that was applicable to both the free-living and parasitic forms. Andrassy (1976) questioned whether a division into two classes reflects the actual evolutionary relationships. Members of the Adenophorea, for example, share only four characters, of which two are negative features-the lack of phasmids and deirids. The remaining synapomorphies are (1) the form and This content downloaded from 157.55. 39.78 on Mon, 20 Jun 2016 07:33:21 UTC All use subject to http://about.jstor. org/terms Kampfer, Sturmbauer, & Ott placement of the amphids, which are large and lie behind the lips, and (2) the excretory organ consisting of a single cell. Because adenophoreans are ecologically and morphologically much more diverse than secernenteans (adenophoreans may live in seawater, freshwater, or soil; secernenteans are mostly parasites), Andrassy suggested that the Adenophorea may not be a monophyletic group. He proposed a new taxonomy subdividing the Nematoda into three groups: Torquentia (= Chromadoria), Secernentia (= Secernentea), and Penetrantia (= Enoplida), and defined the clade Secernentea by 17 synapomorphies. A similar classification was proposed by Inglis (1983), who recognized 3 classes, Enoplea, Chromadorea, and