P. De Ley

Molecular Analysis of Nematode Diversity and the Evolution of Parasitism

A thorough and coherent classification of the phylum Nematoda is essential if the evolution of countless phenotypes is to be understood. Here, Mark Dorris, Paul De Ley and Mark Blaxter discuss how the application of molecular phylogenetics is helping to resolve some of the inconsistencies in morphological classification and phylogeny by establishing relationships between free-living and parasitic groups, showing possible patterns underlying the origins of parasitism and placing key nematode species in an evolutionary context for comparative study.

Ultrastructure of the buccal cavity and the cuticle of three Cephalobidae (Nematoda: Rhabditida)

The ultrastructure of the buccal cavity was elucidated in three species of cephalobids, two belonging to the genus Acrobeloides and one to the genus Chiloplacus. The cuticular differentiations, as well as the identity of the surrounding structures or tissues are used to identify the consecutive buccal regions. Six distinct buccal regions are discernible rather than the five parts generally assumed to be characteristic for many nematodes. The first buccal region is surrounded by the lip cuticle in which three sclerotized plates (two in a dorsosublateral position and one in a medioventral position) are embedded. The second buccal region is surrounded by two consecutive rings of arcade epidermis. The posterior arcade epidermis contains an electron-dense ring that surrounds the buccal cuticle and partly penetrates the junction of the cuticle and the anterior set of pharyngeal muscles. The basal lamina layer separating pharyngeal musculature from the other tissue components in the labial region ends as it contacts the electron-dense ring. The fourth to the sixth buccal regions are surrounded by four consecutive sets of pharyngeal muscles. The two anterior sets (m 1 and m 2 ) are composed of one muscle per pharyngeal sector, while the two posterior sets (m 3 and m 4 ) contain two cells per sector, interradially separated by cell membranes. Marginal cells, containing electron-dense radial fibres, are situated in a perradial position, thus separating the muscle cells. The anteriormost set of marginal cells spans the length of the four anterior muscle sets. The dorsal gland opens into the buccal lumen by means of a narrow canal situated at the anterior margin of the dorsal m 4 muscle set. A neuronal cell process is situated just anterior to the canal. It is proposed that the observed buccal organisation does not fit the current concepts related to the homology of buccal parts in nematodes.

A Revision of the Genus Bathyodontus Fielding, 1950 With the Description of a Male B. Cylindricus Fielding, 1950 (Nematoda: Mononchida)

Soil material collected at Camberene, Senegal yielded two of the three known species of the rare genus Bathyodontus: B. cylindricus Fielding, 1950 and B. indicus (Khan, 1972). A male B. cylindricus is described, having a gubernaculum with lateral guiding pieces. The four females found of B. indicus lack a fully developed anterior reproductive branch. Comparison with collection specimens of B. mirus (Andrassy, 1956) allows a new generic character to be identified: presence of a mononchid-like excretory system. Stoma, pharynx and vagina structure are examined in detail; S.E.M. photographs are included and relationships within the genus and with other genera are discussed.

Metacrobeles Tessella Tus Sp. N., Second Species of a Rare Genus (Nematoda: Rhabditida)

Metacrobeles tessellatus sp. n. is described from Camberene, Senegal, Africa. It is easily differentiated from the type species M. togoensis Loof, 1962 by the tessellated cuticle, the longer cephalic probolae and the nerve ring and excretory pore situated at the base of the procorpus. A thorough morphological study was made of whole and cross-sectioned specimens, both with light microscope and S.E.M. The definition of the genus is emended and its taxonomic position in relation to other cephalobids discussed.