EVOLUTION IN HOPLOLAIMS (NEMATODA: )

BY

E.GERAERT Laboratorium voor Morfologie en Systematiek, K.L. Ledeganckstraat 35, 9000 Gent, Belgium

Genera that show a transitional series are difficult to define: the series discussed is Rotylenchus - Scutellonema Peltamigratus- -Aorolaimus -Hoplolaimus. Two hypotheses are presented. First the evolutionary development of this series is associated with the theory of continental drift. Second the morphology of tylenchs is almost unchanged during the last 200-300 million years, explain- ing the large genetic variation found within some species. Keywords:evolution, variability, genetic diversity, species concept.

Taxonomy of Tylenchida does not seem to have reached a steady state, cf. Siddiqi (1986) and Maggenti et al. (1987). New approaches to elucidate some problems could help. One of these is biogeographical distribution. Ferris et al. (1976) and Vinciguerra (1988) discussed in an authoritative way the distribu- tion of some Dorylaimida (the Leptonchidae and the Actinolaimoidea) relative to plate tectonics. A similar approach for some results in some interesting conclusions (Geraert, 1988).

DISCUSSION

1 ) , morphology. Some genera of the Hoplolaiminae are character- ized by a dorsal overlap over the intestine of the dorsal oesophageal gland: Hoplolaimus von Daday, 1905, Rotylenchus Filipjev, 1936, Scutellonema Andrassy, 1958, Aorolaimus Sher, 1963, Peltamigratus Sher, 1964. Pararotylenchus Baldwin & Bell, 1981 contains the Rotylenchus species with small, non-overlapping glands. Baldwin & Bell (1981) considered Pararotylenchus as a transitional genus between Tylenchorhynchus (family Belonolaimidae) and Rotylenchus (family Hoplolaimidae). Its relationship will not be further dis- cussed. Rotylenchus has punctiform phasmids near the anus level, a plesiomorphic character found in the majority of the tylenchs. The other genera have enlarged phasmids, called scutella, that can be near anus level or positioned elsewhere on the body. Rotylenchus, Scutellonema, Peltamigratus and Aorolaimus were differentiated in Sher's review of the Hoplolaimidae (1965, et seq.) on the basis of several mor- 200 phological characters. The finding of numerous new species and the study of the variation in some species revealed that of all the differences only the posi- tion of the scutella remained useful (see discussion in Rashid et al., 1987 and Fortuner, 1987 who synonymised Peltamigratus with Aorolaimus). Hoplolaimus remained a distinct genus because of the large body (L = 1-2 mm) and because of the anchor or -shaped stylet knobs; H. aorolaimoides Siddiqi, 1972 sharing characters of Aorolaimus (smaller than 1 mm; rounded stylet knobs) and Hoplolaimus (lateral field areolated over entire body) made Hoplolaimus less distinctive. In these genera there is some correlation between the change in phasmids and the structure of the lateral field: the change in shape from punctiform structure to enlarged scutellum is often accompanied by local areolation of the lateral field; a further degradation of the lateral field with more extensive areolation and finally disappearance of the lines occurs in several Peltamigratus and Hoplolaimus species (genera with a more anterior position of the scutella). The tulip shaped stylet knobs and the multiple dorsal oesophageal gland nuclei (in some Hoplolaimus species, by some considered as representatives of a separate genus Basirolaimus Shamsi, 1979) are probably consequent on the much larger body in Hoplolaimus.

2) Geographical distribution. Pararotylenchus apparently has a restricted area of distribution: western U.S.A., Japan, Korea (Asia ?), usually in cool regions, primarily at high elevations. Rotylenchus is found all over the world, more commonly in temperate regions than in tropical areas. In South America Peltamigratus is the predominant hoplolaim (Sher, 1964; Doucet, 1980, 1984; Rashid et al., 1987; Bittencourt & Huang, 1986). In the southern part of North America Peltamigratus is com- monly found (Sher, 1964; Inserra, pers. comm.). In Africa Peltamigratus and Scutellonema are common in West-Africa, more to the east Peltamigratus is less common. In India the genus Scutellonema is present with several species, while in Australia Phillips (1971) described, apart from the species with large scutella, three Scutellonema species with small scutella. In the U.S.A. Scutellonema is very commonly found in samples from anywhere in the country, but most belong to the species S. brachyurus (Fortuner, in litt. ). Aorolaimus is rarely found: four of the seven nominal species are known from the U.S.A., two from the Mediterranean area (Spain, Morocco, Israel) and one from India. Hoplolaimus has about the same area of distribution as Scutellonema, being very common in Africa, India, U.S.A., rare in South America; unlike Scutellonema it is also very common in Sudan and Egypt. The differences in morphology and the differences in geographical distribu- tion are used to develop a hypothesis for the evolution of these hoplolaims.