GENETIC VARIATION OF ASCARIDOID WORMS WITH DIFFERENT LIFE CYCLES.

Abstract

The hypothesis that genetic variation of populations is positively related to the degree of environmental variation they experience was tested by several authors (e.g., Bryant, 1974a, 1974b; Hedrick et al., 1976; Valentine, 1976; Steiner, 1977; Nevo, 1976, 1978; Mitter and Futuyma, 1979; Nevo et al., 1984). These studies yielded contradictory results, partly because of the experimental design and/or the organisms compared. For example, some animal groups are physiologically more buffered than others against environmental variation, and this can affect their levels of genetic variability (Selander and Kaufman, 1973). The effect of environmental variation might be detectable only when the organisms studied are closely related and ecologically similar. In the present study, the genetic structure of several Ascaridoid worms is compared by means of multilocus electrophoresis. Evidence is given that species whose life cycle is carried out on a single homeothermic host show lower genetic variability (by a factor of about 1/3) than do those needing both poikilothermic and homeothermic hosts. Natural selection is suggested to be the major factor promoting and maintaining the different patterns of genetic variability found in these parasites. The 17 species studied belong to the superfamily Ascaridoidea; nine of them (i.e., Ascaris lumbricoides, A. suum, Parascaris univalens, P. equorum, Neoascaris vitulorum, Toxocara canis, T. cati, Toxascaris leonina, Baylisascaris transfuga) are single-host species, while eight (i.e., Anisakis simplex A, A. simplex B, A. physeteris, Phocascaris cystophorae, Contracaecum osculatum A, C. osculatum B, C. rudolphii A, C. rudolphii B) are multiple-host species. The former parasitize terrestrial mammals, whereas the latter live as larvae on poikilothermic hosts (first on crustaceans, later on fishes or squids of various size) and as adults on homeothermic hosts (marine mammals or fish-eating birds). Horizontal starch gel electrophoresis was performed on single specimens, both larvae and adults, for the following enzymes: octanol dehydrogenase (ODH), a-glycerophosphate dehydrogenase (a-GPDH), sorbitol dehydrogenase (SDH), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), malic enzyme (ME), isocitrate dehydrogenase (IDH), 6-phosphogluconate dehydrogenase (6-PGDH), glucose-6-phosphate dehydrogenase (G6PDH), glyceraldheyde-3-phosphate dehydrogenase (G3PDH), xanthine dehydrogenase (XDH), superoxide dismutase (SOD), nucleoside phosphorylase (NP), glutamate oxaloacetate transaminase (GOT), hexokinase (HK), adenylate kinase (ADK), phosphoglucomutase (PGM), esterase (EST), acid phosphatase (ACPH), leucine aminopeptidase (LAP), aldolase (ALD), carbonic anhydrase (CA), triose phosphate isomerase (TPI), mannose phosphate isomerase (MPI), and glucose phosphate isomerase (GPI). The buffer systems used were: 1) tris-versene-borate (Brewer and Sing, 1970) for ODH, G6PDH, SOD, NP, CA, MPI, and GPI; 2) tris-versene-maleate (Brewer and Sing, 1970) for LDH and PGM; 3) phosphate-citrate (Harris, 1966) for a-GPDH, MDH, 6-PGDH, and ACPH; 4) discontinuous tris-citrate (Poulik, 1957) for EST and LAP; 5) continuous tris-citrate-II (Selander et al., 1971) for SDH, ME, IDH, G3PDH, XDH, GOT, HK, ADK, ALD, and TPI. The staining techniques were, with minor modifications, those described by Brewer and Sing (1970; LDH and PGM), Shaw and Prasad (1970; SDH, MDH, IDH, 6-PGDH, G6PDH, and XDH), Selander et al. (1971; SOD, GOT, and GPI), Ayala et al. (1972; ODH, a-GPDH, ME, G3PDH, HK, ADK, EST, LAP, ALD, and TPI), and Harris and Hopkinson (1976; NP, ACPH, CA, and MPI). Each species was tested for a number of loci ranging from 18 to 28, 12 of which (Ldh, Mdh-1, Idh-1, 6-Pgdh, G3pdh, Got-i, Adk-2, Pgm-1, Est-], Est-2, Mpi, and Gpi) were the same for all the species studied. The following parameters were used to estimate genetic variability: expected mean heterozygosity per locus (He); proportion of polymorphic loci, according to