Why Ecologists Need Systematics: Importance of Systematics to Ecological Research

Ecologists are concerned with population dynamics of organisms and with the spatial patterns of single or multiple populations. The goal of the ecologist is usually to explain the observed patterns in terms of processes. Field samples of nematodes from different habitats may contain similar but not identical specimens of a nominal taxon, and the systematist can help the ecologist decide whether the specimens are ecophenotypes of a single taxon or represent distinct species. A correct decision may be important or trivial, depending on the parameters and goals of the ecological study. When a precise identification is crucial to the success of the study, new biochemical methodologies of systematists may provide rapid and accurate diagnoses. Systematists can provide additional help in the assignment of taxa to trophic groups. For clarifying host-parasite associations, often a goal in ecological investigations, modern analytical methods of systematists can facilitate the ordering of systematic relationships.     

Resistant Germplasm in Gossypium Species and Related Plants to Rotylenchulus reniformis

Gossypium hirsutum, G. herbaceum, G. arboreum, G. barbadense, wild Gossypium spp., Hibiscus spp, and other Malvaceae were tested in the greenhouse to identify germplasm resistant to Rotylenchulus reniformis (Rr). Host resistance was based on Rr egg production per gram of root compared with known G. hirsutum susceptible ''Deltapine 16'' as check. G. longicalyx and Sida rhombifolia were nonhosts. High levels of resistance were found in G. stocksii, G. somalense, and G. barbadense ''Texas 110.'' Other cotton lines with potential value in breeding for Rr resistance were G. herbaceum P.I. 408775; G. arboreum P.I. 41895, P.I, 417891, CB 3839; and G. hirsutum 893. All these supported less than 20% of the egg production on the check. Seventy-three percent of the Hibiscus spp. tested were resistant. Female development and egg production reflected host resistance; healthy females and large egg masses were observed on susceptible plants, and degenerated females and small egg masses on resistant plants. Females penetrating nonhost G. longicalyx never matured to kidney shape.     

Susceptibility of Soybean Cultivars and Lines to Pratylenchus hexincisus

Population increase of Pratylenchus hexincisus on 41 soybean cultivars (maturity groups I-VI) and lines was tested under greenhouse conditions. After 3 months, P. hexincisus was recovered from the roots of all plants tesled. Final populations of P. hexincisus per pot were larger than the initial population in 13 cultivars. Pathogenicity of P. hexincisus on five soybean cultivars representing maturity groups (I-V) was demonstrated under greenhouse conditions. An inoculmn of 5,000 P. hexineisus/plant significantly decreased the root and shoot biomass of all five soybean cultivars after 3 months.     

Population Density and Spatial Pattern of Heterodera glycines in Relation to Soybean Phenology

Population dynamics of Heterodera glycines (SCN) were influenced by initial nematode population density in soil, soybean root growth pattern, soil type, and environmental conditions in two field experiments. Low initial populations (Pi) of SCN increased more rapidly during the growing season than high Pi and resulted in greater numbers of nematodes at harvest. Egg and juvenile (J2) populations increased within 2-6 weeks after planting when early-season soil temperatures were 20 C and above and were delayed by soil temperatures of 17 C or below in May and early June. Frequencies of occurrence and number of nematodes decreased with increasing depth and distance from center of the soybean row. Spatial pattern of SCN paralleled that of soybean roots. Higher clay content in the subsoil 30-45 cm deep in one field restricted soil penetration by roots, indirectly influencing vertical distribution of SCN. Shoot dry weight was a good indicator of the effect of SCN on seed yield. Root dry weight was poorly correlated with soybean growth and yield. The relationship of yield (seed weight) to Pi was best described by a quadratic equation at one site, but did not fit any regression model tested at the second site.     

Comparison of Compatible and Incompatible Response of Potato to Meloidogyne incognita

One susceptible (D6) and two resistant (E2 and N4) clones of Solanum sparsipilum × (S. phureja × haploid of S. tuberosum) were used to study the responses of potato roots and tubers to race 1 of Meloidogyne incognita (Kofoid &White) Chitwood. The compatible response was characterized by rapid penetration of large numbers of second-stage juveniles (J2) into roots, cessation of root growth, and occasional curving of root tips. The life cycle of M. incognita in the susceptible clone was completed in 25 days at 23-28 C. The incompatible response was characterized by penetration of fewer J2 into roots, necrosis of feeding sites within 2-7 days, and lack of nematode development. There were no differences in response of tubers from resistant and susceptible clones to nematode infection. Small numbers of J2 were detected in tubers, but they did not develop.     

Protein Ionizable Groups: pK Values and Their Contribution to Protein Stability and Solubility

The structure, stability, solubility, and function of proteins depend on their net charge and on the ionization state of the individual residues. Consequently, biochemists are interested in the pK values of the ionizable groups in proteins and how these pK values depend on their environment. We review what has been learned about pK values of ionizable groups in proteins from experimental studies and discuss the important contributions they make to protein stability and solubility.     

Effect of Soil Temperature and pH on Resistance of Soybean to Heterodera glycines

Soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is a major pest of soybean, Glycine max L. Merr. Soybean cultivars resistant to SCN are commonly grown in nematode-infested fields. The objective of this study was to examine the stability of SCN resistance in soybean genotypes at different soil temperatures and pH levels. Reactions of five SCN-resistant genotypes, Peking, Plant Introduction (PI) 88788, Custer, Bedford, and Forrest, to SCN races 3, 5, and 14 were studied at 20, 26, and 32 C, and at soil pH''s 5.5, 6.5, and 7.5. Soybean cultivar Essex was included as a susceptible check. Temperature, SCN race, soybean genotype, and their interactions significantly affected SCN reproduction. The effect of temperature on reproduction was quadratic with the three races producing significantly greater numbers of cysts at 26 C; however, reproduction on resistant genotypes remained at a low level. Higher numbers of females matured at the soil pH levels of 6.5 and 7.5 than at pH 5.5. Across the ranges of temperature and soil pH studied, resistance to SCN in the soybean genotypes remained stable.     

Sensitivity of Acetylcholinesterases from Aphelenchus avenae to Organophosphorous and Carbamate Pesticides

The sensitivities of acetylcholinesterases (ACHE) from the nematode Aphelenchus avenae and the house fly Musca domestica to various pesticides were compared using a colorimetric assay. ACHE from A. avenae were generally less sensitive than ACHE from M. domestica to inhibition by organophosphorous and carbamate pesticides. Carbamates were somewhat more inhibiting than organophosphorous pesticides to nematode ACHE. In vivo tests with concentrations of various pesticides up to 500 ppm in sand caused less than 100% mortality of nematodes.     

Soybean Lipoxygenase-1 Oxidizes 3Z-Nonenal : A Route to 4S-Hydroperoxy-2E-Nonenal and Related Products

In previous work with soybean (Glycine max), it was reported that the initial product of 3Z-nonenal (NON) oxidation is 4-hydroperoxy-2E-nonenal (4-HPNE). 4-HPNE can be converted to 4-hydroxy-2E-nonenal by a hydroperoxide-dependent peroxygenase. In the present work we have attempted to purify the 4-HPNE-producing oxygenase from soybean seed. Chromatography on various supports had shown that O₂ uptake with NON substrate consistently coincided with lipoxygenase (LOX)-1 activity. Compared with oxidation of LOX's preferred substrate, linoleic acid, the activity with NON was about 400- to 1000-fold less. Rather than obtaining the expected 4-HPNE, 4-oxo-2E-nonenal was the principal product of NON oxidation, presumably arising from the enzyme-generated alkoxyl radical of 4-HPNE. In further work a precipitous drop in activity was noted upon dilution of LOX-1 concentration; however, activity could be enhanced by spiking the reaction with 13S-hydroperoxy-9Z,11E-octadecadienoic acid. Under these conditions the principal product of NON oxidation shifted to the expected 4-HPNE. 4-HPNE was demonstrated to be 83% of the 4S-hydroperoxy-stereoisomer. Therefore, LOX-1 is also a 3Z-alkenal oxygenase, and it exerts the same stereospecificity of oxidation as it does with polyunsaturated fatty acids. Two other LOX isozymes of soybean seed were also found to oxidize NON to 4-HPNE with an excess of 4S-hydroperoxy-stereoisomer.     

Toxicity of Glucosinolates and Their Enzymatic Decomposition Products to Caenorhabditis elegans

An aquatic 24-hour lethality test using Caenorhabditis elegans was used to assess toxicity of glucosinolates and their enzymatic breakdown products. In the absence of the enzyme thioglucosidase (myrosinase), allyl glucosinolate (sinigrin) was found to be nontoxic at all concentrations tested, while a freeze-dried, dialyzed water extract of Crambe abyssinica containing 26% 2-hydroxyl 3-butenyl glucosinolate (epi-progoitrin) had a 50% lethal concentration (LC₅₀) of 18.5 g/liter. Addition of the enzyme increased the toxicity (LC₅₀ value) of sinigrin to 0.5 g/liter, but the enzyme had no effect on the toxicity of the C. abyssinica extract. Allyl isothiocyanate and allyl cyanide, two possible breakdown products of sinigrin, had an LC₅₀ value of 0.04 g/liter and approximately 3 g/liter, respectively. Liquid chromatographic studies showed that a portion of the sinigrin decomposed into allyl isothiocyanate. The results indicated that allyl isothiocyanate is nearly three orders of magnitude more toxic to C. elegans than the corresponding glncosinolate, suggesting isothiocyanate formation would improve nematode control from application of glucosinolates.     

Sympathetic glial cells and macrophages develop different responses to Trypanosoma cruzi infection or lipopolysaccharide stimulation

Nitric oxide (NO) participates in neuronal lesions in the digestive form of Chagasdisease and the proximity of parasitised glial cells and neurons in damaged myentericganglia is a frequent finding. Glial cells have crucial roles in manyneuropathological situations and are potential sources of NO. Here, we investigateperipheral glial cell response to Trypanosoma cruzi infection toclarify the role of these cells in the neuronal lesion pathogenesis of Chagasdisease. We used primary glial cell cultures from superior cervical ganglion toinvestigate cell activation and NO production after T. cruziinfection or lipopolysaccharide (LPS) exposure in comparison to peritonealmacrophages. T. cruzi infection was greater in glial cells, despitesimilar levels of NO production in both cell types. Glial cells responded similarlyto T. cruzi and LPS, but were less responsive to LPS thanmacrophages were. Our observations contribute to the understanding of Chagas diseasepathogenesis, as based on the high susceptibility of autonomic glial cells toT. cruzi infection with subsequent NO production. Moreover, our findingswill facilitate future research into the immune responses and activation mechanismsof peripheral glial cells, which are important for understanding the paradoxicalresponses of this cell type in neuronal lesions and neuroprotection.     

Effects of Temperature on the Pathogenicity of Tylenchorhynchus clarus to Alfalfa and Observations on Feeding

The involvement of Tylenchorhynchus clarus in plant disease is reported. Addition of a suspension of surface-axenized nematodes reduced top and root growth of alfalfa. Reproduction of T. clarus was greater at 24 and 27 than at 21 C. The interaction of nematodes with temperature did not produce significant effects on alfalfa growth in the 4.5-mo experimental period. T. clarus fed endo- and ectoparasitically.