Marcelo E. Doucet

Steinernema ritteri n. sp. (Nematoda: Steinernematidae) with a key to the species of the genus.

Life stages of a new species of Steinernematidae from Cordoba province, Argentina, are described and illustrated. Steinernema ritteri n. sp. is characterized by the presence of a conspicuous double vulval epiptygma in the females. This feature separates it from all the other species of the genus, except from S. bibionis Bovien, 1937 and S. scapterisci Nguyen & Smart, 1990. It differs from S. bibionis by the greater dimensions of the infective larva and from S. scapterisci by having different measurements in all the stages under consideration. A taxonomic key is given for all ten valid species based on the infective larvae.

Usefulness of morphological data for the study of nematode biodiversity

Public interest in biodiversity has never been so high, but the necessary inventory of existing species is hindered by the scarcity of taxonomists able to characterise and describe new species (α-taxonomy). This situation is particularly worrisome in nematology where only a small fraction of the estimated 105 to 108 existing species has been described and where experienced taxonomists are fast disappearing while the few that are still active find it increasingly difficult to publish morphological descriptions and to get financial support for biodiversity studies. The present disregard for α-taxonomy is said to originate from the excessive reliance by funding agencies and academic authorities on the Impact Factors attributed to scientific journals by ISI (Institute for Scientific Information). Molecular studies gave back some support and prestige to taxonomy, but that approach suffers from some limitations and it cannot be used alone for reaching taxonomic conclusions. In addition, any described DNA sequence should always be linked to a named species whose morphology has been correctly described by a trained taxonomist. The authors call for a renewed interest in α-taxonomy. Electronic publication according to the Code of Zoological Nomenclature would be one solution, but journals and authors seem reluctant to accept it. A variant would be to publish on paper only the diagnosis of the new species. The complete descriptions could be deposited in an Internet database but the setting-up of such a database would be a costly enterprise that would require several years of work by a team composed of morphological and molecular taxonomists and computer scientists.

Morphological and molecular characterisation of the entomoparasitic nematode Hammerschmidtiella diesingi (Nematoda, Oxyurida, Thelastomatidae)

A population of Hammerschmidtiella diesingi was detected in specimens of Periplaneta americana collected in Córdoba city (Argentina) was studied. Nematodes were characterized based on morphological, morphometric and genetic (D2–D3 expansion segment) analyses. New data are provided, especially on particular morphological characters that were a matter of controversy in previous studies. H. diesingi is reported in Argentina for the first time.

Assessment of genetic variability in populations of Nacobbus aberrans (Thorne, 1935) Thorne & Allen, 1944 (Nematoda: Pratylenchidae) from Argentina

Nacobbus aberrans produces severe damage to agriculture and is considered to be of quarantine importance. The levels of polymorphism and genetic differentiation among Argentine populations of this species from different hosts and origin were studied using Inter-Simple Sequence Repeat (ISSR) as molecular markers. Second-stage juveniles from nine populations associated with three different hosts (tomato, quinoa and potato) were analysed individually. Three primers were selected because they produced clear and 100% reproducible patterns; 37 fragments were considered for the analysis. The technique employed revealed high levels of polymorphisms, the highest proportion of genetic diversity being found within populations (95%). Populations showed a tendency to group together according to their original host, based on their pairwise FST. Low levels of genetic differentiation among populations were observed, suggesting an extensive gene flow among them. Passive dispersal of nematodes by natural means and anthropogenic activities would probably be, at least partially, responsible for the results observed. This work is the first study of genetic structure of N. aberrans populations at a macrogeographical level using ISSR markers.

Use of the Arbuscular Mycorrhizal Fungus Glomus intraradices as Biological Control Agent of the Nematode Nacobbus aberrans Parasitizing Tomato

ABSTRACT The plant-parasitic nematode Nacobbus aberrans is an endoparasite that induces gall formation in the roots and causes severe losses to diverse crops. Some populations of this nematode show preference for certain hosts, revealing the existence of “races/groups” with diff erent behaviour and making nematode management difficult. A possible biological control alternative to reduce the damage caused by this species may be the use of arbuscular mycorrhizal fungi (AMF). In the present work, the effect of Glomus intraradices on tomato plants inoculated with the nematode at transplanting and three weeks later was tested. At 60 days, the following parameters were estimated: percentage of AMF colonization, root and aerial dry weight, number of galls and egg masses, and reproduction factor (RF=final population/initial population) of N. aberrans . AMF colonization was higher in the presence of the nematode. The use of AMF favoured tomato biomass and reduced the number of galls and RF on the plants inoculated with the nematode at transplanting. Key words: arbuscular mycorrhiza, false root-knot nematode, microbiological control, plant protection

Temperature-induced morphometrical variability in an isolate of Pratylenchus vulnus Allen & Jensen, 1951 (Nematoda : Tylenchida)

The influence of temperature on morphometrical variability of females and males of an isolate of Pratylenchus vulnus was studied. Nematodes were reared monoxenically on carrot disk cultures and incubated for 4 months at 16, 21, 25 and 28°C. The morphometrics of several characters were significantly affected depending on sex and temperature. It is concluded that temperature can significantly influence several morphometrical characters of P.vulnus.

Phylogenetic relationships among populations of the Nacobbus aberrans (Nematoda, Pratylenchidae) complex reveal the existence of cryptic species

The plant‐parasitic nematode Nacobbus aberrans sensu lato is an agricultural pest of quarantine importance. Due to the morphometric, physiological and genetic variability observed within the species, there is no agreement on the taxonomy of this nematode. The objective of this study was to analyse the ITS rDNA region and the D2–D3 expansion segments of 28S rDNA in 10 Argentine populations and one from Ecuador and to establish their phylogenetic relationship with other known sequences from South and North America. Phylogenetic trees of the ITS gene showed seven statistically well‐supported clades; the high and significant Fst values obtained among these groups confirmed this partitioning. The Argentine populations here considered were separated into three clades: one comprising a population from the Andean region and two grouping nematodes from lower altitudes. Three other clades were distinguished for South American populations, which included known sequences of individuals from Peru, Bolivia and north of Argentina. The other clade included sequences from Mexico, Ecuador and two Argentine populations of unknown origin. The important degree of genetic divergence observed among Andean populations suggests that the Andes may have played a crucial role in speciation of Nacobbus, which would have originated in this region. Although D2–D3 segments exhibited lower variation, they were useful for establishing phylogenetic relationships among the Argentine populations considered in this work. As there are no other GenBank sequences available for these segments, it was not possible to make comparisons with other populations from South and North America. The considerable genetic differentiation observed in ITS rDNA region among Nacobbus populations showed evidence of cryptic species within the N. aberrans s.l. complex. Integration of morphological and morphometric studies and molecular analyses considering other genes may aid in the identification of species and their phylogenetic relationships within this genus.

Susceptibility of Spodoptera frugiperda and Helicoverpa gelotopoeon (Lepidoptera: Noctuidae) to the entomopathogenic nematode Steinernema diaprepesi (Rhabditida: Steinernematidae) under laboratory conditions

Spodoptera frugiperda Smith and Helicoverpa gelotopoeon (Dyar) are important agricultural pests of several crops. The aim of the present work was to evaluate the susceptibility of larvae of both insects to an isolate of Steinernema diaprepesi Nguyen & Duncan under laboratory conditions, as well as the capacity of the nematode to multiply on these lepidoterans. Larvae (n = 15) were exposed to 0 (control), 50, and 100 infective juveniles (IJs) per Petri dish. Mortality was evaluated every 24 h during 6 d, and emerging IJs were counted. Mortality of S. frugiperda was 93% and 100% with 50 and 100 IJs dosage, and 87% and 93% in H. gelotopoeon, respectively. The production of IJs was significantly different between doses (P ≤ 005) for S. frugiperda (11 329 with 50 IJs vs. 27 155 with 100 IJs) but not for H. gelotopoeon (19 830 vs. 26 361, respectively). This is the first study evaluating the susceptibility of these lepidopterans to S. diaprepesi. These results encourage the possibility of using this nematode for biological control of both pests.

The Soybean Cyst Nematode Heterodera Glycines Ichinohe, 1952 in Argentina

The damage caused by the soybean cyst nematode Heterodera glycines in Argentina is revised, together with possible management strategies. This nematode emerged in the last decade as one of the most important parasites of soybean in the region. The histopathology, population dynamics and dispersal of H. glycinesand the effects of selected resistant lines and varieties are discussed. Among management tools, observations on the effects of soil fungi on H. glycines densities suggest a possible role of natural suppressiveness and biological control. Recommended actions include development of detailed knowledge about the occurrence of the nematode, trainings of experts, development of sound outreach programs and extension activities, evaluation of the nematode incidence on yields and research related to soybean resistance and possible exploitation of natural antagonists.

Genetic variability estimated with RAPD-PCR markers in two populations of Heterodera glycines Ichinohe, 1952 (Nematoda: Heteroderidae) from Argentina

Knowledge of genetic variability within and between phytophagous nematode populations is important for the selection of suitable control strategies. The nematode Heterodera glycines is the most important pathogen of soybean in many producer countries, causing significant economic losses. The levels of variability were analysed in two populations of the nematode from Argentina, using Random Amplified Polymorphic DNA markers in order to evaluate the genetic population structure of this species. DNA was extracted from single females of race 1 and race 3 populations. Five primers were selected and 50 consistent fragments were considered for further analysis. Populations studied showed high levels of genetic diversity. Most of the total variation detected in allele frequencies was attributed to variability among individuals of the same population (83%) rather than inter-population differences (17%). None of the populations demonstrated exclusive bands. However, 68% of fragments showed significant differences in their allele frequencies. This study revealed an important degree of genetic differentiation between both populations, probably as a consequence of limited gene flow between them or because each population was under different management practices at its site of origin. Results herein represent the first analysis of genetic structure in H. glycines populations using RAPD-PCR technique.