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  • Author or Editor: Philippe Castagnone-Sereno x
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In: Nematology

Abstract

Root-knot nematodes (RKN) of the genus Meloidogyne constitute the most widely distributed and damaging group of plant-parasitic nematodes. Plant resistance (R) is currently the most effective and environmentally safe method to control these pests. The mode of reproduction of the major species, i.e., apomictic (= mitotic) parthenogenesis, should theoretically lead to clonal progenies. However, the recent emergence of new virulent biotypes, able to overcome plant R genes, indicates that variability does exist in these organisms. Experiments showed that selection for virulence was possible in RKN, which has important consequences for the management and durability of natural R genes in the field. To understand the molecular mechanisms involved in such selection, we have developed a differential strategy based on the comparative analysis of pairs of RKN near-isogenic lineages and recent results obtained in the laboratory are presented. Understanding how the genome of these nematodes is modified in response to the selective pressure of a plant R gene should provide further data on the putative role of non-meiotic events leading to stable genetic variation in these apomictic organisms.

In: Nematology

Meloidogyne enterolobii (= M. mayaguensis), the root-knot nematode of the pacara earpod tree, belongs to the group of tropical root-knot nematodes and is considered as one of the most damaging species, due to its wide host range, pathogenicity and ability to develop and reproduce on several crops carrying resistance genes. Moreover, recent reports indicate that the geographic distribution of the parasite tends to extend beyond tropical areas, and the risk of its establishment and spread in Mediterranean regions and southern Europe is now highly probable. Recently, molecular markers have been developed that allow the specific identification of this pest, a prerequisite for the implementation of efficient control strategies. In that respect, plant resistance and biological control are currently being actively investigated but a huge amount of research and development is still required to ensure the successful use of such methods in the field.

In: Nematology

Summary

Plant resistance is currently the most effective and environmentally safe method to control root-knot nematodes. Resistance genes generally act by inducing an hypersensitive reaction at or near the infection site that prevents the parasite installation and/or reproduction. However, the emergence of virulent biotypes able to overcome the plant resistance genes may constitute a severe limitation to this control strategy. To investigate the molecular determinants of (a)virulence in root-knot nematodes, we have been developing differential strategies based on the selection and subsequent comparative analysis of pairs of Meloidogyne incognita near-isogenic lines, just differing in their ability to reproduce or not on tomatoes bearing the Mi resistance gene. The objective of these approaches is to identify genes different or differentially expressed between the avirulent and virulent lines. Results presented are discussed in relation to the mode of reproduction of the nematode, i.e., mitotic parthenogenesis. Elucidating the genetical and molecular mechanisms involved in the selection process of virulent nematodes from avirulent ones should have important consequences for the management and durability of natural resistance genes in the field.

In: Proceedings of the Fourth International Congress of Nematology, 8-13 June 2002, Tenerife, Spain

Abstract

Esterase phenotypes were used to characterise 25 populations of Meloidogyne spp. collected on bananas from different banana producing areas in Brazil, using a simplified technique for routine analysis. Meloidogyne javanica, M. incognita, M. arenaria, and Meloidogyne spp. were detected at percentages of 61.7, 32.2, 4.3 and 1.8, respectively. The Meloidogyne populations represented mixed species in 80% of the samples, M. javanica and M. incognita prevailing under field conditions. Genetic analyses were conducted using RAPD markers which provided, alone or in combination, reliable polymorphisms both between and within species. Based on the presence or absence of bands, RAPD analysis of the data resulted in clustering of species and isolates congruent with esterase phenotype characterisation. The intraspecific variability in M. javanica, M. incognita and M. arenaria represented 29.1, 19.5 and 40.2% of the polymorphic amplified fragments, respectively. A general lack of correlation was observed between banana cultivar group, geographical origin and differentiation of the nematodes.

In: Nematology

Abstract

A survey of Meloidogyne in La Vera (Spain) was carried out during October 1995 and 1997. Isozyme electrophoresis (A-EST, MDH, CAT, GPI, DIA, GOT and SOD) was performed for each nematode isolate, each of which was cultured from an egg mass. In 1995 seven isolates of M. incognita, two of M. javanica and 75 of M. arenaria were found, while in 1997 one isolate of M. javanica and 72 of M. arenaria were found. Taxonomic relationships were established among 151 isolates using enzyme phenotypes by means of Multiple Correspondence and UPGM analysis. Twenty-six Multilocus Enzyme Electrophoresis (MLEE) phenotype variants were detected in ten significant taxonomic units: M. arenaria (seven units), M. incognita (two) and M. javanica (one). A total of 13 loci were identified and allelic frequencies for MLEE phenotype variants were estimated. Genetic distances among them suggested that M. arenaria is a poly- or paraphyletic group formed by at least two or three monophyletic lines of different origins. The evolutionary and epidemiological implications are discussed.

In: Nematology

Root-knot nematodes (Meloidogyne spp.) significantly impact potato production worldwide and in Brazil they are considered one of the most important group of nematodes affecting potatoes. The objectives of this study were to survey Meloidogyne spp. associated with potatoes in Brazil, determine their genetic diversity and assess the aggressiveness of M. javanica on two susceptible potato cultivars. Fifty-seven root-knot nematode populations were identified using esterase phenotyping, including Meloidogyne javanica, M. incognita, M. arenaria and M. ethiopica. Overall, root-knot nematodes were present in ca 43% of sampled sites, in which M. javanica was the most prevalent species, and the phenotypes Est J3, J2a and J2 occurred in 91.2, 6.7 and 2.1% of the positive samples, respectively. Other species, such as M. incognita, M. arenaria and M. ethiopica, were found less frequently and occurred at rates of 6.4, 4.3 and 2.1% of the samples, respectively. Sometimes, M. javanica was found in mixtures with other root-knot nematodes in ca 10.6% of sites containing Meloidogyne. After confirming the identification of 17 isolates of M. javanica and one isolate each of M. incognita, M. arenaria and M. ethiopica by SCAR markers, the populations were used to infer their genetic diversity using RAPD markers. Results revealed low intraspecifc genetic diversity among isolates (13.9%) for M. javanica. Similarly, M. javanica sub-populations (J2a) clustered together (81% of bootstrap), indicating subtle variation from typical J3 populations. The aggressiveness of four populations of M. javanica from different Brazilian states on two susceptible potato cultivars was tested under glasshouse conditions. Results indicated differences in aggressiveness among these populations and showed that potato disease was proportional to nematode reproduction factor.

In: Nematology

Root-knot nematodes (RKN) are important plant pathogens affecting rice in South-East Asia and southern Brazil in irrigated rice fields. In order to investigate the specific diversity of RKN associated with irrigated rice in southern Brazil, Meloidogyne spp. from Rio Grande do Sul (RS) and Santa Catarina (SC) States were characterised biochemically by esterase (Est) and malate dehydrogenase (Mdh) phenotypes. Fifty-six Meloidogyne spp. populations were detected in 48% of rice samples, and a total of five esterase phenotypes were identified, four of which presented as drawn-out bands in different positions. In RS State, M. graminicola (Est VS1), Meloidogyne sp. 2 (Est R2) and Meloidogyne sp. 3 (Est R3) were identified, which corresponded to ca 80, 40 and 10% of samples, respectively. In SC State, M. graminicola, M. javanica (Est J3), Meloidogyne sp. 1 (Est R1), Meloidogyne sp. 2 and Meloidogyne sp. 3 accounted for ca 93.75, 12.50, 62.50, 12.25 and 6.25% of samples, respectively. The esterase phenotypes R1, R2 and R3 are new, never having been detected on rice before. Meloidogyne javanica showed a N1 Mdh phenotype (Rm: 1.0), while four other populations exhibited a N1a (Rm: 1.4) phenotype. All populations were tested with two SCAR markers specific to M. graminicola, which confirmed that, but no specificity was obtained with both markers in relation to the atypical populations analysed. Sequencing and phylogenetic analyses of internal transcribed spacer-rRNA (ITS) were performed to infer the phylogenetic relationship of these atypical Meloidogyne spp. populations. Meloidogyne sp. 1 grouped with the mitotic parthenogenetic species, while the two others (Meloidogyne sp. 2 and sp. 3) clustered with M. graminicola and other meiotic parthenogenetic species. Taken together, these data highlight the unprecedented specific diversity of RKN associated with irrigated rice in southern Brazil. Further morphological and phylogenetic studies involving these atypical isolates will be carried out to identify this complex of species.

In: Nematology

A new root-knot nematode parasitising vegetables, flowers and fruits in Brazil, Iran and Chile, is described as Meloidogyne luci n. sp. The female has an oval to squarish perineal pattern with a low to moderately high dorsal arc and without shoulders, similar to M. ethiopica. The female stylet is robust and 15-16 μm long; the distance from the dorsal pharyngeal gland orifice to the stylet base (DGO) is 3-4 μm. Males have a high, rounded head cap continuous with the body contour. The labial disc is fused with the medial lips to form an elongated lip structure. The head region is not marked by incomplete annulations. Male stylet robust, 20.8-23.0 μm long with rounded knobs; the DGO is 2.5-4.5 μm. The stylet of second-stage juveniles (J2) is 12.0-13.5 μm long and the DGO to the stylet base is 2.3-3.3 μm. The J2 tail is conoid with finely rounded terminus and is 40.0-48.5 μm long. Biochemically, the esterase phenotype L3 (Rm: 1.05, 1.10, 1.25) is unique and is the most useful character to differentiate M. luci n. sp. from all other Meloidogyne species. Reproduction is by mitotic parthenogenesis (2n = 42-46 chromosomes). In a differential host test, the population from Lavandula spica, Caxias do Sul, RS State, Brazil, reproduced on tomato cv. Rutgers, tobacco cv. NC95 and pepper cv. California Wonder. No reproduction occurred on watermelon cv. Charleston Gray, cotton cv. Deltapine 61 or peanut cv. Florunner. In Neighbour-Joining analyses of ITS and D2-D3 rRNA sequences, populations of M. luci n. sp. from Brazil, Chile and Iran clustered together and were clearly separated from other Meloidogyne spp., thus confirming that all three populations are very similar and conspecific.

In: Nematology

Abstract

Isozymes (esterase and malate dehydrogenase), SCAR and RAPD-PCR were studied in 15 populations of three races of Meloidogyne exigua collected in coffee-producing areas in Brazil, Bolivia and Costa Rica and one population from rubber tree plantations in Brazil. This study revealed four esterase phenotypes (E1, E2, E2a, E3) and three malate dehydrogenase phenotypes (N1, N1a, N2) for M. exigua populations. The most common multi-enzyme phenotype was E2N1. The enzymatic phenotypes do not separate M. exigua races. Sixteen populations of M. exigua were tested in Multiplex PCR using SCAR primers ex-D15F/R that allowed the identification of all M. exigua populations. Phylogenetic analyses showed high intraspecific polymorphism (25.9-59.6%) for all M. exigua studied. However, all populations clustered together with 100% bootstrap support, thereby demonstrating the consistency of species identification. In general, no correlation was found between enzymatic profile, race and genetic polymorphism of the studied populations.

In: Nematology