ABSTRACT
A root-knot nematode, Meloidogyne floridensis n. sp., is described and illustrated from peach originally collected from Gainesville, Florida. This new species resembles M. incognita, M. christiei, M. graminicola, and M. hispanica, but with LM and SEM observations it differs from these species either by the body length, shape of head, tail and tail terminus of second-stage juveniles, body length and shape of spicules in males, and its distinctive female perineal pattern. This pattern has a high to narrowly rounded arch with coarsely broken and network-like striae in and around anal area, faint lateral lines interrupting transverse striae, a sunken vulva and anus, and large distinct phasmids. Molecular data from ribosomal IGS illustrate that M. floridensis n. sp. is different from the mitotic species M. arenaria, M. incognita, and M. javanica. Data from RAPDs confirm it and suggest that this new species lies in an intermediate phylogenetic position between the previous species and the meiotic species M. hapla, M. fallax, and M. chitwoodi. Differential host tests based on annual crops and on Prunus accessions are reported.
ABSTRACT
The northern root-knot nematode (Meloidogyne hapla) causes significant damage on vegetables in New York State. This study was conducted to determine if other species of Meloidogyne are also present in New York vegetable fields and to better characterize the root-knot populations in order to facilitate the development of effective rotations for nematode management. Populations of Meloidogyne were collected from 18 vegetable fields across New York and maintained in the greenhouse on tomato cv. Rutgers as both field populations and single-female populations. For the purposes of species identification, nematodes from each population were examined using perineal pattern analysis, scanning electron microscopy, and cytological analysis. All techniques identified the presence of only Meloidogyne hapla in the collected samples. The host range of the populations of Meloidogyne that were collected was investigated using a modified form of the North Carolina Differential Host Test. While host ranges were variable among the collected populations, the host range of all populations closely corresponded to that of M. hapla. The internal transcribed spacer region (ITS1) of the collected populations was amplified and sequenced. All sequences from the collected populations corresponded to the published sequence of M. hapla. Based on these results, it was concluded that M. hapla is the only root-knot nematode species present in New York vegetable fields. Crop rotational strategies for management of the root-knot nematode on vegetables in New York should be devised to reflect this fact.
ABSTRACT
Tests of eight Dutch Meloidogyne chitwoodi isolates to the differential set for host races 1 and 2 in M. chitwoodi provided no evidence for the existence of host race 2 in the Netherlands. The data showed deviations from expected reactions on the differential hosts, which raised doubts of the usefulness of the host race classification in M. chitwoodi. The term ''pathotype'' is proposed for groups of isolates of one Meloidogyne sp. that exhibit the same level of pathogenicity on genotypes of one host species. We recommend that the pathotype classification be applied in pathogen-host relationships when several genotypes of a Meloidogyne sp. are tested on several genotypes of one host species. Three pathotypes of M. chitwoodi were identified on Solanum bulbocastanum, suggesting at least two different genetic factors for virulence and resistance in the pathogen and the host species, respectively. The occurrence of several virulence factors in M. chitwoodi will complicate the successful application of resistance factors from S. bulbocastanum for developing resistant potato cultivars.
ABSTRACT
ABSTRACT The virulence of Meloidogyne hapla, M. chitwoodi, and M. fallax was studied on genotypes of Solanum spp. in a greenhouse. Juveniles of 11 M. hapla race A isolates, 3 M. hapla race B isolates, and 5 mono-female lines of a M. hapla race A isolate were inoculated on S. chacoense, S. hougasii, and S. sparsipilum. Juveniles of eight M. chitwoodi isolates, five M. fallax isolates, and six mono-female lines of a M. chitwoodi isolate were inoculated on S. bulbocastanum, S. chacoense, S. hougasii, S. stoloniferum, and S. tuberosum. Virulence was expressed as nematode reproduction 8 weeks after inoculation. Nematode reproduction was estimated by the number of egg masses and, in one experiment, by the number of hatched second-stage juveniles per inoculated juvenile. Considerable variation in virulence and resistance was observed among M. hapla isolates and plant genotypes, respectively. The M. hapla isolate-plant species interaction was highly significant. The response to M. chitwoodi ranged from susceptible (S. tuberosum and S. chacoense) to highly resistant (S. bulbocastanum and S. hougasii). S. tuberosum was susceptible to M. fallax, whereas all four wild species were resistant. In contrast to M. hapla, no significant isolate-plant genotype interaction was obtained for M. chitwoodi or M. fallax, indicating no or little intraspecific variation in virulence. M. chitwoodi juveniles in species mixtures with M. fallax isolates appeared to be able to break the resistance of S. bulbocastanum and S. hougasii. Significant differences among mono-female lines of M. hapla and M. chitwoodi were observed, indicating heterogeneity of pathogenicity within meiotic parthenogenic Meloidogyne populations.
ABSTRACT
ABSTRACT Hybridization between two meiotic parthenogenetic species of root-knot nematodes, Meloidogyne chitwoodi and M. fallax, was investigated in two different crossing experiments on tomato plants grown in sand. The first experiment was a controlled cross between the two species. The second experiment was a bulk mating in a 1:1 mixture of two isolates. The haploid chromosome number of the parental isolates was n = 18. Successful interspecific hybridization was obtained, and the resulting hybrids produced egg masses. In eggs, cell division was observed, but most of them were without clear differentiation and consequently were sterile. Hatched F(2) juveniles were small in number, not viable, and showed morphological distortions. In the progeny of the isolate mixture of the bulk mating experiment, parental-type females of the two isolates were present in equal numbers, and 10% of all females were nonviable hybrids. Similar ratios of parental-type and hybrid females were detected in roots of test plants grown in soil from a field sample that contained a mixture of M. chitwoodi and M. fallax populations. In the controlled cross experiment, isozyme electrophoresis of malate dehydrogenase was applied to distinguish the two species and their hybrids. In the bulk mating experiment, malate dehydrogenase, esterase, and glucose 6-phosphate dehydrogenase were used as markers, two by two simultaneously on the same individual females, providing conclusive evidence for the occurrence of hybrids. This is the first report on interspecific hybridization in Meloidogyne. The possible role of interspecific hybridization in species differentiation and interspecific exchange of genetic material within Meloidogyne is discussed.
ABSTRACT
The inheritance of resistance to powdery mildew (Oidium lycopersicum) in Lycopersicon hirsutum was investigated by disease tests in segregating populations obtained by hybridising tomato (L. esculentum) cv Moneymaker with the wild relative L. hirsutum G1.1560. One incompletely dominant gene Ol-1 was found to largely control resistance to the disease. To map Ol-1, DNA pools from seven resistant and ten susceptible F2 plants were analyzed for random amplified polymorphic DNA (RAPD). With 32 primers tested, one RAPD, primed with the sequence 5'-GACGTGGTGA-3', was observed between the susceptible and the resistant bulks, which cosegregated with resistance in the F2 population of L. esculentum × L. hirsutum G1.1560. This RAPD was mapped on chromosome 6 by using an F2 (L. esculentum × L. pennellii) already mapped for 49 RFLPs. RFLP analysis of the F2 from L. esculentum cv Moneymaker × L. hirsutum G1.1560 demonstrated that Ol-1 maps near the Aps-1 region on chromosome 6, in the vicinity of the resistance genes to Meloidogyne spp. (Mi) and to Cladosporium fulvum (Cf-2/Cf-5).
ABSTRACT
The contribution of introgressed regions derived from wild species to the genetic variation within the species of Lycopersicon esculentum was investigated by comparing the RFLP patterns of 2 introgression-free, obsolete cultivars ('Moneymaker' and 'Premier') and a modern cultivar ('Sonatine') that carries at least 5 introgressed resistance genes. In this analysis 195 mapped nuclear markers were used in combination with 6 restriction enzymes. Among the 1170 probe-enzyme combinations tested, only 3 showed a polymorphism between the 2 introgression-free cultivars. On the other hand 24 probe-enzyme combinations were found to exhibit polymorphisms between 'Moneymaker' and 'Sonatine'. These represented ten polymorphic loci distributed among 5 linkage groups on chromosomes 1, 3, 4, 6, and 9.On the assumption that most of the polymorphic loci corresponded to introgressed chromosome segments of wild species carrying resistance genes, linkages between these loci and the component resistance genes were examined by RFLP analysis of pairs of near-isogenic lines differing only for one particular resistance gene, and a variety of commercial cultivars having different resistance gene compositions. Two of the polymorphic linkage groups could thus be ascribed to resistance genes whose map positions were already known: Cf2 on chromosome 6 and Tm2a on chromosome 9, whereas another marker, TG301 on chromosome 1, could be assigned to the Cladosporium fulvum resistance gene Cf9 with a hitherto disputable map position. By linkage analysis of a segregating F2 population the genetic distance between the Cf9 gene and the marker TG301 was estimated at 5.5 ± 2.3 cM.
