Apomictic, polyphagous root-knot nematodes: exceptionally successful and damaging biotrophic root pathogens.

Most apomictic root-knot nematodes (RKN; Meloidogyne spp.) have host ranges that encompass the majority of flowering plants, and M. incognita is possibly the world's most damaging crop pathogen. The ancestors, age, and origins of the polyphagous RKN are obscure, but there is increasing evidence that M. incognita, M. javanica, and M. arenaria are closely related, heterogeneous species with a recent, hybrid (reticulate) origin. If so, they must owe much of their current worldwide distributions to spread by agriculture. Host resistance appears to be generally durable in the field, but laboratory studies suggest that apomixis does not prevent evolution in response to selection by a parasitic bacterium (Pasteuria penetrans) and host resistance. Maintaining general fitness may be the evolutionary priority for most populations of polyphagous RKN, and a wide host range, important in the field but not in the laboratory, may be conserved by apomixis. Several factors may help confer a wide host range, including suppression of host resistance, perhaps as a consequence of the strength of the induced susceptible response. Resistance genes effective against RKN appear not to have resulted from coevolution. Rates of juvenile invasion and/or development are low in many wild and some crop plants, with the result that they are both poor hosts and sustain less damage. Overall, it is suggested that greater coordination, particularly of fundamental research, is required.

Cuticle heterogeneity as exhibited by Pasteuria spore attachment is not linked to the phylogeny of parthenogenetic root-knot nematodes (Meloidogyne spp.).

The cuticle is a major barrier prohibiting the infection of nematodes against micro-organisms. The attachment of bacterial spores of the nematode hyperparasite Pasteuria penetrans (PP1) to field populations of root-knot nematodes (RKN, Meloidogyne spp.) from Burkino Faso, Ecuador, Greece, Malawi, Senegal and Trinidad and Tobago were assayed in standard attachment tests. The attachment of spore population PP1 to different field populations of root-knot nematode showed that the rates of attachment differed between countries. Similar tests were also undertaken on P. penetrans spores from these countries against 2 species of RKN, M. incognita and M. arenaria. The results showed a high degree of variability in spore attachment with no clear distinction between the 2 species of nematode. It has been hypothesized that Pasteuria spore attachment is linked to nematode species designations and this study clearly shows that this is not the case. Further tests showed that variation in spore attachment was not linked to nematode phylogeny. The results therefore beg the question of how do parthenogenetic root-knot nematodes maintain cuticle variability in the face of such an aggressive hyperparasite.

Evidence for a Dosage Effect of the Mi gene on Partially Virulent Isolates of Meloidogyne javanica.

The reproduction of single egg-mass isolates of Meloidogyne javanica from Crete that differed in virulence were compared on tomato (Lycopersicon esculentum) genotypes homozygous or heterozygous for the Mi gene. The reproduction of three isolates with partial virulence was much greater on tomato genotypes heterozygous for the Mi gene (cultivars Scala, Bermuda, and 7353) than on two homozygous genotypes (F8 inbred lines derived from Scala). The reproduction of a highly virulent isolate on the homozygous and heterozygous genotypes was similar to that on a susceptible cultivar. These results pose questions regarding the nature of partial virulence and indicate a quantitative effect of the Mi gene in relation to such virulence.

Immunocytochemical localization of neurosecretory amines and peptides in the free-living nematode, Goodeyus ulmi.

Mammalian antibodies to the neuroamines, serotonin and gamma-amino-butyric acid (GABA) and to the neuropeptides, adrenocorticotrophic hormone (ACTH) and FMRF-amide evoked a response to Goodeyus ulmi, a free-living nematode. Serotonin-like immunoreactivity was found in cell bodies in the nerve ring and in the ventral nerve cord in all developmental stages. Neurons in the vulva, implicated in egg-laying, were immunoreactive to anti-serotonin in G. ulmi females, while in males serotonergic nerve fibres was found in the spicular region. Immunoreactivity to ACTH was also seen to differ depending on the developmental stage of G. ulmi, being present only in the ventral cord from the late L3 stage. Anti-GABA immunoreactivity was localized in two cell bodies near the amphids in all life stages and FMRF-amide immunoreactivity was seen in the nerve ring in all developmental stages. No reactivity was found with antibodies to vasointestinal peptide and somatostatin-14.

Effect of Time of Application on the Action of Foliar Sprays of Oxamyl on Meloidogyne hapla in Tomato.

Foliar sprays containing 3,000 or 4,000 ppm oxamyl applied before inoculation with Meloidogyne hapla completely protected tomato plants from intection for up to 36 days but sprays containing 1,000 or 2,000 ppm provided only partial protection. Postinoculation sprays were less effective than preinoculation sprays but they decreased the numbers of females and their rate of development and increased the numbers of males. Similar amounts of oxamyl applied to the soil as a drench or as granules controlled M. hapla more effectively than foliar sprays but the longer treatment was delayed after infection the fewer the larvae that were killed and the more that became male.

Nuclear changes induced by the nematodes Xiphinema diversicaudatum and Longidorus elongatus in root-tips of perennial ryegrass, Lolium perenne.

The DNA content and size of individual nuclei from galls of perennial ryegrass root-tips induced by X. diversicaudatum and L. elongatus were measured. Feeding by X. diversicaudatum increased the DNA content of the nuclei by varying amounts. No regular doubling pattern of the DNA content was discernible. The DNA values varied up to between 32-64C. Generally the size of the nuclei was not increased, although some were larger than control nuclei. The modified nuclei probably have an altered metabolic function, which increases the food value of the gall to the nematode. Some bi-nucleate cells were also observed, which probably result from mitosis without cytokinesis. A preliminary examination of nuclei from galls induced by L. elongatus revealed similar nuclear changes, but no bi-nucleate cells were found.

Development of Four Populations of Meloidogyne hapla on Two Cultivars, of Cucumber at Different Temperatures.

The infectivity and development of four populations of Meloidogyne hapla were compared, at three temperatures, on tomato and two varieties of cucumber. A population from Canada produced few root-galls on cucumber and, except at 24 C, no larvae developed into adult females and produced egg masses. In contrast, a population with 45 chromosomes from America produced many galls on cucumber and small proportions of larvae became females and produced egg masses at 20 and 24 C. At 18 C this population produced no egg masses on cucumber, but a population from Britain and one from America with 17 chromosomes produced more egg masses at this temperature than at 20 or 24 C. Dissection of the galls showed that on cucumber many larvae died or their growth and development was slowed.

Ingestion, retention, and transmission of two strains of raspberry ring-spot virus by Longidorus macrosoma.

The transmission of two strains of raspberry ringspot virus (RRV) by small numbers of nematodes was compared. A strain of RRV from Scotland (RRV-S), originally found in the field associated with Longidorus elongatus, was transmitted frequently by L. elongatus but only once by L. macrosoma. A strain from England (RRV-E) associated with L. macrosoma in the field was transmitted infrequently by each species of nematode. The reasons why L. macrosoma infected only a small proportion of bait plants with virus were investigated, and it was found that most of the nematodes tested had fed on the source plants and many had ingested virus. Most nematodes exposed to RRV-E or RRV-S had fed on the roots of the bait plants and, when thin sections were examined by electron microscope, had retained particles (thought to be those of the virus) in the region of the anterior odontostyle, Thus, most nematodes seem to have had ample opportunity to transmit virus, and the low frequency of transmission may have been due to a failure of the virus particles to be released from the site of retention or to a lack of infectivity of the virus when L. macrosoma was the vector and Petunia hybrida was the host.