Trade-off between dispersal and reproduction in Aphelenchoides besseyi (Nematoda: Aphelenchoididae) harbored in Oryza sativa seeds in paddy fields

Abstract

The white tip nematode, Aphelenchoides besseyi, is a seed-borne ectoparasite of rice, Oryza sativa. Population parameters were compared with relation to seed specific gravity between infested and non-infested paddy fields to evaluate the effect of a nematode infestation on dispersal and survival of both host plant and nematode. When seeds from infested and non-infested paddy fields were floated on water, the proportion of light seeds (identified by floating at the water surface) was greater for infested field than for non-infested field. Light seeds showed smaller mean swelling, developed a smaller proportion of seminal roots, and required longer to produce the roots than did the heavy seeds from infested and non-infested fields. Nematode mortality was greater in light seeds than in heavy seeds. These results suggested that there was a trade-off between both the dispersal and competition of rice seeds and between dispersal and reproduction of nematodes harbored in the seed. Intriguingly, light seeds from infested fields showed a larger mean degree of swelling than did those from non-infested fields and light seeds harboring many nematodes had a welldeveloped endosperm, suggesting a host manipulation by the nematode.Key words: Host manipulation; seedling growth; nematode load; specific gravity of seeds; white tip nematode Appl. Entomol. Zool. 44 (3): 387-396 (2009) http://odokon.org/ * To whom correspondence should be addressed at: E-mail: togashi@fr.a.u-tokyo.ac.jp † Present address: Laboratory of Forest Zoology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan. DOI: 10.1303DOI: 10. /aez.2009 387 cific gravity Ն1.0), irrespective of their source, whether derived from nematode-inoculated or uninoculated plants (Togashi and Hoshino, 2003). The experiment also showed that nematode-inoculated plants produced a greater proportion of light seeds than un-inoculated plants and that nematode mortality was greater in light seeds than in heavy seeds. These results indicated a trade-off between dispersal and reproduction of nematodes harbored in the seed through seed specific gravity.As the reproduction of A. besseyi after dispersal depends on the rice plant survival, the nematode might force the parent plant to supply a large amount of hydrocarbons and proteins to the light seeds harboring them. Intriguingly, the inoculation experiment indicated that as the number of nematodes in a light seed increased, the nematode mortality decreased and the degree of seed swelling increased, the latter suggesting a host manipulation by A. besseyi (Togashi and Hoshino, 2003). To our knowledge, similar type of host manipulation has never been reported for other nematode species that parasitize plant seeds.The results of the inoculation experiment (Togashi and Hoshino, 2003) have not been confirmed for A. besseyi-infected rice seeds harvested in paddy fields yet. Thus, the objective of this study was to determine the relationships between initial seed specific gr...

Assessment of a three-stage sampling strategy to investigate the spatial distribution and population density of Aphelenchoides besseyi among Oryza sativa seeds

“…The proportion of rice seeds harbouring live nematodes in each panicle (prevalence) increased and approached the upper limit as the mean density of nematodes per seed increased. The upper limit of 76.3% for living and dead nematodes combined in a seed, which was estimated from a regressed curve of y = 39.68x/(1 + 0.52x), was higher than 66.2% from y = 45.87x/(1 + 0.69x) for living nematodes in a seed, which was caused by the likely death of a small number of nematodes in individual seeds (Huang & Huang, 1974;Togashi & Hoshino, 2003;Hoshino & Togashi, 2009). An upper limit of the proportion of nematode-infested seeds, which meant the presence of nematode-free seeds under heavy infestation, would contribute to the persistence of the rice plant population.…”

“…In addition, living nematodes showed clumped distributions among seeds on the three spatial scales of panicles, plant hills and paddy fields, which may be related to a positive relationship between the survival rate and number of nematodes within seeds (Allee effect) (Huang & Huang, 1974;Togashi & Hoshino, 2003;Hoshino & Togashi, 2009). …”