Pathogenicity of the entomopathogenic nematode Steinernema carpocapsae depends on its symbiotic bacterium Xenorhabdus nematophila. To understand the pathogenicity of the nematode itself, and its influence on insect tissues, we compared nematode invasion, insect mortality, nematode development in insects, and differences in the effects on the insect tissues between infection by axenic or monoxenic nematodes. Invasion of axenic infective juveniles (IJs) was lower than that of monoxenic IJs. Axenic as well as monoxenic nematodes killed both axenic and xenic Galleria mellonella and Spodoptera litura larvae, although it took a longer time for axenic nematodes to kill the insects, especially axenic ones. Axenic nematodes grew and reproduced in insects; however, their growth and reproduction were delayed as compared to that of monoxenic nematodes. Tissue destruction of axenic IJ-infected insect larvae was delayed as compared with monoxenic IJ-infected insect larvae, and suppression of insect hemolymph melanization was not observed in the axenic nematodes. These results suggest that S. carpocapsae is able to kill insects and partly degrade insect tissues in the absence of any bacteria; however, the pathogenicity of axenic nematodes is weaker than that of monoxenic nematodes.
Beneficial effects of winter flooding in rice paddies have been proved for birds and other above-ground wildlife. However, there have been few studies that accounted for soil organisms. In this study the effects of winter flooding together with an organic amendment on soil fauna as represented by nematodes were examined. Soil samples were taken from experimental rice paddies in northern Japan with one of three treatments, i.e., winter flooding with organic farming (WFO), no winter flooding with organic farming (NFO), and no winter flooding with conventional farming (CVN, only chemical fertilizers and chemicals applied). During this study period of over one year and a half, nematodes were extracted and counted to estimate their density. Then to evaluate nematode diversity, a polymerase chain reaction-denaturing gradient gel electrophoresis analysis of the nematode 18S rRNA gene was conducted， which found 34 operational taxonomic units with Tobrilus spp. and Hirschmanniella sp. as the three most dominant taxa. Statistical analysis showed the following trends: 1) nematode density in the 0–5 cm soil layer was WFO > NFO > CNV, and 2) nematode diversity, H′, in the 5–10 cm layer was WFO < CNV < NFO. The results of our nematode and soil physico-chemical analyses suggest, 1) winter flooding and organic farming could increase nematode density at least in the upper soil layer with increased fertility, and 2) on the contrary, winter flooding could be harmful to maintain nematode diversity in the deeper part of soil, probably due to an increased soil reduction level.
Development of Pratylenchus kumamotoensis eggs was observed in distilled water at 15–35ºC. Egg developmental duration was shortest at 32.5ºC (5.6 days) and longest at 15ºC (28 days). Base temperature and thermal constant estimated from data between 20ºC and 30ºC were 11.8ºC and 108 degree-days, respectively. Population development from a single female inoculated to chrysanthemum was observed at 20, 25 and 30ºC. Pratylenhus kumamotoensis reproduced bisexually and the number of total progeny from the inoculated female sharply increased 15–18 days after inoculation (dai) at 30ºC, 24–27 dai at 25ºC and 40–44 dai at 20ºC, indicating onset of oviposition by next generation females. These results indicated that P. kumamotoensis is closer to tropical or subtropical plant-parasitic nematodes in thermal requirements rather than to temperate species such as P. penetrans.
The rice paddy fields in Asia are being recognized as biodiversity hotspots. We have reported the seasonal dynamics of nematode communities in a conventional type of paddy field (CPF) in Japan, which is flooded only in spring and summer. The CPF nematode fauna included the typical taxa of bacterial feeders (BAC) and algal feeders (ALG) that inhabit the sediment of freshwater ponds or lakes, as well as fungal feeder + facultative root feeder (FFR) and obligatory root feeders (ORF). Here, we report on the nematode fauna of paddy fields flooded all year round (PFF), which were developed to imitate“ yatsu” paddy fields located at the bottom of a valley. We examined the fauna at soil depths of 0–50 mm in May, August, and November, 2008. One half or more of the total nematode abundance was concentrated into the 0–15 mm layer. This concentration was enhanced after the physical disturbances of puddling and rice transplanting in June. Nevertheless, the compositions of feeding groups and nematode taxa were stable across the study seasons, and generally identical across soil layers. PFF fauna comprised typical sediment taxa with two dominant BAC, Paraphanolaimus (Aphanolaimidae) and Paraplectonema (Leptolaimidae) (60–90% of total density), whereas the FFR and ORF taxa were scarce. Not surprisingly, the PFF fauna was quite different from those in the adjacent terrestrial habitat (ATH), which was composed mainly of FFR and ORF taxa. PFF fauna, however, also had some taxa in common with ATH; e.g., BAC of Monhysteridae, Aphanolaimus, and Chronogaster. Patches of wet peat moss in ATH might be important for the occurrence of these taxa.
The species diversity and abundance of soil nematodes can be an indicator reflecting environmental changes in a given soil ecosystem. As a rather simple type of forest ecosystem, coastal pine forests make a good model system for characterizing nematode communities. The purpose of this study was to determine the heterogeneity and community structure of soil nematodes in a coastal Japanese black pine (Pinus thunbergii) forest. We examined nematodes from sandy soils in the forest over one year and divided them into trophic groups based on morphological traits. The abundance of nematodes (per g dry soil) in all plots ranged from 0.4 ± 0.2 (mean ± SD) in February to 8.3 ± 4.4 (mean ± SD) in July. There was a significant interaction in the abundance of nematodes between the plots and months. Among 7,295 nematodes detected, the 18 taxa retrieved were dominated by the genera Aphelenchoides (24.3%), Ditylenchus (19.0%), and Acrobeloides (18.6%). The trophic structures of nematodes were continuously dominated by bacterivorous and fungivorous ones. These findings suggest that nematode distribution was spatially heterogeneous, their abundance varied with time, and the rather limited taxa of these trophic groups likely play important roles in the coastal soil ecosystem.
SHORT COMMUNICATION (IN JAPANESE WITH ENGLISH SUMMARY)
In order to clarify the parasitic behavior of Hirschmanniella diversa and host reactions in Indian lotus roots, we observed naturally infected roots from a lotus field using light and scanning electron microscopy. Light microscopy revealed that various stages such as adults, juveniles and eggs of H. diversa were present in the cortex, but none were present in the stele of the root. These results suggest that H. diversa spends a large part of the life cycle in Indian lotus roots. Scanning electron microscopy also confirmed that most H. diversa were present in the cortex cells, and revealed that in cortex cells with much smaller volumes and with lower nutrient content than the usual ones, a few H. diversa were turning with several twists of the body, suggesting these nematodes were seeking cells which could supply better feeding sites.
On February 15, 2015, a stunted rooted cutting of Okinawan spinach or Gynura bicolor (Roxb. ex Willd.) was found in a vinyl house nursery at Suemachi, Kanazawa City, Ishikawa Prefecture (36º30'55.8"N 136º41'38.4"E, 152 m). After visual examination of the damage, nematodes were collected from root samples using a Baermann funnel method and a large number of root-lesion nematodes, Pratylenchus sp., were obtained. The cult ivated f ield of Okinawan spinach at Kashimimachi, Kanazawa City (36º29'20.3"N 136º42'02.4"E, 265 m), where the rooted cuttings were transplanted, was also later revealed to be contaminated by the nematode species. The isolated root-lesion nematode materials were sent to the NARO, Agricultural Research Center, and the species was concluded to be a Pratylenchus species previously unrecorded from Japan. The morphological characters and measurements are given for the species herein.
The southern root-knot nematode Meloidogyne incognita is an important pathogen of solanaceous plants worldwide. To assess resistance and suppressiveness to the nematode of the Japanese cultivars of eggplant rootstocks, we conducted greenhouse experiments. Numbers of nematode egg masses were significantly lower on Solanum torvum rootstock cultivars, Tonashimu, Torero, and Torvum vigor, than the cultivars of the other Solanum species after 45 days of cultivation. Next, we examined the effect of Tonashimu on the population density of the nematode in soil. After 116 days of cultivation, the second-stage juvenile density of the nematode in soil was reduced, and was significantly lower in pots of Tonashimu, than in those of Solanum melongena cultivars. These results suggest that the Japanese rootstock cultivars of S. torvum are resistant to the nematode, and could suppress nematode density in soil.
ABSTRACT OF PAPERS PRESENTED AT THE TWENTY-FOURTH ANNUAL MEETING OF THE JAPANESE NEMATOLOGICAL SOCIETY
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