- Relationships of six species of saprozoic nematodes, Pristionchus
lheriteiri (Maupas, 1919) Paramonov; Panagrolaimus subelongatus
(Cobb, 1914) Thorne, 1937; Cephalobus persegnis Bastian 1865;
Pelodera (Rhabditis) lambdiensis (Maupas, 1919) Dougherty, 1953;
Rhabditis sp. I and Pelodera chitwoodi (Bassen, 1940) Dougherty,
1955 and a nitrogen-fixing bacterium, Rhizobium japonicum (Kirchner,
1895) Buchanan, 1926 were investigated, These nematodes were
found reproducing, feeding on cultures of R. japonicum grown on
Rhizobium agar medium but the density of the populations varied,
depending on the species of nematode. It was demonstrated that these
nematodes could carry the bacteria both in the alimentary canal and
on the body surface, The average numbers of bacterial cells carried
internally, ranging from 12 cells in Rhabditis sp. I to 9, 034 cells in P. lheriteiri, did not correlate with nematode body size. By contrast,
the numbers carried externally, ranging from 100 cells on Rhabditis
sp. I to 37, 776 cells on Pelodera chitwoodi, had a direct relationship.
The relationship between P. lheriteiri and R. japonicum was
studied in detail. The saprozoic nematode, P. lheriteiri, could not
survive and reproduce on Rhizobium agar medium containing R.
japonicum culture broth or an inoculum that were previously autoclaved
or sonicated but only could survive if the food source was a living
bacterial suspension. Females of P. lheriteiri were found to carry
2, 622 bacterial cells in the alimentary tract while only 1, 004 cells
could be found in males. The bacteria still were viable after they
were ingested and defecated by the nematodes.
The nodule-forming ability of the bacteria, after passing through
nematode alimentary tract, did not change, Bacteria carried internally
by the nematode could develop nodules on Lee soybean, Glycine max
(Linn.) Merr. , roots in water agar as well as those in suspension.
There was no relationship between numbers of nodules and numbers
of bacteria-carrying nematodes in any treatment. Nodules were small
in size, ranging from 0. 5 - 2. 0 mm in diameter, and few in number
per plant. Sixty percent of the soybean plants, including 13 percent
in the control treatment which were inoculated with a bacterial suspension,
formed nodules on roots 30 days after inoculation. Almost
92 percent of nodules were found on or near plant hypocotyls; only 8 percent of the total nodules were seen deep in the agar medium.
This investigation indicates that many kinds of saprozoic nematodes
can become carriers and disseminators for an agriculturally
beneficial bacterium, R. japonicum. P. lheriteiri, Panagrolaimus
subelongatus, Pelodera chitwoodi and Pelodera (Rhabditis) lambdiensis
were demonstrated to be efficient bacterial vectors. These nematodes,
including Rhabditis sp. I and Cephalobus persegnis could carry the
bacteria internally as well as externally but this capability varied with
the species of nematode.