RESULTS
Determination of attraction and repulsion
A typical result of an attraction test with the nematophagous fungus Arthrobotrys
musiformis is shown in Table 1. On average, 75 % of the nematodes were found under the
discs of attracting fungi in quadrants I and I11 after 24 h.
Of the 14 nematophagous fungi tested, 10 attracted P. redivivus (Table 2). Only one
fungus, A. arthrobotryoides, with spontaneously formed traps, repelled the nematodes,
Arthrobotrys dactyloides and D. gracilis, both with constricting ring traps, neither attracted
nor repelled the nematodes. The same result was found with A. conoides, which did not form
traps spontaneously. All three strains of A. oligospora showed moderate attraction,
irrespective of the presence or absence of spontaneously formed traps.
Of the nine non-nematophagous fungi tested, five attracted and one repelled the nematodes
(Table 2).
Determination of attraction intensity
The three groups of nematophagous fungi were tested for their attraction intensity. The
endozoic parasite (H. anguillulae) always showed the highest attraction intensity (slope > 6).
Group 2, with the slow-growing attracting fungi M. ellipsosporum, M. cionopagum and
D. candida, showed a somewhat lower attraction (slope 3 to 6), while the fast-growing species
(group 1) had the lowest attraction intensity (slope < 3). The non-nematophagous fungi
tested (Trichothecium roseurn, Verticillium dahliae, Mortierella parvispora and Penicillium spinulosum) also showed low attraction intensities. Figure 1 shows examples of these
differing responses.
Results from one experiment with several members of groups 1 and 2 are shown in Fig. 2.
The difference in attraction intensity between the two groups is evident. In a parallel
experiment, each slow-growing fungus (group 2) was tested against a fast-growing fungus
(group 1) on the same plate (e.g. M. ellipsosporum versus A. oligospora, M. cionopagurn
versus A. mus$ormis etc.). After 6 h the nematodes under each disc were counted. Once
again, members of group 2 attracted more nematodes than those of group 1 (P < 0.01, five
replicate plates). Although the slow-growing group 2 fungi had traps, which the fast-growing
group 1 fungi lacked, very few nematodes were captured during this 6 h period. The higher
attraction intensity of the fungi in group 2 could therefore not have been due to capture by
the traps.
Eflect of u.v.-irradiation
To determine if the attraction/repulsion pattern of the nematophagous fungi shown in
Table 2 was confined to living fungi, attraction tests were done after killing the fungi with
u.v.-irradiation. After irradiation, group 1 fungi and non-nematophagous fungi no longer
attracted nematodes but the slow-growing fungi (groups 2 and 3) still did so (Table 2).
Repulsion and neutral responses to the other fungi remained unchanged.
DISCUSSION
The assay used in this investigation allowed us not only to detect attraction or repulsion,
but also to determine differences in the attraction intensity of different fungi; in both cases
it gave reproducible results.
Field & Webster (1 977), studying attraction to living mycelium of five nernatophagous
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Attraction of nematodes to fungi 93
fungi with different types of traps, showed that fungi stimulated to trap formation with horse
serum or nematode extract attracted both a Rhabditis sp. (bacteria-feeding) and the fungusfeeding
Aphelenchus avenae, whereas the unstimulated fungi did not. They concluded that
attraction was dependent on the presence of traps. -We have shown that the presence of traps
does not necessarily cause attraction, and one trap-forming fungus, A. arthrobotryoides,
even repelled the nematodes. Further, the constricting ring forms (A. dactyloides and
D. gracilis) neither repelled nor attracted the nematodes (Table 2). An explanation for these
differing results might be that substrate composition affects the attractiveness of the fungi.
Addition of proteinaceous trap-inducing materials, such as horse serum or nematode
extract, may result in attracting excretion products, not necessarily coupled to trap formation.
When the fungi were killed by u.v.-irradiation before nematodes were added, the
attraction of group 1 fungi was completely abolished. The attracting fungi of groups 2 and 3
were still attractive, although somewhat less so, after such treatment. This suggested that
a volatile substance, or a small rapidly diffusing compound, continuously produced, was
responsible for attraction to the fast-growing fungi. With the fungi of groups 2 and 3, larger
or less volatile, slowly diffusing compounds might be responsible for attraction. The
non-nematophagous fungi resembled the fast-growing nematophagous fungi in this respect.
However, the substances responsible for attraction are unknown.
Cooke (1963) concluded that rapid growth rate and good saprophytic ability were
accompanied by lower predacious efficiency. In our study, the attraction intensity was
lowest in fungi with moderate or high saprophytic ability (Table 2, group 1 and the nonnematophagous
fungi). The u.v.-irradiation tests also showed a similarity between these
fungi, Attraction intensity increased with increasing parasitism of the fungi in groups 2 and
3. It therefore appears that the attraction intensity reflects the dependence of the fungi on
nematodes for nutrients.
The gift of Harposporiurn anguillulae from Dr G. L. Barron, University of Guelph,
Canada, is gratefully acknowledged. This investigation was supported by the Swedish
Natural Science Research Council.
R
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