Preypredator Interactions

3.1. Diversity of Antagonists and Parasitism

Most of nematode-parasitic prokaryotes include unicellular rickettsiae, with binary fission, viruses (whose pathogenic effects on nematodes remain unclear), and bacteria, among which only one group is currently recognized for its parasitic action: Pasteuria penetrans sensu lato (Sayre & Starr, 1988). Among eucaryotes, only Hyphomycete and Zygomycete fungi are nematode parasites. They were commonly classified according to their predation activity: endoparasite fungi of juvenile and adult stages, such as Catenaria anguillulae, or trapping fungi, such as Arthrobotrys oligospora (Gray, 1988), and ovicide fungi, such as Pochonia chlamydosporia (Morgan-Jones & Rodriguez-Kabana, 1988). Nematophagous fungi can be obligate parasites of nematodes (Catenaria anguillulae), or can be opportunists as trapping fungi. The growth of opportunist fungi in soil can be influenced by plants (root exudates), and traps can be stimulated by the presence of nematodes. Consequently, dependence of fungi on their environment is very wide.

The Gram+ bacterium P. penetrans has three dependence levels with respect to its environment. Present in the soil (first level), this organism is an obligate parasite of nematodes (second level). P. penetrans was never detected on other organisms than on soil nematodes. Moreover, its parasitoid behavior makes it absolutely dependent on its nematode hosts. Spore germination is induced when the nematode begin to feed on the plant (third level). P. penetrans presents three phases in its life cycle: a free step in the soil, as sporanges, commonly called spores (survival form); an attachment step when the spores attach to the nematodes (aggression form) and a penetration/development step in the nematode (parasitic form).

Unfortunalely, systematics of both P. penetrans and nematophagous fungi remain poor, and their diversity and specificity to their nematode hosts are insufficiently evaluated. Up to now, research mainly focused on "nematode -predator" relationships and environment as factors mainly restricted to soil temperature, moisture and acidity. Concerning nematophagous fungi, the impact of organic matter and minerals on their growth was often highlighted. But, these studies focused on biological requirements for growth and predation with the aim of biopesticide production.

P. penetrans was detected on soilborne nematodes only, on more than 90 genera and 200 species. P. penetrans was initially classified in the genus Duboscqia (Cobb, 1906), then in the genus Bacillus (Mankau, 1975), and finally in the genus Pasteuria (Sayre & Starr, 1985). However, the characterization of this bacterium by description of its vegetative and reproduction structures only never made it possible to classify in the Bacillaceae family or in the Actinomycetaleae one. Data on ribosomal genes, however, clearly indicated its affinity with Bacillaceae (Charles et al., 2005). Six groups of Pasteuria were set up at species rank: P. ramosa, parasite of daphnia (Metchnikoff, 1888); P. penetrans, parasite of Meloidogyne nematodes; P. thornei, parasite of Pratylenchus nematodes (Starr & Sayre, 1988); P. nishizawae, parasite of Heterodera and Globodera nematodes (Nishizawa, 1986), and two other species, one parasite of Heterodera goettingiana (Sturhan, Winkelheide, Sayre, & Wergin, 1994) and one parasite of Belonolaimus longicaudatus (Gibkin-Davis, Williams, Hewlett, & Dickson, 1995). However, pure culture of this bacterium and more informations proceeding from biochemical and genomic taxonomy (Anderson, Preston, & Dickson, 1999; Atibalentja, Noel, & Domier, 2000; Bird, Opperman, & Davies, 2003) are required to understand its evolutive speciation and to characterise this bacterium.

3.2. Nematode-antagonists Specificity

Most of the known nematophagous fungi are able to trap and parasitize very different nematode species as bacteriophagous, plant-parasitic and insect-parasitic species (Rosenzweig, Premachandran, & Pramer, 1985). Some specificities were, however, observed in the composition of the fatty acids produced by fungi in culture, depending on the generic affiliation of the predaceous species and its predation mechanisms (Radzhabova, Gasanova, Mekhtieva, & Bekhtereva, 1987). A wide investigation reported specificities in morphological adaptations, host preference and prey recognition by nematophagous fungi (Nordbring-Hertz, 1988). Antagonistic effects of several Arthrobotrys oligospora and A. conoides strains isolated in Burkina faso and Senegal on three Meloidogyne species (M. mayaguensis, M. incognita, M. javanica) appeared very specific (Duponnois, Mateille, Sene, Sawadogo, & Fargette, 1996) (Table 1). This observation suggests that efficacy of biocontrol of major nematode pest communities requires mixed inocula, including more than a single fungus isolate/species.

Table 1. Fraction (%) of Meloidogyne spp. juveniles trapped in vitro by different Arthrobotrys spp. isolates (adapted from Duponnois et aL, 1996)1.

Species

Isolate

M. mayaguensis*

M. incognita*

M. javanica*

Arthrobotrys sp.

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