Rev: 01/01/2020
Plant and soil nematodes have evolved a suite of adaptations that confer fitness in a variety of spatio/temporal niches and that enhance their probability of survival under adverse conditions. Here are some examples.
Although the feeding habits of many plant and soil nematodes have not been determined, a great deal of information is available from experiment, observation, and inference from feeding structures and organisms associations. The classic paper of Yeates et al. (1993) summarizes the information available to that time. More recent contributions by Moens et al (2004) add to that information. Yeates et al (1993) categorized six clear feeding types among soil and plant nematodes:
They also recognized two other categories, the ingestion of substrate incidental to feeding by open-mouthed morphotypes such as bacterial feeders and certain predators, and dispersal or infective stages, often in phoretic relationships with insects, that may not be feeding.
The term "omnivore" is usually applied to certain nematodes of the Dorylaimida for which omnivory has been observed or for which feeding habits are unknown. Clearly, some dorylaims are plant feeders (e.g., Xiphinema) and some are predators (e.g., Labronema).
True omnivory is an adaptation to unreliable food sources that may be seasonally or spatially sparse. Besides its inferred occurrence in the Dorylaimida, it has been observed in Mononchida where juveniles may be sustained on bacteria (Yeates, 1987) and in certain Aphelenchina (e.g., Aphelenchoides spp. ) and Tylenchina (e.g. Ditylenchus spp.). In the case of plant-feeding species of Aphelenchoides and Ditylenchus, survival between plant hosts by feeding on fungi is of obvious adaptive significance.
Many plant-feeding nematodes that are successful in annual crop agriculture have wide host ranges. There is a high probability that they are able to feed on a variety of plants that might be exposed to through crop sequence selections. Also, weeds are always an available resource.
While a wide host range is an advantage in annual crop agriculture, there are other successful strategies. Some nematodes with quite narrow host ranges are successful because they remain in a dormant state until stimulated to emerge by root exudate signals recognized from a host plant. The non-feeding dormant stage might technically be considered a dauer stage (see below) without the morphological features of extra cuticle and closed mouth. In some cases the dormant stage is the second stage juvenile retained in the egg (e.g., Heteroderinae) and in other cases it may be a pre-adult juvenile (e.g., Paratylenchus spp.).
Many soil nematodes, particularly bacterial feeders of the Rhabditidae, Panagrolaimidae and Diplogastridae, have a metabolically-suppressed specialized survival stage.
Schneider (1866) reported the existence of a life stage of rhabditid nematodes with a cuticle differing from that in other stages; he considered this form to be a molting stage but was uncertain of its role. According to Maupas (1899), Pérez (1866) recognized an “encysted” stage in Rhabditis teres and indicated that larvae easily encysted at the end of the second stage. Experimentally, Maupas (1899) determined that always the same life stage entered encystment when nutrients were lacking. He showed that emergence from the encysted stage occurred with enrichment and noted that encysted nematodes survive for weeks and are often a dispersal stage.
Later, Fuchs (1915) in his description of rhabditids associated with bark beetles, coined the term dauerlarva for the persistent or enduring stage of these nematodes. Many of the nematodes that have phoretic relationships with insects are in a dauer stage during the phoresy. Likewise, rhabditid nematodes that are entomopathogens await their insect hosts in a dauer stage.
We now know that dauerlarva induction in Caenorhabditis elegans is mediated by the ratio between a dauer-inducing pheromone, which is constantly produced by the nematode, and the magnitude of a carbohydrate signal from the bacterial prey. The ratio forms a net measure of population size in relation to food availability. When the dauer-inducing pheromone is significantly greater than the food signal, dauer formation commences (Riddle, 1988).
The term dauerlarva is sometimes applied more broadly to recognize persistence of non-feeding life stages across a broad range of Nematoda (Bird and Opperman, 1998).
Cryptobiosis (literally, hidden life) is an attribute of certain nematodes that enables their survival without detectable metabolic activity. The most commonly recognized form of cryptobiosis is anhydrobiosis, survival in a metabolically inactive state in the absence of water. Other forms, e.g., anoxybiosis, have also been described.
The first report of anhydobiosis, although not recognized as such at the time, was that of Needham 1742, when he opened the seed galls of Anguina tritici on wheat. Anhydrobiosis is a common attribute of nematodes that are successful in habitats that are subject to seasonal drying and to those that feed on the aboveground parts of plants. For example, fourth stage juveniles of Ditylenchus dipsaci enter anhydrobiosis, usually in large masses, on or below the surface of plant tissue. The term "eelworm wool" is coined to describe the appearance of the dried nematodes. Similarly, the second-stage juveniles of Aphelenchoides besseyi enter anhydrobiosis under rice hulls.
Nematodes capable of anhydrobiosis when subjected to slow drying. There is insufficient time for the necessary physiological changes and membrane structural changes to take place if the drying is rapid. Experimentally, slow drying is controlled by subjecting the nematodes to atmospheres of controlled humidity.
Dormancy is a general term applied to the condition of lowered metabolism and various categories have been recognized in nematodes (Evans and Perry, 1976; Evans, 1987):
Facultative quiescence: dormancy under unfavorable conditions with development readily resumed as conditions become favorable.
Obligate quiescence: required dormancy for a life stage with development readily resumed under favorable conditions.
Facultative diapause: dormancy initiated by environmental factors with delayed resumption of development under favorable conditions.
Obligate diapause: dormancy initiated by endogenous factors with delayed resumption of development under favorable conditions after specific requirements are satisfied, e.g., Meloidogyne naasi.
Delayed egg hatch: delayed development of eggs despite favorable conditions.
The non-dormant condition, eggs that hatch rapidly in water.
Eggs that hatch rapidly in host root diffusate.
Eggs that hatch slowly in water over a long period of time.
Eggs that hatch slowly over a long period of time in host root diffusate.
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