with permission from Office of Rights/Permissions, Plenum Publ.
233 Spring Street, New York, NY 10013, signed Georgia Prince, dated
A NEW DESCRIPTION OF THE PROCESS OF IDENTIFICATION OF PLANT-PARASITIC NEMATODE GENERA
California Department of Food and Agriculture
1220 N Street
Sacramento, California 94271-0001, USA
Accurate identification is a prerequisite to an understanding of the effect of nematodes as parasites of plants. It is obvious that no significant study on plant pests should be initiated until the identity of the parasite has been accurately established. For example, members of the genus Rotylenchulus in the past have been erroneously classified in the genus Helicotylenchus (H. elisensis, H. parvus), or they were proposed as new genera (Spyrotylenchus, Leiperotylenchus). In the first case, potentially dangerous parasites were wrongly identified as members of a relatively innocuous genus. In the second example, costly studies may have been initiated to define the biology of a supposedly new parasite, whereas the biology of Rotylenchulus is well known.
As shown by the examples above, errors sometimes are made by nematode taxonomists working within their field of expertise. Nematologists in other branches of the science also may err when they attempt to identify nematodes. The present review investigates the origin of such errors in the hope of defining a better method for more correct identifications.
Identification at the genus level and the species level each has its own problems and difficulties. To limit the scope of this study, it will address only identification at the genus level. Specific identification will have to be studied later.
A growing number of researchers are studying new approaches to identification: monoclonal antibodies, nucleic acid probes, characterization of proteins by immunoelectrophoresis, etc. These methods will allow the identification of a single species or even a race of a species. Similar tests already available for home medical diagnosis (pregnancy test) can be performed by persons with no knowledge of biology. These new techniques will soon provide easy means for identification of selected nematode pests. However, the development of a test for the identification on one species or one race requires lengthy and costly studies. This is economically feasible for important parasites, but it is doubtful that similar tests will ever be developed for all nematode genera, far less for all nematode species. Routine identification will long (or maybe forever) rely on morphological characteristics as seen with light microscopy (for the moment, scanning electron microscopy - SEM - is too complicated and costly for routine use).
Nematode identification, even limited to the genus level, is a difficult process which is not mastered by all plant nematologists. Professional identifiers, taxonomists, farm advisors, workers in private identification labs, employees of regulatory agencies, ecologists engaged in extensive faunistic surveys, etc., identify nematodes daily, or almost daily. Most have an expertise limited to selected nematode groups; a handful are comprehensive experts, able to recognize all genera. It should be noted that even those identifiers with a limited area of expertise can extend this area, if and when necessary, because their knowledge of nematode morphology, and their familiarity with the process of identification, make it easy for them to learn to identify new groups. Other nematologists are occasional identifiers. They only know the "agricultural genera", i.e., the genera that are most commonly found associated with cultivated plants (e.g. Meloidogyne, Pratylenchus, Xiphinema, etc.). They lack the practice for across-the-board identifications, and they have difficulty in learning to identify unfamiliar groups. Those shortcomings are even more obvious for nematologists who are working on a single species (for example Caenorhabditis elegans), and who are unfamiliar with other nematodes. Students are a case apart, at first they may have little or no knowledge of nematodes or of nematode morphology but, in the course of their studies, they are expected to become well acquainted with all nematode taxa.
No matter what their level of expertise and practice may be, most nematologists occasionally have to identify unfamiliar forms. This happens very rarely with the best experts, whereas every specimen presents a new and difficult challenge to the beginner. In such circumstan-ces, the success of iden-tification depends on available identification aids.
TRADITIONAL IDENTIFICATION AIDS
Because of the amount of data related to identification (100 to 150 morphological criteria differentiating 125 to 200 plant nematode genera), it is necessary to use an identification aid when identifying an unfamiliar form. Dichotomous keys are the traditional identification aids in nematology.
With Baylis & Daubney (1926), Tom Goodey (1933), Filip'ev (1936), Thorne (1949), Chitwood (1951), J.B. Goodey (1963), Siddiqi (1971; 1986), Golden (1971), Andrássy (1976), etc., many nematologists have published identification keys encompassing all plant-parasitic nematode genera. Most are dichotomous line keys. Notable exceptions include J.B. Goodey's (1963) tabular keys and Mai's (1975) pictorial key. Traditional identifica-tion aids have been used successfully for a hundred years. However it will be shown that their usefulness is greatly reduced when the user has no preconceived idea of the identity of the specimens.
Dichotomous and tabular keys rely on an existing classification and require the user to identify successively order, superfamily, family and subfamily before reaching the genus level. This poses several problems:
A NEW APPROACH: DEFINITION OF EXPERT IDENTIFICATION
In practice, expert identification differs from the process described above by two characteristics:
Keys follow a step by step approach, starting with the entire group (usually at the order level) and eliminating successively whole categories one at a time (sequential process), using one or two characteristics (monothetic process). By contrast the expert uses a simultaneous, polythe-tic process to "recognizes" at a glance the form to be identified.
For an expert, the "identification landscape" is quite different from the hazy swamp that confronts the beginner. There may still be some patches of fog, and some unknown quicksand may still be lurking here and there, but the identification country is covered with brightly lit, well indicated freeways going directly to the answer ("this is a Pratylenchus") or at least in the vicinity of the answer ("this looks like a Ditylenchus"; "this belongs to the tylenchorhynchid family of genera"). Or if the expert does not recognize the specimen he can somewhat connect it to a known form ("This looks like a Dolichodorus, but with only one genital branch"; i.e., it has all the characteristics of the genus Dolichodorus, but it has only one genital branch, instead of two). He can recognize a higher category (family) but narrow down the possible genera by identifying a striking characteristic that exists in only one or a few of the taxa in this family, e.g., this is an hoplolaimid, and it has scutella on body (i.e., it can only be an Hoplolaimus or a Aorolaimus/ Peltamigratus).
The expert immediate conclusion is often reached at low magnification, either with the dissecting microscope, or with low power objectives of the compound microscope. The dissecting microscope generally is used with live or freshly killed nematodes in a small dish filled with water, before they are mounted on slides. The compound microscope is used either as a second step in the identification of specimens already seen under the dissecting microscope, or it is used directly with specimens previously mounted on permanent slides. It can be noted that the highest magnification attainable with a dissecting microscope (about 40X) is similar to the low power of a compound microscope. Using this material, the expert recognizes at a glance, even before looking at fine morphological details, what is now defined as a promorph.
(Note : Promorphs and nests of species (below) were originally named protomorphs and nucleus, respectively. Both terms created conflicts, the first because os its shaky etymology (proto means primitive as pointed out by Dr. Coomans), the second because of possible confusion with biological elements (cell nuclei). Following lengthy discussion among the participants , Dr. Doucet proposed promorph, and Dr. Siddiqi proposed nest, and both terms were accepted and are used in the present volume.)
The Concept of Promorph A promorph (pro before; morph morphology) is a form that can be recognized at low magnification powers, before observation of detailed morphology.
The concept of promorph has no standing with the traditional zoological nomenclature codified in the International Code of Zoological Nomenclature. To clearly mark the difference between names of promorphs and other names, but at the same time to make it easy to associate a promorph name and a well known shape, promorphs will be named by P- and the abbreviated name of a representative genus as given in Table 1.
|Name of Promorph||Example|