Morphological Variation in Pratylenchus Penetrans

JOURNAL OF NEMATOLOGY VOLUME 8 JULY 1976 NUMBER 3 Morphological Variation in Pratylenchus penetrans RODRIGO TARTE and W. F. MAI ~ Abstract: Variability of morphological characters used to separate Pratylenchus penetrans from other species of the genus was studied in a population originating from a single gravid female. Pronounced heteromorphism was observed and characterized. About 30% of females had a crenate-tail terminus. Several shapes of styler knobs were characterized; 50% of them were anteriorly flattened to indented. The outer margin of the cephalic framework extended into the body from one-half to two annules. The shape of the spermatheca varied from round to oval. A fifth lateral line was observed in many specimens. Environmental factors, and particularly the host plant, influenced such morphometric characters as body length, width, esophagus length, stylet length, V value, a and b" ratios, as well as qualitative characters such as tail terminus, growth of ovary, and shape of median bnlb. Nematodes reared on pea and cabbage had a higher percentage of females with a crenate-tail terminus than those from totnato, rye, beet, and alfalfa callus culture. Nematodes from peas were longer aml wider; they often had gonads that extended to esophagi, but they had shorter esophagi amt stylets than those from callus culture. Popula- tions from different geographical locations also exhibited variahility in morphological characters, as did the Cornell population. The validity of many characters used in species identification is discussed, and the possibility that other related Pratylenchus species are conspecific with P. penetrans is suggested. Key Words: taxonomy, crenate tail, smooth tail, host effect, selective response. Knowledge of intraspecific variability of during nematode development (4, 7, 9, 19). many morphological characters used in tax- Induced variability has been considered in onomy of plant-parasitic nematodes is often relation to population density (10), method lacking. As a result, the taxonomic value of of killing and fixation (13), environmental many such characters is doubtful, and factors (10), host species or growth medium proper identification is made very difficult. (1, 10, 11, 12, 14), and host physiology (3). Investigators concerned with morphological Differences related to geographical location variation have conducted research mainly have also been suggested in a few cases (17, on quantitative characters. The magnitude 29). of variations cannot be estimated unless de- The genus PratyIenchus contains ap- tailed studies on morphology and frequency proximately 40 described species, many of of variants, as they occur naturally within which differ only in small morphological different populations and as they are af- features, lntraspecific variation has been fected by ecological factors, are conducted. suggested by Taylor and Jenkins (30) and Naturally occurring, intraspecific varia- by Loo[ (19), and it has been carefully stud- bility in nematodes is considered in few tax- led by Roman and Hirschmann (25). No onomic reports and reviews (4, 5, 6, 15, 23, attempt has been made to study variability 25, 31). Variation has been associated with in living populations under a wide range of some aspects of nematode behavior (2, 4, 7, environments, or to determine the extent of 18, 22), as well as with changes occurring variability in different geographical populations. Received for publication 9 September 1974. The present study was undertaken to de- 1Graduate Assistant and Professor, respectively, Department of Plant Pathology, Cornell University, Ithaca, New York termine tile extent of morphological varia- 14853. Present address of senior author, Facultad de Agl"on- tion in Pratylenchus penetrans (Cobb, 1917) omia, Universidad de Panama, Panama, R.P. The authors wish to thank the following persons for supplying cultures Filip. & Schuurm.-Stekh., 1941, especially of P. penetran.~: J. L. Townshend, Canada; D. C. M. with respect to those diagnostic characters Corbett, England; H. Inagaki, Japan; M. Oostenbrink, The Netherlands; N. D. Singh, Trinidad; A. C. Tarjan, used at the species level. The effects of dif- Connecticut; D. E. Stokes, Florida No. 1; A. J. Overman, ferent environments and the geographic Florida No. 2; G. Thorne, Kentucky; and K. R, Barker, North Carolina. source of populations were studied. The JOURNAL OF NEMATOLOGY for April (8: 97-184) was issued 20 April 1976. 185 186 Journal of Nematology, I'olume 8, No. 3, July 1976 MATERIALS AND METHODS Variation of 8 quantitative and 10 qualitative and meristic characters, most of A population of P. penetrans originat- them commonly used in species differentia- ing in 1963 from a single gravid female was tion, was studied. Quantitative data were propagated on alfalfa callus tissue growing analyzed by the analysis of variance and by on modified ~Vliite's medium at approxi- Tukey's honestly significant-difference pro- mately 23 C in the dark (24). Extraction for cedure (THSD) at tlle 5% probability inoculation and microscopic observation level. Coefficients of variability were also was carried out by placing pieces of infected calculated. callus on a fiber filter supported by a screen in a shallow layer of water in a pie-pan. Cultures used were not necessarily of the RESULTS same age nor were they inoculated at the same time in the various experiments. CHARACTERIZATION OF VARIATION IN THE Additional fixed specimens of P. pene- CORNELL POPULATION OF P. penetrans: The trans were obtained for study from: The diagnostic characters of P. penetrans (28) Netherlands, Japan, Canada, Trinidad, consist of three lip annules, broadly England, and from the states of Maryland, rounded stylet knobs, lack of striations New York, North Carolina, Florida, Ken- around tile felnale-tail tip, round sper- tucky, and Washington of the U.S.A. matheca, short post-uterine branch, ovary not extending to esophagus, four lateral In experiments designed to determine lines, and outer margins of sclerotized labial the influence of environmental factors on framework extending posteriorly into the quantitative and qualitative characters, body about one body annule. Microscopic seeds of Wando peas, Pisum sativum L., observations of more than 500 females ob- were germinated in sterile sand and the tained from alfalfa callus tissue, however, seedlings transplanted 3 days later to 5-cm revealed extensive heteromorphism in most clay pots which contained a sandy loam characters. orchard soil (pH 6.7) with a low nitrogen Stylet knobs varied in shape from slop- and phosphorus content. Pratylenchus pene- ing anteriorly to being indented (Fig. 1), trans was then added by placing a suspen- with about 50% being anteriorly flattened sion of 500-1,000 nematodes close to the to indented. root tip of each seedling. One week after Female tails were usually rounded at the inoculation, intact soil balls were trans- tip, but truncated, pointed, or somewhat ferred to 20-cm clay pots where they re- clavate tails were also found. The tail mained for the duration of tlle experiment. terminus varied from smooth to distinctly Unless indicated otherwise, peas were re- crenate. About 30% of the nematodes planted twice during an experiment. No possessed coarsely annulated or crenate-tail fertilizer was applied except when the ef- tips in which one or more indentations, fect of nutrition was tested in a particular forming annules usually wider than the treatment. body annules, occurred at any point around At harvest, nematodes were extracted by the terminus but mostly toward the dorsal cutting the roots in pieces 1/2 to 1 cm long, side (Fig. 2). Coarsely annulated tail tips, blending them with water for 60 sec, pour- independent of number and location of ing the suspension on a paper filter, and indentations, were classified as crenate. collecting the nematodes after 48 h. When Crenate-tail tips, with one and two indenta- nematodes were extracted from soil, 100 ml tions oriented toward the dorsal side, were of soil were spread evenly over a fiber filter most common. The remaining 70% of the supported by a screen in a shallow layer of nematodes had typically smooth tails. Al- water and the nematodes collected after though, in some cases annulation continued 24 h. posteriorly on one side, it was never coarse, Adult females picked at random were and the tail tip was smooth. used in morphological observations. They The outer margin of the sclerotized were killed by being immersed in hot water cephalic framework occasionally extended (65 C) for 2 rain and fixed and mounted in from one-half to two annules posteriorly 2.5% formalin. on the body, but in most specimens, it Variation in P. penetrans: Tarte, Mat 187 MORPHO~fETRICS: Effects of host.--Ten ran- domized replicates of five host plants: 'Wand& pea (Pisurn sativum L.), rye (Secale cerale L.), 'Rutgers tomato' (Ly- copersicon esculentum Mill.), 'Round-up' cabbage (Brassica oleracea capitata L.), and 'Ruby Queen' beet (Beta vulgaris L.) were inoculated with a suspension of P. penetrans and kept for 6 months in the gTeen- house at an average temperature of 21 C. After completion, 50 randomly picked females fi'om each treatment were measured and studied. Treatments were compared ( ) ( with each other and with 50 females previ- ously preserved from the initial inoculum. Differences among treatments were found in body length, width, esophagus length, stylet length, and ratios a and b' (Table 1). The longest and widest nematodes were re- covered from peas, and the next longest (in descending order) were from the initial inoculum, rye, tomato, cabbage, and beet, respectively. The stylets in nematodes from all host plants were slightly shorter than the stylets in nematodes from the initial inocu- ltnn. Although esophagus length was about the same in nematodes reared on tomato and pea, tile b' ratio differed because of differences in body lengtla of the nematode in the two treatments. No differences were found in V. value. Observations of certain qualitative characters revealed that, in 30% of females FIG.

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