Allozyme Diversity at the Pgi-2 Locus of Citrullus lanatus from Zimbabwe

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Cucurbit Genetics Cooperative Report 14:104-106 (article 36) 1991

D. Benscher and R. Provvidenti
Department of Plant Pathology, Cornell University, new York State Agricultural Experiment Station, Geneva, NY 14456

Toll and Gwarazimba reported in a983 to have collected 134 landraces of watermelon (Citrullus lanatus) in Zimbabwe (5). In 1986, we received from the USDA Germplasm Resources, 56 accessions of that collection kindly provided by V. Gwarazimba of the Harare Research Station. The primary purposeof our request was to find sources of resistance to strains of zucchini yellow mosaic virus (ZYMV).

A few isozyme loci have been demonstrated to be ideal molecular markers for viral resistance genes in some vegetables (6, 7). Using the information generated by Navot and Zamir (2), regarding the linkage relationship of 19 isozyme genes in C. lanatus and C. colocynthis, we directed our efforts toward finding a locus which might be useful as a marker for ZYMV reistance. In a preliminary study involving single plant selections resistant to the Florida strain of ZYMV and those of several commercial cultivars, we noted that although all lines possessed two loci for phosphoglucoisomerase (Pgi-1 and Pgi-2), two different banding patterns were evident, for the Pgi-2 regionl. One band was comon to all the commercial cultivars, while a second band occured in plants resistant to the virus. Since multiple allelism for the Pgi-2 locus had been reported for other crops (3), we extended our tests to all accessions from Zimbabwe, as well as a number of watermelon cultivars of domestic and foreigh origin. Enzyme extraction, starch gel electrophoresis, and enzyme staining were performed according to known protocols (4, 8). Individual samples of cotyledon or leaf tissue of six plants of each line were homogenized in 0.01M potassium phosphate extraction buffer (pH 7.0) and crude buffer, 11% starch gel at 250 VDC, for 3 to 4 hr at 4C.

The results can be summarized as follows:

  1. All PI’s, from Zimbabwe, as well as commercial watermelon cultivars, contained Pgi-1 and Pgi-2 loci;
  2. The Pgi-1 locus appeared to be monomorphic, a condition which has been reported for other plant species (1). Polymorphism was noted in the Pgi-2 region, where three bands were noted: a, b, and c. In each individual plant they occured either as a single-band (homodimer) for homozygous condition, or as a triple-band (heterodimer) for heterozygous condition;
  3. The fast homodimer band (aa) was confined to PI’s: 482273, 482308, and 482361;
  4. The intermediate homodimer band (bb) was noted in PI’s: 482252, 483353, 482261, 482273, 482286, 482289, 482293, 482398, 482399, 482316, 482319, 482321, 482322, 482333, 482342, 482356, and in the cultivar Knight from Taiwan.
  5. The slow homodimer band (cc) was present in PI’s: 482250, 482253, 482256, 482267, 482269, 482318, 482334, 482341, 482342, 482343, 482345, 482346, 482347, 482348, 482349, 482362, 482363, 482364, 482365, 482366, 482368, 482369, 482370, 482371, 482371, 482373, 482375, 482376, 482377, 482378, and 482381. It occured also in the domestic cultivars Charleston Gray, Crimson Sweet, Dixie Queen, Florida Giant, Keckley Sweet, Jubilee, New Hampshire Midget, Sugar Baby and in the Taiwanese Early Klondike, Empire No. 2, Farmer’s Giant, Fengshan No. 1, Flower Dragon, Grand Baby, Jumbo, New Dragon, Petite Yellow, and Yellow Baby.
  6. The original seeds (collected in Zimbabwe) were from open pollinated fruits, hence, a certain degree of heterogeneity was anticipated. Consequently, some plants contained triple-banded patterns resulting in a combination of ab, bc, or ac. The combination of ab was present in a few plants of PI’sL 482273, 482286, 482289, 482308, and 482333; the combination bc was evident in some plants of PI’s: 482253, 482316, and 482356; and the combination ac was encountered in one plant of PI 482348. The generation of active heterodimers between monomers is typical of allelic products.
  7. All the plants posessing aa or bb bands shared a number of common charcteristics including large size, late maturity, large fruits with a yellowish flesh and low sugar content. Although fruit shape and skin color varied considerably, all had long storage capabilities. In addition, most of the plants appeared to be cold tolerant. Our designation a, b, and c implies that there is a multiple allelic system at the Pgi-2 locus of C. lanatus. Genetic studies are presently under way to confirm this hypothesis and determine whether homodimers aa or bb can be used as markets for ZYMV resistance and cold tolerance.

Literature Cited

  1. Gottlieb, L.D. 1981. Electrophoretic evidence and plant populations. Prog. Phytochem. 7:1-46.
  2. Navot, N. and D. Zamir. 1986. Linkage relationship of 19 protein coding genes in watermelon. Theor. Appl. Genet. 72:274-278.
  3. Parfitt, D.E. and S. Arulsekar. 1989. Inheritance and isozyme diversity for GPI and PGM among grape cultivars. J. Amer. Soc. Hort. Sci. 114-468-491.
  4. Tanksley, S.D. and T.J. Orton (Eds.) 1983. Isozymes in plant genetics and breeding. Elsevier/North Holland Press. Amsterdam, Holland.
  5. Toll, J. and V. Gwarazimba. 1983. Collecting in Zimbabwe. Plant Genet. Resources Newsl. FAO/IBPGR 53:2-5.
  6. Weeen, N.F. amd R/ Provvidenti. 1988. A marker locus, Adh-1, for resistance to pea enation mosaic virus in Pisum sativum, J. Hered. 79:128-131 .
  7. Weeden, N.F., R. Provvidenti and G.A. Marx. 1984. An isozyme marker for resistance to bean yellow mosaic virus in Pisum sativum. J. Hered. 75:411-412.
  8. Wendel, J.F. and N.F. Weeden. 1989. Visualization and interpretation of plant isozu,es. p 5-45. In D.E. Soltis and P.S. Soltis (eds). Isozymes in Plant Biology. Dioscorides Press, Portland, OR.