Cucurbit Genetics Cooperative Report 3:66-67 (article 36) 1980

Taxonomy of Cucumis callosus (Rottl.) Cogn. - The Wild Melon of India

V. A. Parthasarathy and C. N. Sambandam

Division of Horticulture, ICAR Research Complex for NEH Region, Shillong - 793 003, India (first author); Faculty of Agriculture, Annamalai University, TN-608 101, India (second author)

Cucumis callosus, a feral species of India, has attracted the attention of muskmelon breeders, as this species is reported to possess genes for resistance to fruitfly and leaf-eating caterpillars (9). But this species remains to be a controversy for taxonomists. The synonym given for this species is C. trigonus (2). While cytologists have shown that C. callosus (syn: C. trigonus) has 14 somatic chromosomes (3, 10), the breeding experiments of Naudin (6), Vavilov (11), and of Sambandam and Chelliah (9) have given evidence of free compatibility between C. callosus and C. melo. Hence, to obviate the confusion regarding the chromosome number as well as the relationship with other Cucumis species, the present study was undertaken.

Seeds of stocks maintained in the Division of Horticulture, Annamalai University, were used in the study. Counts of somatic chromosomes were made from the root tip cells following the procedure of Roy (8) using proppiono-orcein stain. To ascertain the compatibility with other Cucumis species, crosses were made (including reciprocals) with seven Cucumis species, namely, C. metuliferus, C. anguria, C. longipes (= C. anguria var. longipes), C. zeyheri, C. myriocarpus, C. dipsaceus, and C. melo. The success of each cross was determined by the percent fruit set and the number of well developed seeds. The F₁ fertility was determined by pollen fertility and viability (germination). Meiotic studies were carried our in the PMCs of F₁ plants to study the behavior of chromosomes during diakinesis.

The metaphase plates revealed the somatic chromosome number of 24 for C. callosus. The crosses with seven Cucumis species indicated that C. callosus was crossable with only C. melo. The number of well developed seeds per fruit in the cross C. callosus x C. melo was, on an average, 266 seeds and the seeds showed about 93% germination. The pollen mother cells showed normal bivalent formation, indicating cross compatibility between C. melo and C. callosus.

The somatic chromosome number observed from the study (2n=24) is disagreement with that of Singh and Roy (10) who reported the 2n number to be 14. It is interesting to note the report of Brown et al. (1) who stated that C. trigonus from India was mislabeled and in reality was C. hardwickii. Cucumis trigonus is the synonym for C. callosus (2). But the confusion in the taxonomy of Cucumis, especially those found in India, is due to the fact that names have been given based on the morphological differences. To substantiate this, the following statements of Hooker (5) and Gamble (4) are presented. While Hooker and Gamble described C. trigonus and C. pubescens as synonyms, chakravarthy (2) placed C. trigonus under C. callosus and C. pubescens under C. melo. Watt (12) made a statement very long ago which still holds good for Indian Cucumis. He stated much confusion exists regarding the Indian so called wild and cultivated species and varieties of Cucumis.

Based on the chromosome number, its free compatibility with C. melo, and normal behavior of chromosomes during diakinesis, we conclude that C. callosus does not warrant a separate species status and is nothing but a progenitor of C. melo. This confirms the earlier breeding experiments of certain workers (6, 7, 9, 11).

Literature Cited

1. Brown, G. B., J. R. Deakin and M. B. Wood. 1969. Identification of Cucumis species by paper chromatography of flavanoids. J. Amer. Soc. Hort. Sci. 94:231-234
2. Chakravarthy, H. L. 1959. Monograph on Indian Cucurbitaceae - Taxonomy and distribution. Records of the Botanical Survey of India 17: 98-112.
3. Darlington, C. D. and A. P. Wylie. 1955. Chromosome atlas of flowering plants. Allen and Unwin Ltd., London.
4. Gamble, J. S. 1923. Flora of the presidency of Madras. Adlard & Son Ltd., London, Vol. I, 377 pp.
5. Hooker, J. D. 1879. Flora of British India. L. Reeve & Co. Ltd., U.K., Vol. II: 619-620.
6. Naudin, C. 1859. Revue des Cucurbitaces cultivees au Museum en. Ann. Sci. Nat. Ser. 4 Bot. 12: 79-164.
7. Robinson, R. W. and E. Kowalewski. 1978. Interspecific hybridization of Cucumis. Cucurbit Genetics Coop. Rpt. 1:40.
8. Roy, R. P. 1972. Cytogenetical investigations in the Cucurbitaceae. USDA PL-480 Res. Project (Final Report). Patna Univ. 263 pp.
9. Sambandam, C. N.and S. Chelliah. 1972. Scheme for the evaluation of cantaloupe and muskmelon varieties for the resistance to the fruit fly (Dacus cucurbitae C.). USDA PL-480 Res. Project. Annamalai Univ. 67 pp.
10. Singh, A. K. And R. P. Roy. 1973. Karyological studies in Cucumis. Caryologia 27: 153-160.
11. Vavilov, N. I. 1931. Mexico and Central America as the principal centres of origin of cultivated plants of the New World. Bull. Appl. Bot. and Pl. Breed. 16: 135-199.
12. Watt, G. 1898. Dictionary of Economic Products of India. Vol. II: 626. Periodical experts, New Delhi, India.

The senior author thanks the Indian Council of Agricultural Research for the award of Senior Fellowship to him during the study.