Isozyme polymorphism in melon (Cucumis melo L.), and application to seed purity test of F₁ cultivars

Faculty of Agriculture, Okayama University, Okayama 700- 8530, Japan Isozyme polymorphism was studied for melon cultivars, in order to identify suitable markers for seed purity test of commercial F₁ cultivars of Japanese muskmelon. Intraspecific variation was detected by the analysis of acid phosphatase (APS) isozymes. A total of 108 melon cultivars, covering five varieties of C. melo, was classified into five zymogram types according to APS zymogram. As for pure line cultivars, D type was commonly found in all the varieties, while C type was specific to var. inodorus. F₁ cultivars of muskmelon were all classified as A type which was considered to be homozygous at APS loci, and thus it was impossible to test seed purity by the analysis of APS isozymes. On the other hand, the analysis of peroxidase (POX) isozymes enabled to classify melon cultivars into three major groups (Px(2A), Px(2B), and hetero-type). Breeding lines for disease resistance belonged mostly to the Px(2A) type, and pure line derivatives of a high fruit quality cultivar 'Earl's Favourite' were all classified as Px(2B) type. It was thus suggested that the demand for both fruit quality and disease resistance resulted in the breeding of hetero-type cultivars, which accounted for half of the F₁ cultivars. For hetero-type cultivars, it is possible to detect the contamination by selfed seeds of the female parent by the analysis of a single POX marker, and it takes only nine days for analyzing seed samples. Purity test for seed samples of four commercial F₁ cultivars showed that contamination sometimes occurred at a low frequency. The results clearly indicated that this method is practically applicable to seed purity test for many cultivars of Japanese muskmelon.