Mechanisms for Hybrid Development in Vegetables
SUMMARY. Availability of cost effective mechanism/method to pro duce large-scale F,seeds utilizing selected parental lines is an important factor, which ultimately determines the commercial viability of the hy brid varieties. In vegetables, although experimental crosses (few seeds for research purpose) can be developed through manual emasculation (in case of hermaphrodite crops) followed by manual pollination of emascu lated flowers or pistilate flowers (in case of monoecious crops with sepa rate staminate and pistilate flowers) seed production of commercial hybrids (large quantity of seeds for cultivation) based on such methods is economically feasible only in tomato, eggplant, sweet pepper, cucur bits and few other vegetables, in which a large number of F, seeds are obtained from one manually pollinated crossed fruits. Ever since (since 1930s) the discovery of male sterility (in onion) and self-incompatibility (in cabbage) mechanisms and their proposed utilization in hybrid seed production, several mechanisms and methods have been evolved for the development of experimental and commercial hybrids. This chapter de scribes genetic (inherited) and non-genetic mechanisms utilized for hybrid development in selected vegetable crops with special reference to male
KEYW ORDS. Apomixis, auxtrophy, CHA, gynoecism, hybrids, male sterility, self-incompatibility, sex regulation, vegetables
The successful demonstration of maize hybrids in 1920s promoted studies to examine possible exploitation of heterosis in the form of F, varieties in several crop species. Among the vegetables, first F, hybrid of eggplant was re leased during 1924 in Japan (Nishi, 1967). Subsequently, hybrids of water melon (1930), cucumber (1933), radish (1935), tomato (1940), and cabbage (1942) were developed (Liedle and Anderson, 1993). Seeds of most of these hybrids were produced through natural crossing exploiting competitive fertil ization between self and cross pollen. Thus only 40-80% of the seeds were ac tual hybrids (Liedle and Anderson, 1993), which were far below than the current acceptable level for contamination in hybrid seeds. Therefore, search for methods to produce pure hybrid seeds at commercial scale was realized. Pearson (1933) using self-incompatibility mechanism in cabbage and Jones and Clarke (1943) using cytoplasmic male sterility mechanism in onion, pro posed the methods to produce large scale pure hybrid seeds. Now a day, F, hy brid breeding method is commonly utilized to exploit heterosis in several economically important vegetables including tomato, eggplant, hot and sweet peppers, onion, cabbage, cauliflower, other cole crops, radish, carrot, melons, etc. Vegetable breeders prefer to select hybrid breeding because it is compara tively easy to incorporate resistant genes for biotic and abiotic stresses in F 1 hybrid and right of the bred variety is protected in terms of parental lines. Moreover, despite high cost of hybrid seeds, there has been increasing concern of the farmers on the cultivation of hybrids. This is because under optimum crop production and protection management, crop raised from the seeds of Fj hybrid has several advantages like better yield, adaptability, uniformity and reactions to certain stresses in comparison to crop raised from the seeds of im proved pure line or population.