Molecular Analyses and Heterosis in the Vegetables: Can We Breed Them Like Maize?
- Michael J. Havey
Advantages of hybrid-vegetable cultivars include superior performance over inbreds or open-pollinated populations, greater uniformity for maturity or quality, and the commercial benefit of annual seed sales. Hybrid cultivars have been developed for many vegetable species with different reproductive systems and are produced using large-scale emasculation, cytoplasmic-genic male sterility, and genetic or chemical modification of sex expression. In maize (Zea mays L.), hybrids generated by crossing among inbreds from different heterotic groups usually perform significantly better than those generated by crossing among inbreds within the same group. Genetic distance estimates based on molecular markers have been used to assign maize inbreds to previously characterized heterotic groups and occasionally correlated with single-cross performance. For crosses among more closely related inbreds, genetic distance estimates did not predict single-cross performance for grain or forage yields. The failure of molecular-based genetic distance estimates to predict superior hybrid performance also has been reported for oat (Avena sativa L.) and soybean [Glycine max (L.) Merr.]. Compared with maize, many vegetable crops have a restricted genetic background and molecular markers reveal relatively few polymorphisms. For cucumber (Cucumis sativus L.) and onion (Allium cepa L.), phenotypically distinct populations show a significant reduction in variability at molecular marker loci. Vegetable crops with a narrow genetic background, such as cucumber, show little inbreeding depression and no significant heterosis. Although no direct comparisons between genetic-distance estimates and hybrid performance in vegetables have been reported, I expect that the narrow genetic background of many vegetable crops will restrict or preclude the identification of naturally occurring heterotic groups.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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