- Wehner and Jenkins (1985) measured a natural outcrossing rate of 53% (36% between row).Some have reported heterosis from specific hybrid combinations (Ghaderi and Lower, 1979), especially when crossing diverse parents (Hayes & Jones, 1916).However, others have found no heterosis when parents were of similar type (Hayes & Jones, 1916). In a study involving three years, two replications and eight locations in the USA (Wehner et al., unpublished data), the total once-over harvest yield of the popular gynoecious hybrid ‘Calypso’ was 60 fruits per plot, while the gynoecious inbred parent Gy 14 had 61 fruits per plot.Where breeders have worked to develop elite hybrids, heterosis may average only a 5% advantage.
- In cucumber gherkins and melons hybrids are produced commercially using a gynoecious inbred as the female parent crossed with a monoecious inbred as the male parent, and honey bees as pollen vectors.Gynoecious inbreds can be developed by self pollination of plants that have been treated with silver nitrate or other ethylene inhibitors (Tolla & Peterson, 1979).One advantage of producing F1 with gynoecious female parent by crossing with monoecious male parentis that the resulting hybrid will be gynoecious, having pistillate flowers at every node.If the F1 hybrid is not parthenocarpic for commercial crop a monoecious pollenizer must be mixed in with the hybrid.Gynoecious cultivars are earlier and sometimes higher yield than monoecious cultivars since they have pistillate flowers at every node (Wehner & Miller, 1985).
- Melon has no inbreeding depression, but some crosses were reported to express 3% heterosis for earliness and 8% for yield (Lippert & Legg, 1972). Andromonoecious lines had only 20 to 35% natural outcrossing (Whitaker & Bohn, 1952), so the lack of inbreeding depression might be explained in the same way as for cucumber.Hybrids are produced by hand pollination of emasculated perfect flowers on the female parent using staminate flowers from the male parent. Gynoecious inbreds have been produced, but fruits from pistillate flowers are oval, rather than round like those produced from perfect flowers. Also, gynoecy is controlled by several genes and is complex to work with.The use of cytoplasmic genic male sterility would reduce hand labour requirements for crossing in hybrid seed production blocks.
- Summer squash (Cucurbita pepo L.) has been shown to have heterosis for yield, with 11 to 84% hybrid advantage over open pollinated cultivars in zucchini types and 0 to 82% advantage for yellow types (Elmstrom, 1978).Hybrids are produced by crossing two monoecious inbred lines using honey bees as the pollen vector. One of the inbreds is made gynoecious for the first two to three weeks of flowering by spraying the plants with ethephon at the two and four-leaf stage. Hybrids are used in 52% of the U.S. crop. On the other hand, winter squash (Cucurbita maxima Duch and C. moschata Duch) is difficult to cross using monoecious inbreds since ethephon does not work well to change sex expression. Heterosis for yield in hybrid cultivars may be 40% higher than for the open pollinated ones, and an increasing proportion of the crop is planted to hybrids.
- Watermelon is a monoecious, annual outcrossing species with little inbreeding depression.Heterosis is expressed for yield in some parental combinations, perhaps averaging 10%; however, the main use of hybrids has been to protect the parental inbreds and to produce seedless fruits. Seedless hybrids are triploids produced by crossing a tetraploid female parent with a diploid male parent. Tetraploids are produced by doubling the chromosome number of elite diploid inbred lines. Once the triploid F1s are planted for commercial production of water melon fruits, a diploid cultivar must be planted in alternating rows with such triploid cultivar to provide the pollen required for fruit set, so seeded watermelons are produced along with the seedless ones.
- The benefits provided to the world by heterosis in the vegetable crops is dependent on the crop. In the case of bean, pea and lettuce, there has been no effect of heterosis. In other cases, such as tomato, onion, cabbage and asparagus, there have been significant savings of agricultural land and a large increase in our ability to feed people as a result of heterosis.
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