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A hybrid swarm is a population of hybrids which contains parent types, and offspring types, and intermediate types all mingled together and back-crossing and cross pollinating each other. Hybrid swarms are characterized by huge phenotypic differences between individuals.Uses
I use my hybrid swarms primarily as a source for new breeding material. Secondarily I use them as a means of preserving genetic diversity for future breeding projects. I maintain hybrid swarms for two species: Zea mays, and Cucumis melo. When I am looking for a trait that could be incorporated into a new variety or into an existing landrace I start by screening the swarm. For example in the winter of 2010/2011 I planted huge numbers of corn swarm seed in the fall, and several times during the winter, looking for seed that grows well after being frozen in soil. Some plants survived the frost and cold temperatures. After a couple of years of this treatment they will be good candidates to incorporate into a land-race of sugary enhanced sweet corn to add better cold soil emergence to a variety that suffers severe losses when planted in cold soil. An equally clever result of these tests would be to find seeds that carry perennial traits or are extremely frost tolerant.Example
On my farm I maintain a few sweet corn populations, and a popcorn population, and a flour corn population, and a decorative corn population, and a wild teosinte population. I also grow a hybrid swarm of corn which contains genes from all of those populations and many more. The only selection pressure that is put on this population is that it must produce harvestable seed in my garden in spite of my weeding habits, and the weather, and the soil, and the pests, etc. If anything interesting shows up in the hybrid swarm, it may be selected for further breeding work, or for inclusion into one of my landrace populations. My landraces undergo selection to conform to a set of expectations about how the plant should grow, or look, or yield, or taste, etc. My hybrid swarm only has to reproduce in my garden, and seed is saved from every plant that successfully reproduces.Problems
The genetic and phenotypic differences in a hybrid swarm can be huge, even to the point of causing incompatibility issues. For example my corn swarm contains parents from the tropics which are day-length sensitive and long season, and it also contains northern flints which are very short season and mature regardless of day-length. Staggering planting times, and hand pollination can help to bridge some of these differences. Natural or assisted back-crossing to intermediate types can help create a genetic bridge between phenotypes.
Blog: Mother Earth News -- Landrace Gardening.
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