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Some plants produce flowers but use them to reproduce asexually rather than sexually. Dandelions, blackberries, some citrus trees, and some other plants reproduce by the asexual production of seeds, which is called apomixis. In sexual reproduction, seeds form from the union of haploid gametes in the embryo sac. But in apomixis, one of two other things happens:
The megasporocyte in the ovule that is supposed to undergo meiosis fails to do so, resulting in a diploid egg cell, which then goes on to form an embryo and seed.
Diploid cells from the integument surrounding the embryo sac form a diploid embryo sac, and the sac goes on to form an embryo and seed.
In both cases, seed and fruit development proceed normally. But the genetic consequences are profound: apomixis produces clones.
Some agriculturally important plants are hybrids that are derived by crossing two genetically different varieties. Because these hybrids have two sets of chromosomes that are not homologous to one another, they cannot undergo meiosis and are sterile. This means they cannot form the seeds and fruits that are desired by people. In citrus, apomixis occurs naturally, which gets around this problem. Kentucky bluegrass, a mainstay of lawns, reproduces in this manner as well.
Many important crops, such as corn, are grown as hybrids because the progeny of a cross between two inbred, homozygous genetic strains are often superior to either of their parents, a phenomenon called hybrid vigor (see Key Concept 12.2). Unfortunately, once farmers have obtained a hybrid with desirable characteristics, they cannot use those plants for further crosses with themselves (selfing) to get more seeds for the next generation. You can imagine the genetic chaos when a hybrid, which is heterozygous at many of its loci (e.g., AaBbCcDdEe, etc.), is crossed with itself: there will be many new combinations of alleles (e.g., AabbCCDdee, etc.), resulting in highly variable progeny. The only way to reliably reproduce the hybrid is to maintain populations of the original parents to cross again each year. That is exactly what seed companies do. For farmers, it means buying new seeds every year—
If a hybrid carried a gene for apomixis, however, it could reproduce asexually, and its offspring would be genetically identical to itself. New hybrids could be developed that would be adapted to specific environments and could be propagated by the farmers on the spot. So an intensive search is on for genes for apomixis that could be introduced into desirable crops and allow them to be propagated indefinitely. Such a gene has been identified in corn, but the yield of the variety that contains it is low.