A hybrid is a cross between two genetically different plants.
Hybrid plants that were deliberately developed by humans — as opposed to occurring naturally in the wild — first made a significant impact on society when development of superior hybrid corn varieties during the first half of the 20th century led to a spike in corn production that has continued to the present day.
Just as improvements in corn revolutionized farming and the American diet, genetic improvements in turfgrasses have significantly altered and enhanced golf course management practices. For many years, the turfgrass breeding program at the University of Georgia’s facility in Tifton, Ga., has focused on developing superior bermudagrasses for turf and for forage (Figure 1, above). In this article, we present some basic information about bermudagrass hybrids and the work that has been done at the University of Georgia.
Hybrid bermudagrass biology
Usually, better hybrids result from crossing more genetically diverse parents. Whether a hybrid is better than a non-hybrid in a specific plant species depends to a large degree on the genetic makeup of the parents of the hybrid. Consequently, just making crosses is no assurance of developing superior hybrids. The production of thousands of experimental hybrids may result in only a single superior hybrid (3).
Figure 2. Production of common hybrid bermudagrass.
Figure 3. Production of triploid interspecific hybrid bermudagrass.
Because bermudagrass is a sexual species with perfect flowers (male and female organs in the same flower), it can set seed through two mechanisms or a combination of these mechanisms. Bermudagrass plants can be either self-fertile or self-incompatible. In self-fertile plants, pollen from a flower on a plant can fertilize the female organs (stigmas) on the same plant and produce seed.
When plants are self-incompatible, pollen from a flower on a plant cannot fertilize the stigmas (pollen will not grow properly on the stigma) on the same plant to produce seed. To set seed, self-incompatible plants need to be fertilized by pollen from genetically different plants nearby. Many grasses, including bermudagrass, have various degrees of self-fertility or self-incompatibility. Self-incompatibility is controlled by several genetic and environmental mechanisms (2,3).
Crosses can be made within a species, and resulting plants are usually referred to as hybrids. Crosses between species of a particular genus are interspecific hybrids. The purpose of this manuscript is to clarify the difference between a hybrid (Figure 2) and a triploid interspecific hybrid (Figure 3) in the bermudagrass genus.
Common vs. hybrid bermudagrass
Common bermudagrass (Cynodon dactylon) is predominantly tetraploid (2n = 4x = 36 chromosomes) and produces outcrossed seed because of various levels of self-incompatibility (4). Because of this self-incompatibility, a large portion of the bermudagrass plant populations around the world consists of natural hybrids. Depending on the genetic makeup of the parents, some of the natural hybrids can be better than other plants in the immediate population. Improved plants are sometimes selected from these populations. Plants with improved and/or desirable plant characteristics can be marketed as commercial cultivars.
Figure 4. Pollen- and seed-sterile interspecific hybrid bermudagrasses are much easier to maintain than hybrids of common bermudagrasses, no matter whether they are in an athletic field, golf course, home lawn or landscaped area. Photo by Wayne Hanna
Turf and forage bermudagrass breeding programs usually focus on producing hybrids. University of Georgia programs have produced hybrid cultivars, such as Coastal and Tifton 85 forage bermudagrasses, that were superior hybrid plants selected from thousands of experimental hybrids produced by selecting, developing and crossing outstanding parents.
Examples of vegetatively propagated turfgrass hybrid bermudagrass cultivars are Tiflawn (a product of intentional crosses of superior parents), VaMont, Celebration and Tifton 10 (a fertile hexaploid with 54 chromosomes) bermudagrasses, which were selected from natural populations. Their identity is maintained through vegetative propagation of rhizomes, stolons and/or stems (1).
These hybrid cultivars have various levels of seed production potential from selfing, and from crossing, if pollen is available from different bermudagrass plants in the surrounding area. The seed production by these cultivars can easily produce off-types that can contaminate the purity of the planted cultivar. Although they are hybrids, these cultivars are technically common bermudagrasses with improved characteristics that are maintained by vegetative propagation (4).
The released common bermudagrass forage and turf hybrids have made important contributions to animal nutrition and improved landscapes (1). However, it should be remembered that common bermudagrass, especially C. dactylon, can produce deep rhizomes that are difficult to control and seed that is easily dispersed. These two characteristics can make these hybrid cultivars invasive in certain situations.
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