The release of ‘Tifgreen’ bermudagrass in 1956 launched the era of vegetatively propagated turfgrasses using a production method that preserved the initial hybrid vigor of new cultivars barring genetic instability (Hanna and Anderson, 2008). ‘Tifgreen’ originated from a cross between an Egyptian Cynodon transvaalensis introduction and a common bermudagrass (Cynodon dactylon) from a North Carolina golf course green (Hein, 1961). This cultivar can be mowed daily at a plant height of 4.7 mm, which at the time of its release represented a breakthrough in putting green quality. The ability to uniformly mow ‘Tifgreen’ at lower heights spurred golf courses across the southeast to replace their common bermudagrass greens (Hanna and Anderson, 2008).
Well-defined areas with noticeable differences in plant morphology (i.e., plant color, leaf size, and internode length) were observed on ‘Tifgreen’ putting surfaces soon after its release. Several of these “off-types” were isolated from putting greens in Florence, SC, and Sea Island, GA, and further evaluated at lower mowing heights than ‘Tifgreen’ could tolerate. The term off-type refers to a plant that has a different growth habit, density, texture, and color that disrupts uniformity in monoculture plantings (Capo-chichi et al., 2005).The off-type selected from Florence Country Club was released as ‘Tifdwarf’ (Burton, 1966; Elsner, 1966) and became one of the most widely used grasses on greens in the warmer regions of the world.
Off-types found in ‘Tifgreen’ or ‘Tifdwarf’ make up the latest generation of bermudagrass for putting greens. These cultivars include Champion, Classic Dwarf, FloraDwarf, Jenson, Jones Dwarf, MiniVerde, MS Supreme, Pee Dee 102, Quality, Reesegrass, and TifEagle (Harris-Shultz et al., 2010; McCarty and Canegallo, 2005). ‘Champion’, ‘FloraDwarf’, ‘Jones Dwarf’, ‘MiniVerde’, ‘MS Supreme’, ‘Tifdwarf’, and ‘TifEagle’ were highly genetically similar to ‘Tifgreen’ when DNA fingerprinting was used and appear to be somatic mutants (Capo-chichi et al., 2005; Harris-Shultz et al., 2010; Wang et al., 2010). Although many cultivars have been derived from ‘Tifgreen’ and ‘Tifdwarf’, most mutations are deleterious. Inconsistencies in appearance, playability, response to environmental conditions (i.e., nutrient and water availability), herbicide application, etc., have resulted in severe problems and millions of dollars in loss to the golf course industry and sod farms (Caetano-Anolles et al., 1997).
Off-types in bermudagrass can be generated by either clonal variation or by contamination from an unrelated bermudagrass through seed germination or sprig introduction. Clonal variation can be the result of epigenetic modification in response to the environment, from the presence of plant pathogens, or, more commonly, from mutations that occur during growth (Pelsy, 2010). Genetic mutations can be induced by chemicals such as herbicides that affect microtubule formation during mitosis (Capo-chichi et al., 2005) or can occur at random by a natural process such as transposon activity and problems with DNA repair (Leroy and Leon, 2000; Wessler, 2001). A mutation in one cell of a layer of the shoot apical meristem increases by mitosis and produces a mutated sector (Hocquigny et al., 2004). These plants are periclinal chimeras because one or two entire cell layers of the apical meristem are genetically distinct from the adjacent layers. Periclinal chimeras are often stable and can be maintained by vegetative propagation of stems (Dermen, 1960) and are common in long-lived clonally propagated crops (Franks et al., 2002).
Grapes (Vitis vinifera), like bermudagrass, have cultivars that are maintained by vegetative propagation to create clones that are genetically identical to the parent plant assuming that no somatic mutation occurred in the regenerating cells that gave rise to the clone, yet similar to bermudagrass, grape clones as well as many species propagated vegetatively for commercial horticulture often display phenotypic variation (Gill et al., 1995; Pelsy, 2010). When microsatellite markers were used among clones from many grape cultivars (which are diploids), three to four alleles were detected (Franks et al., 2002; Hocquigny et al., 2004). This increase in allele number was the result of expansion or contraction of the repeat motif in a cell layer of grape forming a periclinal chimera (Hocquigny et al., 2004). When the cell layers of the periclinal chimera ‘Pinot Meunier’ were separated by somatic embryogenesis, the regenerated plants not only had distinct genotypes, but had novel phenotypes as compared with the parental plant (Franks et al., 2002). It has been noted that it is easy to miss instances of microsatellite chimerism because minor peaks/bands are often generated (Franks et al., 2002). The genotypes of the cell layers in grape have been determined by studying the roots and sexual progeny because both are descended from the L2 inner tissues and the leaf is derived from L1 and L2 of the plant meristem (Thompson and Olmo, 1963). To our knowledge, the number of cell layers in the tunica of the shoot apical meristem of bermudagrass has not been determined. In monocots, the numbers of tunica cell layers vary from one to four with one and two predominating (Khurana et al., 2004). For example, the shoot apex of wheat (Triticum aestivum) consists of both the L1 and L2 cell layers (Simmonds, 1997), whereas the shoot apex of maize (Zea mays) is composed of one layer of L1-derived cells (Jackson and Hake, 1997).
The close genetic similarity of cultivars within the ‘Tifgreen’ family has made it very difficult to distinguish each cultivar from one another using morphological and DNA fingerprinting methods (Harris-Shultz et al., 2010). This creates problems in the protection of cultivar proprietary rights and plant stock certification. The objectives of this study were to identify SSR markers that can distinguish between genotypes in the ‘Tifgreen’ family and to characterize these polymorphisms in each ‘Tifgreen’-derived cultivar.
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