Guayule, a shrub native to the Chihuahuan Desert that produces a variety of secondary compounds, is currently being investigated as a source of hypoallergenic natural rubber latex (Cornish et al., 2001; Ray et al., 2005). Although guayule has been known as a source of high-quality rubber since the beginning of the twentieth century, research on guayule has been very intermittent (Ray et al., 2005). Due to these intermittent research efforts, improvement of guayule has been slow (Dierig et al., 2001; Ray et al., 2005; Thompson and Ray, 1989).
Multiple strategies to improve guayule have been used during the sporadic periods of intensive research. Sexual diploids and a few naturally occurring sexual polyploids of guayule have been used in breeding programs for generation of intraspecific and interspecific hybrids (Esau, 1944; Gardner, 1946; Hashemi et al., 1989; Kuruvadi et al., 1997a; Naqvi et al., 1984). However, the most successful methods have used mass, family, or single plant selections (Ray et al., 1995, 1999, 2005; Thompson and Ray, 1989). A more recent tool for improvement of guayule is transformation (Pan et al., 1996) to insert genes from the rubber biosynthetic pathway into the plant in an attempt to increase rubber concentration and rubber yield (Veatch et al., 2005). This can be problematic because many of these products are also used in other plant pathways (Oh et al., 2000).
The economics of guayule production will improve significantly if higher-yielding lines can be developed using reliable and rapid methods of selecting plants with the best possible traits that will be passed faithfully to their progeny. One prediction method is to examine the linear correlation between different morphological traits. Correlations between traits aid in selection by using nondestructive measurements, such as plant height or width, to indirectly select for traits like rubber and resin content, which would otherwise involve destructive sampling of the plant. For example, rubber yield has been found to be positively correlated with fresh weight, dry weight, height, and width (De Rodríguez et al., 2001; Dierig et al., 1989b; Ray et al., 1993; Thompson and Ray, 1989). On the other hand, rubber concentration generally has a low positive or even a negative correlation with the traits that are posititively correlated with rubber yield (Dierig et al., 1989b; Ray et al., 1993).
Successful breeding programs focus on characteristics with high heritabilities, indicating low environmental effects. Heritability estimates of the traits associated with rubber concentration and rubber yield have been variable. In a study by Dierig et al. (2001), variance from measured traits of clones represented environmental variance and were compared with variances from traits of open-pollinated progeny, representing the phenotypic variance (G × E). They found that height, resin concentration, and rubber concentration had high broad-sense (BS) heritabilities at 2 years of growth, but by the third year, BS heritability was almost zero as environmental effects compounded. In another study, using path-coefficient analysis, all traits measured at 1 year were found to be highly heritable in California but much less heritable in Arizona (Estilai et al., 1992). In a third study, when progeny and parents from single plant selections were compared when they were 2 and 3 years of age, respectively, no significant regressions were found between parents and progeny, which is indicative of low heritabilities (Ray et al., 1993). Additionally, resin and rubber concentration have been difficult to predict from year to year, possibly because of large environmental effects (Coffelt et al., 2005; Dierig et al., 1989a).
Improvement of guayule is further complicated because of its relatively long generation time (2–5 years) and its complex genetic and reproductive system (Ray et al., 2005; Thompson and Ray, 1989). Guayule has a haploid chromosome number of 18, with a natural ploidy series ranging from diploid to tetraploid or higher (Bergner, 1944; De Rodríguez et al., 1993; Powers, 1945; Thompson and Ray, 1989). In native populations ≈95% of the accessions surveyed are polyploid, with the majority being tetraploid (Kuruvadi et al., 1997a). Diploid guayule plants reproduce sexually, but tetraploid guayule reproduces predominantly by facultative apomixis, which is embryo development without fertilization. However, even in apomictic plants pollination is required for endosperm development (Esau, 1944; Ray et al., 1990). The exact frequency of apomixis in guayule has yet to be determined, but it is known to vary by line and season (Keys et al., 2002; Thompson and Ray, 1989). Sexually reproducing plants are self-incompatible with a sporophytic system of self-incompatibility (Gerstel, 1950; Ray et al., 1993). This system of enforced outcrossing promotes variation within cultivated and native populations.
Most of the guayule germplasm, upon which the present University of Arizona and USDA-ARS breeding program is based, comes from only a few plants (Thompson and Ray, 1989). Despite this apparently narrow genetic base, there appears to be a large amount of variation in cultivated guayule (Ray et al., 2005), not only among lines but also within lines (Dierig et al., 1989a; Naqvi, 1985; Ray et al., 1990). There is also a great deal of unexploited variation available in native populations (De Rodríguez et al., 1993; Kuruvadi et al., 1997b). This variation could be a valuable tool in guayule improvement; however, methods that determine the genetic contribution to this variation and subsequent response to selection need to take into account guayule's unusual mode of reproduction. The objective of this study was to estimate narrow sense heritability in guayule using a method that more accurately accounts for the contributions of apomictic and sexual reproduction.
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