Genome-Wide Genetic Diversity among Components Does Not Cause Cultivar Blend Responses

vars had greater grain yield than their component pure lines on average. Furthermore, they reported that blend Genetically diverse plant populations may be better able to exploit response varied among different combinations of culecological resources and reduce interplant competition than genetitivars. cally homogeneous populations. Cultivar blends can have greater proMost oat, soybean, and wheat cultivars are pure lines. ductivity and yield stability than pure lines; however blend effects are not consistent. The varying levels of genetic diversity represented in The available pure-line cultivars represent a wide range blends may confound the interpretations and comparisons of the reof unique, homozygous genotypes often adapted to spesults of different blend studies. We tested the hypothesis that genetic cific environmental conditions. A genetically variable diversity of blend components is related to blend performance by plant population will often have a greater chance of evaluating blends of a set of five early-maturing and a set of 10 successful adaptation across a range of environments midseason-maturing oat (Avena sativa L.) cultivars in two separate than a genetically homogeneous population (Allard and experiments at eight Iowa environments. Within each experiment, Bradshaw, 1964). A cultivar blend capitalizes on this pure lines and all possible two-way blends were evaluated for grain principle by using a mixture of two or more pure-line yield and test weight means and stability and adaptability parameters. cultivars grown in the field at the same time in an atThe genetic diversity of each blend was estimated by pedigree diversity [1 coefficient of parentage (COP)], amplified fragment length polytempt to achieve greater yield and yield stability. morphism (AFLP)-derived genetic distances (1 Dice coefficient), Ecologists have demonstrated that increasing species and phenotypic diversity (based on height and heading date differdiversity contributes to greater ecosystem productivity ences). Blend response was limited in these experiments and was not and stability (Tilman et al., 1997; van der Heijden et correlated with any diversity measure, and blend stability parameters al., 1998; Hector et al., 1999). The benefits of genetic were not consistently related to diversity measures across experiments. diversity within single-species populations have also We also investigated the relationship between pedigree diversity and been demonstrated. Overcompensation, the ability of blend performance in other crops by computing the coefficients of genetically diverse populations to use resources more parentage of cultivar pairs used in previous blend studies in maize, efficiently and successfully than monomorphic populasoybean, and wheat. Pedigree diversity was correlated with higher blend response only in two of 10 experiment–environment combinations, has been documented in Drosophila melanogaster tions tested. Genome-wide genetic diversity alone does not cause (Peng et al., 1991). Similarly, Cole and Wiernasz (1999) positive crop blend responses. found there was a positive relationship between colony fitness and genetic diversity measured at two isozyme loci in harvester ant [Pogonomyrmex occidentalis (CresS of cultivar blends over pure-line cultison)] colonies. It is not known whether this principle vars has been observed in numerous crops, includalso extends to within-species genetic variation in crops. ing soybean [Glycine max (L.) Merr.], oat, wheat (TritiIf it does, we expect that increasing the genetic diversity cum aestivum L.), barley (Hordeum vulgare L.), sorghum among blend components should result in greater blend [Sorghum bicolor (L.) Moench], cotton (Gossypium hirresponse and yield stability. We hypothesized that a sutum L.), and rice (Oryza sativa L.) (Smithson and confounding factor contributing to the variability among Lenné, 1996); however, research in oat, soybean, barley, results of previous blend experiments was the level of maize (Zea mays L.), and wheat has also indicated that genetic diversity among cultivars included in the blends the effects of blending cultivars vary. In some cases, under study. If high levels of genetic diversity between blending may result in no significant gain in yield or the components of a blend are important for increasing no reduction in disease damage, and may even have blend response, the response may be variable among negative effects (Smithson and Lenné, 1996). For examblends and among experiments. ple, Frey and Maldonado (1967) found a significant inThe objective of this experiment was to determine crease in mean blend yields over component pure-line the relationship between genetic diversity (as estimated yields in oat only in high-stress environments. Similarly, by genetic distance, COP, and phenotypic differences) Helland and Holland (2001) reported that blends of and blend response, blend stability, and blend adaptabilearly-maturity, but not midseason-maturity, oat cultiity in oat. Additionally, we investigated the relationship between the COP and blend response in other crops by analyzing blend data from previously published experiS.J. Helland, Dep. of Plant Pathology, 351 Bessey Hall, Iowa State ments in soybean (Gizlice et al., 1989), maize (Hoekstra Univ., Ames, IA 50011; J.B. Holland, USDA-ARS Plant Sci. Research Unit, Dep. of Crop Sci., Box 7620, North Carolina State Univ., Raleigh, NC 27695-7620. Received 23 Aug. 2002. *Corresponding author Abbreviations: AFLP, amplified fragment length polymorphism; ([email protected]). BYDV, Barley yellow dwarf virus; COP, coefficient of parentage; GSE, genetic similarity estimate; NTSYS-pc, Numerical Taxonomy Published in Crop Sci. 43:1618–1627 (2003).  Crop Science Society of America and Multivariate Analysis System personal computer program; QTL, quantitative trait loci. 677 S. Segoe Rd., Madison, WI 53711 USA