RAPD Marker Variation among Divergent Selections for Fiber Concentration in Smooth Bromegrass

regulation) or hormonal signals indicating satiety, a fulfilling of current nutritional requirements (metabolic Neutral detergent fiber (NDF) is considered the laboratory mearegulation) (Van Soest, 1994). The concentration of sure most closely correlated with voluntary intake of forages by rumiNDF is the most common selection criterion to improve nant livestock. The objectives of this study were to create smooth bromegrass (Bromus inermis Leyss.) populations divergent for NDF the intake potential of forage crops. Also, preference concentration in four smooth bromegrass germplasm pools and to or palatability has been found to be associated with low identify random amplified polymorphic DNA (RAPD) marker changes fiber concentration and high digestibility (Falkner and in these divergently selected populations. Two cycles of divergent Casler, 1998; Gangstad, 1964). phenotypic selection led to significant linear responses in NDF among Genetic studies and germplasm research through inselection cycles for all germplasm pools. Within-population variation tensive selection and breeding efforts have provided for RAPD markers was large, reflecting the outcrossing reproduction a solid scientific basis for improving forage quality of and the complex inheritance of smooth bromegrass. However, analysis smooth bromegrass. However, selection designs have of molecular variance revealed significant genetic differences among been inefficient in estimating independently the effects selected populations. Analysis of genetic distances showed that each of some nutrients or constituents on forage nutritional cycle of selection created additional divergence, both among cycles value or separating negative associations between some and among germplasm pools. Up to 15 RAPD markers were associated with selection in each population, but only one marker was forage quality traits and forage yield or disease resisconsistently associated with selection for NDF across all four germtance (Casler, 2001; Casler and Vogel, 1999). Therefore, plasm pools (linear, homogeneous slopes, no drift). Some of the a long-term divergent selection program for NDF was RAPD markers appear to have utility for marker selection or markerundertaken initially to create more variability in smooth assisted selection (MAS) to modify NDF concentration. bromegrass populations, and to establish relatively unconfounded populations differing in NDF concentration capable of addressing some important genetic and breedD feed consumption, and energetic effiing questions. ciency are the three components of forage nutriOver time and selection cycles, population phenotional value theoretically and/or empirically related to types change as a result of changes in the frequencies animal performance (Raymond, 1969). Digestibility, preof favorable alleles. Allele frequencies may change as dicted simply and effectively by standardized in vitro a result of selection or drift. Selection responses will laboratory techniques, has received the most attention occur for alleles under direct selection pressure and by forage grass breeders. Quantitative genetic changes those within their linkage blocks. Although most molecin in vitro dry matter digestibility (IVDMD) are generular markers are considered to be selectively neutral, ally due to changes in chemical composition of individchanges in marker frequencies associated with changes ual plant parts (Buxton and Casler, 1993) and only rarely in population performance have been reported (Keithdue to changes in plant morphology (Kephart et al., ley and Bulfield, 1993; Ollivier et al., 1997; Stuber and 1989) or relative maturity (Buxton and Casler, 1993). Moll, 1972; Stuber et al., 1980). Consistent frequency A forage sample consists of a cell-contents fraction changes in molecular markers, without effects of drift, soluble in neutral detergent and an insoluble cell-wall may be a mechanism to identity linkages between markfraction (neutral detergent fiber, NDF), largely coners and quantitative trait loci (QTL), potentially improving selection efficiency. taining cellulose, hemicellulose, pectin, and lignin. The The objectives of this study were (i) to create diverconcentration of NDF is the single most effective laboragent populations for NDF concentration and (ii) to identory predictor of an animal’s ability to consume a forage tify RAPD marker changes during two cycles of diad libitum (Van Soest, 1994). However, it is not clear vergent selection for NDF concentration, for eventual whether the mechanism of intake is regulated by negaapplication of MAS to improve the efficiency and costtive feedback from ruminal tract distension due to the effectiveness of phenotypic selection for reduced NDF high bulk density of high-fiber forages (physiological concentration. M. Diaby, Dep. of Agronomy, Univ. of Wisconsin-Madison, WI 53706MATERIALS AND METHODS 1597 USA; M.D. Casler, USDA-ARS, U.S. Dairy Forage Research Center, 1925 Linden Dr. West, Madison, WI 53706-1108. Mention of Germplasm a trademarked or proprietary product does not constitute a guarantee Divergent selection for NDF concentration was conducted or warranty by the USDA-ARS and does not imply its approval over other suitable products. Received 8 Oct. 2003. *Corresponding author in four germplasm pools representing three different levels of ([email protected]). Abbreviations: IVDMD, in vitro dry matter digestibility; MAS, marker-assisted selection; NDF, neutral detergent fiber; PCR, polyPublished in Crop Sci. 45:27–35 (2005). © Crop Science Society of America merase chain reaction; QTL, quantitative trait loci; RAPD, random amplified polymorphic DNA. 677 S. Segoe Rd., Madison, WI 53711 USA