Agricultural Fitness of Smooth Bromegrass Populations Selected for Divergent Fiber Concentration

relation is undesirable, as is the case with NDF and forage yield. The consequences, resolution, and/or amelioSelection for reduced neutral detergent fiber (NDF) concentration ration of these various factors in agricultural crops are has been used as a mechanism of improving intake potential of perendramatically different. nial grasses by ruminant livestock. However, reduced NDF concentration is typically associated with reduced forage yield, although the Undesirable linkages can be alleviated by recombinareasons for this genetic correlation are unknown. The objectives of tion, if time and resources are sufficient. Multiple cycles this study were to determine the relative contributions of pleiotropy, and large population sizes may be necessary if loci are linkage, and drift to the genetic correlation of NDF concentration with tightly linked. Pleiotropy, by definition, cannot be corfour agricultural fitness traits: forage yield, survival, seed yield, and rected by any methodology known today. Drift, which lodging. These traits were measured on four smooth bromegrass (Broresults in inbreeding, can be corrected by the use of hymus inermis Leyss.) populations that had undergone one cycle of dibrids or strain crosses to restore heterosis, but only vergent selection for NDF concentration. Selection responses for forif selection was conducted in multiple populations of age yield were linear and homogeneous, suggesting pleiotropic effects. reasonably divergent pedigrees. Drift can be minimized Growth and expansion of the cell wall appears to be essential for acby increasing effective population size, but this is costly, cumulation of forage yield, lending a certain allometry to these two traits. However, natural selection within swards appeared to regulate either in increased effort or decreased selection intenthis response, because forage yield responses disappeared by the third sity. Nonequilibrium allele frequencies may cause asymproduction year. Selection responses for survival and lodging were metrical selection responses in positive vs. negative dilinear, but nonhomogeneous, suggesting linkage. Selection responses rections, but is probably a minor factor, as its effect is for seed yield appeared to be regulated by all three phenomena, with generally small relative to selection coefficients (Faldrift (asymmetry) the most important. Seed yield is highly sensitive to coner and Mackey, 1996). inbreeding depression, which occurs as a result of drift. Short-term diCasler (1999) applied the method of orthogonal convergent selection experiments, analyzed by a factorial ANOVA model, trasts to test and estimate two independent effects of provide a mechanism to identify genetic phenomena responsible for divergent phenotypic selection for NDF. Divergence beobserved genetic correlations. tween high and low Cycle-1 populations estimated the direct effect of selection for NDF on the correlated trait, forage yield, which would be due to linkage and pleioB for increased intake potential of perentropy together. Because effective population size was nial grasses for ruminant livestock, by selection for identical for both directions, asymmetry of response bereduced NDF concentration, have been plagued by pertween the high and low populations (the difference of sistent reductions in forage yield (Casler, 1999; Han et al., their mean relative to the mean of the base population) 2001; Surprenant et al., 1988). Because NDF is approxiestimated the direct effect of drift combined with the mately equal to cell-wall concentration (Van Soest, 1994), effect of nonequilibrium allele frequencies (Falconer, a positive genetic correlation between NDF and for1953; Falconer and Mackey, 1996). The causal relationage yield may be a biological necessity. The pertinent ship between drift and asymmetry of selection responses questions are how large is this genetic correlation, what is well documented in animals (Falconer, 1977). In WBgenetic phenomenon underlies it, and is it pliable? The RP1 smooth bromegrass, approximately half of the varithree questions are interrelated because the cause will ation in forage yield among selection cycles was due to determine the size of the correlation and whether it is linkage and/or pleiotropy of loci controlling NDF and static or stochastic. forage yield and half due to drift and/or nonequilibrium Selection typically results in changes to traits that were allele frequencies (Casler, 1999). not specific selection criteria or under directed selection In a single population or selection line, separation of pressure, i.e., correlated responses. These correlated relinkage and pleiotropy is impossible by quantitative gesponses can occur by one or more of four mechanisms: netic approaches and extremely difficult with molecular linkage, pleiotropy, drift, or nonequilibrium allele fremarkers. However, multiple populations or replicated quencies. The distinction among these various genetic selection within a single population may be used to causes is extremely important, particularly when the corpartially separate these two genetic phenomena (Casler, 2002). The main effect of selection, across populations USDA-ARS, U.S. Dairy Forage Research Center, Madison, WI or replicates, is a partial measure of the effect of pleio53706-1108. This research was supported by the College of Agricultropy. Pleiotropy, common effects of a single locus on tural and Life Sciences, Univ. of Wisconsin-Madison through the use multiple traits, would be constant across populations if of Hatch formula funds. Received 21 Nov. 2003. *Corresponding author ([email protected]). the same pleiotropic loci are segregating in each population. This assumption may not be necessary if we assume Published in Crop Sci. 45:36–43 (2005). that each trait is controlled by a large number of quanti© Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA tative trait loci, each with relatively small effects. Quan-