The uniformity of PSRI responses to divergent selection for NDF suggested either pleiotropy
or tight linkages as genetic factors.
A fraction of the viability mutations, either positive or negative, might have an opposite effect on other fitness components, so that they might produce some kind of over-dominance for fitness due to antagonistic pleiotropy
(Rose 1982, 1985).
Furthermore, if the trait correlated with resistance is undesirable (e.g., tall, lodging-susceptible plants), then it will be necessary to determine if the correlation is due to pleiotropy
or tight linkage.
Several mechanisms could, in theory, maintain variation for quantitative traits: mutation-drift balance of selectively neutral alleles (Lynch and Hill 1986), overdominance (Robertson 1956), mutation-selection balance (Barton and Turelli 1989), antagonistic pleiotropy
(Rose and Charlesworth 1981), and environmental heterogeneity (Gillespie and Turelli 1989).
Furthermore, in both species, only one cross was used as source population, thus limiting our understanding of the genetic basis (linkage and/or pleiotropy
) of correlated responses to selection.
Direct benefits (such as increased fecundity or resources) and pleiotropy
have also been proposed to explain the mointenance of female preferences (e.g., Kirkpatrick 1987; Ryan 1990; Kirkpatrick and Ryan 1991; Endler 1992).
Molecular markers provide a powerful tool for identifying genes controlling complex traits, determining gene action, and obtaining evidence for and against pleiotropy
(Paterson et al., 1991; Stuber et al., 1992; Tanksley et al., 1989).
These include: (1) the number of genes causing species differences; (2) the magnitudes of allelic effects at these loci; (3) the modes of gene action (additivity, dominance, epistasis); and (4) pleiotropy
versus linkage of effects on correlated traits.
The relationships between yield, its components, product quality, and stress resistances are next examined under the hypothesis of a "near-constant capacity system." Causes of negative associations, such as pleiotropy
and competition for internal resources, are discussed.
Early one- and two-locus models of ecological specialization, based on antagonistic pleiotropy
, invoked trade-offs in performance in different environments, and predicted that performance in different environments should be negatively correlated (Maynard Smith 1966; Felsenstein 1981; Rausher 1984; Diehl and Bush 1989).
(1999) suggested that pleiotropy
or linkage, or the presence of additional loci affecting these traits may explain these correlations and the identification of a few RICLs having the CNN(WI3A) phenotype for one trait, but the CNN phenotype for the other correlated trait.
Mutations affecting life-history traits include the following classes: (1) deleterious mutations of large effect, generally showing widespread pleiotropy
; (2) mildly detrimental mutations; [TABULAR DATA FOR TABLE 4 OMITTED] and (3) mutations showing antagonistic pleiotropy