Fluxes and metabolic pools as model traits for quantitative genetics. I. The L-shaped distribution of gene effects

The fluxes through metabolic pathways can be considered as model quantitati ve traits, whose QTL are the polymorphic loci controlling the activity or q uantity of the enzymes. Relying on metabolic control theory, we investigate d the relationships between the variations of enzyme activity along metabol ic pathways and the variations of the flux in a population with biallelic Q TL. Two kinds Of variations were taken into account, the variation of the a verage enzyme activity across the loci, and the variation of the activity o f each enzyme of the pathway among the individuals of the population. We pr oposed analytical approximations for the flux mean and variance in the popu lation as well as for the additive and dominance variances of the individua l QTL. Monte Carlo simulations based on these approximations showed that an L-shaped distribution of die contributions of individual QTL to the flux v ariance (RS) is consistently expected in an F-2 progeny. This result could partly account for the classically observed L-shaped distribution of QTL ef fects for quantitative traits. The high correlation we found between R-2 va lue and flux control coefficients variance suggests that such a distributio n is an intrinsic property of metabolic pathways due to the summation prope rty of control coefficients.