Reaction norm functions and QTL-environment interactions for flowering time in Arabidopsis thaliana.

Many plant traits are phenotypically plastic in response to resource levels that vary continuously among environments. To be able to predict phenotypes in new environments, it is useful to model reaction norms as functions, rather than as a collection of discrete character states. Flowering date and rosette leaf number were measured in 100 recombinant inbred lines of Arabidopsis thaliana, grown on a gradient of light intensity. The results show that there is genetic variation among the recombinant inbred lines for parameters of the reaction norm functions. Genetic variances for leaf number and flowering date are highest under low light conditions. Underlying quantitative trait loci (QTLs) affecting the shape of the reaction norm functions were mapped by modifying Haley & Knott (1992) regressions to include polynomial effects of the environment. Quantitative trait loci of large effect were generally insensitive to the resource gradient. Seven QTLs affecting flowering date and eight QTLs for rosette leaf number were identified, of which only two had significant effects on the linear and quadratic components of the reaction norm function. These results suggest that the genotype-environment interactions for flowering time are controlled by many minor genes, whose effects are below the detection limit in most mapping experiments.

THE EFFECT OF INFLORESCENCE SIZE ON MALE FITNESS: EXPERIMENTAL TESTS IN THE ANDROMONOECIOUS LILY, ZIGADENUS PANICULATUS.

We studied the relationship between inflorescence size and male fitness in the andromonoecious lily Zigadenus paniculatus, using experimentally manipulated inflorescences to eliminate possible correlations between flower number, resource availability, and other floral traits. Allozyme markers were used to determine the siring success of large versus small plants in 14 arrays of plants, each array containing five large and five small plants. The inflorescence size of small plants was held constant both within and among arrays; the size of large plants was held constant within an array but was varied among arrays. Large plants sired more than half the seeds in 12 of the 14 arrays, and significantly more than half in six of these 12. However, in eight of the arrays, large plants sired significantly fewer seeds than expected on the basis of their size advantage. Furthermore, there was no significant relationship between relative size and relative siring success in comparisons among arrays. A maximum-likelihood model estimated that 28% of seeds were sired by imported pollen, with 95% confidence limits of 13% and 50%. Within these limits, high import rates tended to mask the relative success of large plants in several arrays. These results suggest that the evolution of inflorescence size in Z. paniculatus is at least partly driven by selection for increased male success, assuming genetic variation for flower number. However, the data also support a growing body of evidence that estimates of male fitness in plants can be highly variable. We discuss the sources of this variability and the possible effects of inflorescence design on the relationship between inflorescence size and fitness.