HERITABILITY AND DISTRIBUTION OF VARIATION IN DEGREE OF COPPER TOLERANCE IN MIMULUS-GUTTATUS AT COPPEROPOLIS, CALIFORNIA

The variation in degree of copper tolerance, its heritability and relationship with copper contamination of the soil and geographical location of sample, was studied on samples of seed collected from a large abandoned copper mine at Copperopolis, California. The realized heritability of variation in degree of tolerance at a high level of copper, 2 mug cm-3, is shown to be about 0.40. The F1s of crosses between an upward and downward selection line and a non-tolerant line give intermediate values for tolerance. All seed samples taken from contaminated soils, and a number from populations downstream of the mine, have 100 per cent tolerant plants. Samples taken from upstream of the mine, and many populations- in the vicinity, have a proportion of tolerant plants. The degree of tolerance of the samples is more variable, with rather steep clines for this character within the mine. The variation in degree of tolerance is related more to the location of the sample than to the absolute amount of copper contamination of the soil. Populations along the stream running through the centre of the mine wastes are more tolerant than populations taken from the edge of the wastes, which are more tolerant than populations collected from uncontaminated soils near the mine. The difference in steepness of the clines for the two characters, tolerance and degree of tolerance, suggests that selection against tolerance (the 'cost' of tolerance) acts more on degree of tolerance than on tolerance per se. These results are discussed in relation to the concept of genostasis.