SELECTION FOR TOLERANCE TO COPPER DURING POLLEN FORMATION IN MIMULUS-GUTTATUS FISCHER EX DC

In Mimulus guttatus, copper tolerance is determined largely by a singl e gene and is expressed in both the sporophyte and microgametophyte. T his study explores the extent to which selection during pollen formati on affects copper tolerance in the sporophytic generation. Two sets of plants heterozygous for copper tolerance, produced by reciprocal cros ses between different copper-tolerant or sensitive families, and the p lant on which the original observations were based, were cloned and gr own in control or copper-supplemented solutions. Pollen viability and the number of tolerant progeny produced in backcrosses to sensitive pl ants were compared. In addition, the effect of copper treatment on pol len viability in vitro was compared for plants tolerant, sensitive and heterozygous for copper tolerance. The extent to which in vitro polle n viability decreased in response to copper treatment corresponded to the copper tolerance of the pollen source. When grown with added coppe r, four of the five plants showed significant reductions in pollen via bility, ranging from 18% to 48% of control values. The reductions in p ollen viability were correlated with an increase in tolerant progeny ( r = 0.679, P = 0.004). Increases in tolerant progeny could be large, r anging from 119% to 170% of that of controls, but were usually smaller than was predicted from the reductions in viable pollen. In addition, plants derived from reciprocal crosses differed significantly in the extent to which pollen viability was decreased and sporophytic toleran ce increased. Thus, while selection during pollen formation could incr ease sporophytic tolerance, sporophytic factors, perhaps including cyt oplasmic or epigenetic ones, moderated the effectiveness of pollen sel ection for copper tolerance.