RECURRENT SELECTION FOR MICROGAMETOPHYTIC VIGOR IN ALFALFA AND CORRELATED RESPONSES AT THE SPOROPHYTIC LEVEL

In alfalfa (Medicago sativa L.), ovules closest to the stigma (apical) were more likely to be fertilized than the ones closest to the pedice llar end of the ovary (basal). Based on the hypothesis that basal ovul es are fertilized by more vigorous microgametophytes, differential sel ective pressures for microgametophytic vigor were applied by selecting the basal seed and the apical seed in alfalfa pods. Ten pods containi ng at least eight seeds from each of 100 plants were used for each of three cycles. Plants from apical and basal seed were separately interc rossed. Pollen tube lengths of apical and basal plants from the second and third cycle of selection differed. The tubes of plants from apica l seed were shorter than the tubes of plants from basal seed after 2, 5, and 22 h of in vitro growth. Divergence increased from the second t o the third selection cycle, indicating that additive genetic effects are involved in the expression of microgametophytic vigor. Pollen germ ination percentage was not affected by selection. Sporophytic traits o f second-cycle plants from the selected populations were evaluated for 2 yr. Plant height, dry matter yield at the first harvest, and seed y ield were significantly lower for plants from apical seed, showing gen etic overlap between the gametophytic and the sporophytic phases for v igor-related traits. Also, coefficients of variation of plants from ba sal seed were lower than those from apical seed for seed of yield. The absence of an unselected control prevented us from concluding whether or not selection of basal seed can increase sporophytic vigor, but ou r results encourage further study to assess the practical interest of this type of selection in alfalfa breeding.