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Outcrossing Rates of Alfalfa Populations Differing in Ease of Floret Tripping

Dep. of Agronomy and Range Sci. Univ. of Calif. Davis, Davis, CA 95616-8515

^* Corresponding author.

Breeding alfalfa (Medicago sativa L.) with florets that are easier for honey bees (Apis mellifera L.) to trip has been proposed as a means of increasing the efficiency of pollination and improving seed yield, but the impact of selection for ease of tripping on the mating system is not known. This study was conducted to evaluate ease of tripping, self-fertility, and outcrossing rate of alfalfa populations selected for ease of tripping. Easy-to-trip, hard-to-trip, and ‘CUF101’ parent populations were planted in a randomized complete block design with three replicates at Davis, CA. Ease of tripping of the populations was measured, self-fertility evaluated, and open-pollinated seed harvested in each of two years. The mode of inheritance of the Peroxidase-2 and Fluorescent Esterase-1 allozyme loci was verified. Allozyme genotype at these two loci was determined for progeny arrays grown from the open-pollinated seed, and the data used to calculate outcrossing rate of each population using multilocus maximum likelihood estimation procedures. The easy-to-trip population was easier to trip and the hard-to-trip population was harder to trip than the unselected parent population. Self-fertility did not differ among populations. Both Peroxidase-2 and Fluorescent Esterase-I segregated as expected for monomeric enzymes at single tetrasomic loci. The outcrossing rate of the easy-to-trip population (0.76) did not differ from the outcrossing rate of the hard-to-trip population (0.75) or the CUF101 parent population (0.77). Selection for ease of tripping is an easily utilized indirect method which may improve alfalfa seed yield and does not alter the proportion of self-pollinated seed produced.

Received for publication October 5, 1992.