THE GENETICS OF SELF- AND CROSS-INCOMPATIBILITY IN BRASSICA OLERACEA

Abstract

OUR genetic schemes have been proposed to account for previous observations on F self-and cross-incompatibility in Brassica oleracea: (1) oppositional S alleles a t one locus, with the pollen behavior determined gametophytically by the genotype of the pollen itself (ODLAND and NOLL 1950); (2) system (1) modified by sympathetic T alleles at a second locus to account for instances of self-compatibility (KAKIZAKI 1930); (3) system (1) modified by what we interpret to be a polygenic system (ATTIA and MUNGER 1950); and (4) a system with oppositional alleles a t two loci, with gametophytic control of pollen behavior (SEARS 1937; MIZUSH~XA and KATSUO 1953). BATEMAN (1954, 1955) showed that two members of the Cruciferae, Iberis amara and Brassica camaestris, have a sporophytic incompatibility system whereby pollen behavior is determined by the genotype of the pollen parent. BATEMAN suggested a similar system for Brassica oleracea and the present study using 'Calabrese Green Sprouting' broccoli shows this view to be correct. PARENT PLANTS Five flowering broccoli plants were moved from the field to an insect free greenhouse maintained at temperatures above 65'F. Compatibility relationships were determined during the winter of 1954-55 when the plants were six to nine months from seed At first, at least 30 fresh flowers of each plant were selfed with fresh pollen. After about three weeks it was evident from the presence or absence of silique development that plant 1 was self-compatible and the others were self-incompatible. The five plants were then crossed to each other in all combinations. Emasculation was used only when plant 1 was the female, and for each cross except one over 20 flowers were pollinated. The results varied considerably from plant to plant but there was no difficulty in deciding whether or not a cross was compatible. Incompatible pollinations resulted in the development of fewer siliques and fewer seeds per silique than compatible pollinations. Thus the average number of seeds per flower pollinated provided a good index of compatibility. This value is referred to as the "compatibility index". Self-compatible plant 1 had a compatibility index of 4.0 when selfed, compared to 4.4 when crossed. Self-incompatible plants 2, 3, 4 and 6 had compatibility indexes of 0.0, 0.03, 0.28 and 0.01 respectively when selfed, compared to 6.7, 11.9, 13.8 and 9.2 in compatible crosses. In crosses, plants 1 and 2 were compatible with each other and with the three other plants in both directions. The relationships among plants 3, 4 and 6 were the most 1 Present address: Horticulture Division, Central Experimental Farm, Ottawa, Canada.