The complex S-gene system for control of self-incompatibility in the buttercup genus Ranunculus

Abstract

Continued research into the S-gene mechanisms in the genus Ranunculus, their genetic architecture and mode of cooperation, applied a backcross analysis in two steps to the three diploid, perennial, closely related, but reproductively perfectly isolated species R. acris. R. bulbosus. and R. polyanthemos. Parental plants were preferably taken from remote provenances, an F1 plant was selected to be crossed with pollen from the maternal parent, and the backcross offspring, altogether about 2300 plants in five backcross families, were tested individually with their pollen onto this parent. Backcross plants failing with their pollen on their backcrossed parent were selected and tested to one another in a diallel scheme of cross pollinations. The analyses based on these selected backcross plants showed 4 S-loci to be at work in all three species; in no case was it necessary to presume more than four. There were no indications that the higher levels of heterozygosity were favoured in any way. The main influence on the frequencies with which the different S-genotypes appeared among the selected backcross plants was seen to be associated with individual S-genes and not with specific S-gene constellations. The data are interpreted to testify to the ancient state of the complex S-gene system, to indicate that the 4 loci form the genuine number, at least for these three species, and to favour the view that they cooperate through a complex recognition molecule, at least a heterotetramer, where each of the four S-loci contribute by its specific polypeptide. The presumption of a restrictive supply of alleles in the team is supported by four of the five F1 plants showing S-gene identities between the crossed parental plants.