K. Kakeda et al., A self-compatible mutant S allele conferring a dominant negative effect onthe functional S allele in Ipomoea trifida, SEX PLANT R, 13(3), 2000, pp. 119-125
A spontaneously occurring self-compatible mutant has been identified in Ipo
moea trifidia, a species possessing sporophytic self-incompatibility contro
lled by a single multiallelic S locus. Analysis of the segregation of compa
tibility/incompatibility phenotypes in selfed and crossed progenies of the
self-compatible mutant plant indicated that the self-compatibility trait wa
s caused by a mutation at the S locus; the mutated S allele was therefore d
esignated Sc. RFLP analysis of progeny plants segregating for the Sc allele
using the SSP gene (a gene linked closely to the S locus of I, trifida) as
a probe confirmed that the mutation was present at the S locus. Self-incom
patibility responses were examined in F-1 progenies obtained from crosses b
etween the self-compatible mutant and self-incompatible plants homozygous f
or one of three S alleles, S-1, S-3 and S-22, where the dominance relations
hip is S-22>S-1>S-3. All F-1 progeny plants from crosses with S-22 and S-1
homozygotes were self-incompatible and exhibited the respective phenotypes
of each self-incompatible parent (either S-22 or S-1) in both stigma and po
llen. However, of the F-1 progeny plants from the cross with the S-3 homozy
gote, those carrying the genotype ScS3 were all self-compatible and cross-c
ompatible as both female and male parents with the S-3 homozygote. These re
sults indicate that the dominance relationship between the four S alleles i
s: S-22>S-1>Sc>S-3 and so reveal the unexpected finding that the mutated Sc
allele is dominant over a functional S-3 allele. A possible explanation fo
r this observation is that the gene product encoded by the Sc allele confer
s a dominant negative effect on the S-3 gene product.