Phases in the S-gene Expression, and S-allele interaction in the control of interspecific incompatibility

Abstract

Plants of 19 progeny families, grown from seeds obtained from crosses between five different 5 genotypes of JSficotiana bonarimsis, were studied for their interspecific incompatibility behaviour as female parents with pollen of certain S.I. and S.C. species. The relationship between 5 genotypic constitution and interspecific incompatibility behaviour, first shown by the plants of the parental families studied earlier, was confirmed without exception in all the progenies. All plants of the same 5 genotypic constitution always behaved alike. No dissociation of specific intra- and interspecific incompatibility reactions has been found in a total of 1200 plants so far tested in N. alata and TV. bonarimsis. Study of the 10 genotypes possible from four different 5 alleles, six heterozygous and four homozygous, taken in conjunction with the earlier studies in this species, showed that interspecific incompatibility, including unilateral incompatibility, was the function of 5 alleles. Each 5 allele had its own specific property of accepting or rejecting a class of pollen. There is a truly sporophytic determination of interspecific incompatibility in the style. In styles heterozygous for 5 alleles there was either independent action, or allelic interaction between the accepting and the rejecting allele showing dominance of one or the other. The allelic interactions in the style are specific for pollen of particular species, and have no relationship with allelic behaviour in connection with pollen of other species. Generally, all pollen of a species behaved alike, polymorphism occurring only in the style behaviour. The tester S.I. plant GM8 of otherwise S.C. species TV. glauca is an exception. The pollen of this plant differed in interspecific incompatibility behaviour from a number of other S.I. plants of this species tested, which were, as expected, all alike in this respect. The S.I. plant GM8 is 5 heterozygous with two distinct forms of 5 alleles, a usual self-incompatibility allele Si, and a self-compatibility allele Sc which when present alone in the pollen causes that pollen to be rejected by a S.I. style. The unusual behaviour of the plant GM8 (5I5c) is due to S allele interaction in the pollen. All other S.I. plants, with usual behaviour of their pollen, are S homozygous with two different S alleles of the same Si form. The results suggest a sporophytic action of the Sc gene in the pollen of GM8. There are two phases in the physiology of the S-gene action: (i) a primary S-gene action phase which occurs independently in the pollen and style and (ii) a secondary, incompatibility reaction phase which occurs after the pollen and style have come in contact with each other. Allelic interaction may occur in one or both phases. There are stepwise differences in the extent of rejection of the pollen of S.C. plants having different forms of S alleles, starting at the one end with full acceptance of the comparatively young self-compatible “S0“ class of pollen by S.I. styles to the other end with complete rejection of the comparatively ancient self-compatible “SCn“ class of pollen (SC1, SC2,…, etc.) by S.I. styles It is suggested that the S0 class of self-compatible pollen, along with the usual S1 class of pollen, has a gametophytic action of the S gene whereas the Sf, Sc and SCn classes of pollen containing a relatively” eroded” form of the S gene have a sporophytic or partially sporophytic action. © 1973 The Genetical Society of Great Britain.