Facts and Hypotheses on the Origin of S Mutations and on the Function of the S Gene in Nicotiana alata and Lycopersicum peruvianum

Fragments in certain pollen-part mutants are suspected to originate from duplications of satellited regions. Linkage tests demonstrate that, if this interpretation is correct, either the specificity segment of the S locus is not on the fragment or two unliked specificity segments operate respectively in pollen and style. As electron microscopy and the effects of giberellic acid on selfing suggest that one manifestation of incompatibility reactions is a cessation of protein synthesis in pollen tubes, it is possible that the fragment does not carry an S locus but ribosomal cistrons necessary for the maintenance of activity. Ancestry tests are under way for testing the hypothesis that different specificity segments are organized as tandem repeats and can be switched on and off in inbred backgrounds. Electron microscopy analyses of self- incompatible hybrids indicate that a single gametophytic factor governs interspecific incompatibility in hybrid pollen and is allelic or linked to the S locus.