ONE-LOCUS SPOROPHYTIC S-GENE SYSTEM WITH TRACES OF GAMETOPHYTIC POLLEN CONTROL IN CERASTIUM-ARVENSE SSP STRICTUM (CARYOPHYLLACEAE)

Cerastium arvense ssp. strictum (2n = 36), obtained from the Swiss alps, was investigated by fluorescence microscopy of pollen tube growth in 52 population plants, 77 F1-plants in four F1-families, and 51 backcross plants from two backcrosses F1 X parent. All plants were self-incompatible with pollen tubes arrested on the stigmatic surface. One disomic S-locus with sporophytic pollen control and independent action of the S-alleles was indicated by the four groups segregating in all four F1-families. The data are interpreted to show that any of the two S-substances produced in the pollen and style is sufficient alone to cause incompatibility, and that a fraction of the pollen grains react as if one of the two imprints were absent. It is suggested that, by occasional successive cell wall formation at microsporogenesis, S-gene transcription, even if initiated before S-gene segregation in the first anaphase of the pollen meiocyte, may be insufficient to prevent the S-gene of the dyad cell to put its imprint alone on the two microspores formed in the second meiotic division. This gametophytic imprint on the pollen grain may form a necessary substitute to a system of dominance relationships in the pollen, cross-compatibility being promoted without loss of self-incompatibility. Estimates of the number of S-alleles in the sample of population plants ranged from 7 to 19; the weighted means was 10. -The presence of an S-gene lacking dominance interaction supports the opinion that the orders Caryophyllales and Ranunculales form a phyletic lineage.