6. Male sterility.
A case of cytoplasmic male sterility has been analyzed which exhibits a unique type of inheritance. Sterility is complete so that no viable pollen is produced. The behavior of the male sterile plants in crosses is peculiar in that sterile plants are recovered in the progeny only when the inbred line Kys is used as the pollen parent. The F1 population shows a good 1:1 segregation of male sterile and normal plants. When male steriles are outcrossed to a large number of unrelated lines, the F1 progeny are all normal. Furthemore, continual backcrossing to male sterile plants for four generations failed to produce any male steriles. When these normal backcrossed plants were used as the female parent in crosses with Kys, sterility was recovered.
The F1 hybrids of outcrossed Kys yield only normal plants when crossed with male steriles. These hybrids were backcrossed to Kys for six generations, using Kys as the female, and still they gave only normal plants in crosses with male steriles even though they are 98.5% pure Kys. However, when Kys was used as the male in the backcrosses, sterile plants were produced.
The inheritance of this male sterile condition was found to involve an interaction between the cytoplasm and a dominant gene for sterility (Ms), in addition to a male gametophyte factor (SGa) which is associated with a suppression of male sterility. Kys is homozygous recessive for ms and sga. Thus, crosses of male sterile plants (Msms sgasga) with Kys (msms sgasga) segregate for the sterility gene (Ms) and yield normal and male sterile offspring in a typical backcross ratio. All of the unrelated lines studied were found to be MsMs SGaSGa. The F1 progeny of an outcrossed male sterile plant is composed of only normal individuals since the suppressor (SGa) is present. sga pollen can not compete successfully against the pollen carrying the dominant gamete factor (SGa) so that in the subsequent backcrosses to male steriles only SGa pollen effects fertilization and the offspring are all normal due to the action of the suppressor gene. On the other hand, when these male fertile backcrossed plants, which are heterozygous SGasga, are used as the female parent in crosses with Kys, there is no gametophyte competition. Half of the progeny lack the suppressor gene and the male sterile condition can be expressed.
This same scheme explains the action of the Kys outcross hybrids in backcrosses to Kys. When the heterozygous SGasga hybrids are used as the male parent only SGa pollen functions and all of the individuals in the progeny carry the suppressor gene. However, when the hybrid is used as the female parent, the suppressor segregates and some of the offspring yield sterile plants when used in crosses with male steriles.