Genetical studies in pears

Summary

1. Self-incompatibility in diploid pears is due to arrested pollen-tube growth. Three days after pollination the tubes are approximately one-third of the way down the style and their ends are swollen.

2. In a cross between two diploid varieties compatible and incompatible tubes are present indicating gametic determination of incompatibility.

3. The somatically doubled autotetraploid Fertility is self-fertile, but both compatible and incompatible tubes are present in selfed styles. Compatible tubes of the tetraploid grow faster than compatible tubes of diploids both in tetraploid and in diploid styles.

4. The breakdown of the self-sterility mechanism following chromosome doubling is reviewed and found to be of fairly widespread occurrence in plants. A working hypothesis to explain the effect of polyploidy on self-sterility is given: it is based on the assumption thatS ₁ S ₂ pollen is compatible andS ₁ S ₁ andS ₂ S ₂ pollen incompatible inS ₁ S ₁ S ₂ S ₂ styles.

5. Pollen tubes of the diploid variety Beurré Bedford grow as fast as those of a tetraploid variety, penetrate the style and enter the embryosac; but only rarely do they effect fertilization. Beurré Bedford pollen grains are unreduced pollen mother cells with haploid, diploid, triploid, and tetraploid nuclei. The high rate of growth of the pollen tubes is due to the nature of the pollen cell and the failure to fertilize the eggs to the lack of balance between the sperm nuclens and the cell.

6. The result of this normal pollen-tube growth without fertilization is the formation of seedless fruits.