Cytological Effects of a Gene in Datura which Causes Dyad Formation in Sporogenesis

Abstract

1. Dyad formation in sporogenesis in Datura, owing to a recessive gene, was found among the F2 offspring from radium-treated pollen. 2. During megasporogenesis the first meiotic division is present, but the second meiotic division is regularly absent. As a result only two megaspores (dyads) are formed. 3. One of the megaspores disintegrates; the other undergoes three successive mitotic divisions. Its nucleus contains 12 pairs of chromosomes instead of 12 single chromosomes. 4. An embryo sac is formed containing eight nuclei, each with 24 chromosomes. 5. During microsporogenesis the same meiotic abnormality is found in the pollen mother cell. There is a failure of the second division, and after the first division a thick cell wall develops between the two nuclei, forming a dyad of microspores. 6. Each microspore, containing 12 pairs of chromosomes, develops into the male gametophyte. 7. A small percentage (about 1 to 5 per cent) of megasporocytes and microsporocytes in dyad plants develops normally. Since both meiotic divisions are present, a "tetrad" of microspores instead of a "dyad" is formed. Each gamete that results from these exceptional cases contains 12 single chromosomes. 8. The formation of 2n and 3n normal offspring from the cross dyad x normal confirms the occurrence of both 1n and 2n egg cells. Dyad 2n offspring have come from parthenogenesis of a 2n dyad egg. 9. The occurrence of large and normal sized pollen grains in dyad plants confirms the occurrence of both 1n and 2n sperms.