Cytolegical Studies of Genus <i>Petunia</i>

Abstract

1. The tetraploid plant of giant variety of Petunia was selfed. Among 31 plants of the F1 progeny 29 were tetraploid, one 4n-1 and one 4n-3. No significant morphological differences were noticed among them.2. The tetraploid plants of the giant variety and the diploid plants of small variety were crossed reciprocally. Among the populations of F1 progenies two gave a high percent of triploids besides each one plant of heteroploids and a diploid. The third gave no triploid but respectively one plant of 2n, 3n+2 and 4n. Among these three diploid plants, one was completely different from both the parents, while the other two were complete replicas of the diploid parent. The triploid plants showed approximately intermediate characters more or less tending toward the tetraploid parent.3. The triploid progeny was cytologically studied. In the heterotypic metaphase 7 sets of trivalent were not infrequentiy found. Almost all the trivalent configurations possible in auto-triploids were observed excepting one. Besides these the ring of three or triangle, and the multivalents were sometimes met with. These may be interpreted by the segmental interchange hypothesis.4. The 2n×4n crossing gave only the diploid progeny in one Gase. These progeny exhibited exactly the same characters morphologically as well as physiologically as the diploid parent. Another mating resulted in three tetraploids and one 4n-1 plant.5. The triploid hybrids selfed gave 26, 27, 28 and 29 chromosome plants in the F, progeny among which 27 chromosome plants were of the highest fequency. The majority of these progenies had one or two fragments of chromosomes. A trabant derived from the diploid ancestor is identified at one end of one of the chromosomes.6. The triploid hybrid was back crossed to the diploid parent. Three plants were obtained with the somatic number respectively of 19, 20 and 21. This shows that the gametes of the triploid with the chromosome numbers of 12, 13 and 14 function.7. The tetraploid-triploid crossings gave the F1 progenies of 24, 26, 27, 28 and 29 chromosomes with respective frequencies of 1, 6, 17 and 29.8. The reciprocals of the above crossings gave one 29 chromosome plant and twenty 28 chromosome plants.9. The appearance of diploid progenies in the crossings between 2n and 4n plants may be explained by the diploid parthenogenesis.10. As to the appearance of diploid progenies in 4n×2n crossings, two different Gases must be distinguished: (1) diploid parthenogenesis and (2) merogony. Both of these are supposed to occur.11. The occurrence of tetraploid plants among the F1 progeny of 4n×2n crossings probably was induced by the fertilization of an egg nucleus by a generative nucleus with double number of chromosomes.12. The tetraploid and triploid plants of Petunia must be autotetraploid and auto-triploid.