CYTOGENETICS OF GAMETOGENESIS IN HAPLO 9 OF GOSSYPIUM HIRSUTUM L. (MEGASPOROGENESIS, MEIOSIS).

Abstract

Plants monosomic for chromosome 9 of Gossypium hirsutum, in addition to Haplo 9 plants, produce many kinds of aneuploid progenies consisting of monosomics, trisomics, multiple monosomics, and different monosomic-trisomic combinations. Cytological analysis of megasporagenesis was conducted to determine the stage and the mechanism that would account for the production of the different kinds of aneuploid progeny. Two new cytotypes involving chromosome 9 were isolated and identified as monotelodisome 9S (Telo 9S) and monoisodisome 9S (Iso 9S), both of which produce different kinds of aneuploids very similar to Haplo 9. Monotelodisomic 9L plants do not produce various kinds of aneuploid progeny like Haplo 9, Telo 9S and Iso 9S. These observations indicate that control of chromosome segregation is on the long arm of chromosome 9. There was no significant difference between Telo 9S and Iso 9S in the kinds and frequencies of aneuploids, but there was a significant difference between these two and Haplo 9. Haplo 9, Telo 9S and Iso 9S showed no significant difference in the chromosomes subtracted or added to the chromosome complement of their respective aneuploid progenies. Analysis of female gametogenesis in TM1 or control plants showed that premeiotic division occurred when bud size is 6.0 - 6.6mm; meiosis occurred when bud size is 6.3 - 7.2mm; the first mitotic division of the megaspore occurred when bud size is 6.9 - 7.8mm, and the second and third mitotic divisions of the megaspore occurred when bud size is 7.8 - 8.5mm. The premeiotic, meiotic, and post meiotic divisions were normal. Female gametogenesis was cytologically analyzed in Haplo 9 plants and it was observed that there was a higher frequency of gametophytes with fewer post meiotic divisions and that there was nonsynchrony of the post-meiotic divisions in a number of the developing female gametophytes. It is assumed that these irregularities are due to the absence of chromosome 9. It is concluded that the long arm of chromosome 9 has control over chromosome segregation, and that most likely nondisjunction occurs throughout female gametogenesis, but confined primarily to the three mitotic divisions following meiosis. Genetic tests showed that eight mutant genes were not located on chromosome 9. The aneuploids had lower seeds per boll than normal disomic plants.