| While unreduced gametes are concerned with fertilization, sexual polyploidization will occur in angiosperm, which is considered to be the dominant routine of spontaneous polyploid types arising. Breeders are enthusiastic in germplasm enhancement via 2n gametes. But in many fruit crops, the frequency of spontaneous 2n gametes is too low to reach the requirement in the practical breeding or germplasm enhancement. In our study, 2n gamete was induced with heat shock or low temperature due to meiotic disturbance, and the cytogenetic mechanism of 2n pollen formation was determined. Both in vitro and in vivo germinations of 2n pollen, its ability of contribution to fertilization and its purification were also tested. Some new germplasm at different ploidy arose from crosses involving 2n pollen. 1. Either heat shock or low temperature treatment was efficient to induce 2n gametes. 2n pollen of Prunus persica and Prunus salicina was induced with the 12-hour-heat shock at 32℃during male meiosis on in vitro flower shoots. In plum (cv. Niuxin), 16.06% dyad formed during microsporogenesis after heat shock whereas 0.114% in 20℃control, and 2n pollen mounted to 20.37% of stainable pollen after heat shock while no 2n pollen was observed either in pollen samples from field or in 20℃control. In peach (cv. Qingfeng), 14.02% dyad and 16.74% 2n pollen formed after heat shock, no dyad or 2n pollen was observed in field or 20℃control. The 4-hour-heat shock of 35.5℃and the 48-hour-low temperature treatment under 11℃were available to induce 2n pollen in Capsicum annuum (cv. Xianjiao). The frequency of dyad formation and the percentage of 2n pollen were 35.4% and 12.17% respectively after heat shock. After low temperature treatment, 20.55% dyad and 13.0% 2n pollen were observed, while the frequency of dyad formation and the percentage of 2n pollen in control were below 1%. The technique for inducing 2n pollen was feasible in 11 varieties of 6 species of horticultural crops. 2. The cytogenetic mechanism of 2n gametes formation was determined. The cytogenetic mechanism of 2n pollen formation after treatments was determined on the basis of the behavior of meiotic chromosomes and sporads during microsporogenesis. The cytological responses to extreme temperature differed depending upon the stages of male meiosis. The formation of synaptic complex was partially or even entirely inhibited in meiotic cells treated with low temperature or heat shock during meiotic leptotene to zygotene, which would lead to first division restitution of nuclei and the occurrence of none cross over 2n pollen. The stability of synaptic complex was affected either by heat shock or by low temperature treatment applied to male meiotic cells during pachytene to diakinesis, which resulted in disaggregation of bivalents, restitution of nuclei in part of cells, and formation of cross over 2n pollen. Cells at meiotic anaphase I -metaphase II also responded to the treatments. One of the two cytological phenomena observed was the laggards at anaphase I which led to second division restituted nuclei and SDR 2n pollen. Another phenomenon was the abnormal cytokinesis and the subsequent binucleate dyad, but no evidence sufficient to determine unambiguously the 2n pollen developed from binucleate dyads equivalent to FDR or SDR type.3. The optimum method of 2n pollen purification was provided. The percentages of 2n pollen in plum and chile were increased from 14.6% and below 0.1% to 26.47% and 9.09% respectively after sieving in media, xylene or mannitol buffer. 4. The fertilizing ability and the competition of 2n and n pollen were studied. It took longer time for 2n pollen induced with heat shock to germinate in vitro than n pollen in peach and plum, but 2n pollen tubes grew faster. 2n pollen induced with low temperature or heat shock in chile germinated and its tubes grew in vitro quite similarly with n pollen. But 2n pollen germinated later and pollen tube grew slower than n pollen in vivo in chile, suggesting that the fertilizing ability of 2n pollen was weaker than that of n pollen and 2n pollen was less competent than n pollen. 5. Induced 2n pollen was utilized in germplasm creation of horticultural crops. No triploid plant was obtained in progenies of 2n pollen induced with heat shock in peach and plum, owing to the low fruit set after pollination. But in chile, 4 trisomics and 1 triploid plant arose from the progenies of 2n pollen induced with low temperature, and triploid plant accounted for 1.49% of the offsprings. Its would be concluded that new polyploid germplasm in horticultural crops could be created employing 2n gametes induced with low temperature and heat shock. 6. Biological and genetic characters of newly created germplasm were evaluated. The crown, leaves and flowers of the triploid chile plant were larger but the berries were significantly smaller than that of the diploid plant. Abnormalities presented on seed set and male meiosis in the triploid. The trisomics of chile presented similarity with its diploid type. A... |
PDF Full Text Request