The distribution of microtubular cytoskeletons and organelle nucleoids during microsporogenesis in Populus simonii Carr. x P. nigra L. ‘Tongliao’ was studied by indirect immunofluorescence and 4’,6-diamidino-2- phenylindole (DAPI) staining. Our purpose was to reveal the relationship between changes in microtubule and chromosome behavior and cytokinesis and to explain the mechanism of 2n pollen production. We observed stable frequencies of 2n pollen production, ranging from 1.56% to 2.29%, between 2006 and 2008. Moreover, conjoined pollen grains were also observed. Meiotic abnormalities, including univalents, lagging chromosomes and micronuclei, were observed during microsporogenesis. Triads and dyads were also detected in meiotic products. Parallel spindles in metaphase II were unable to fuse, owing to the existence of an organelle band. Regularly, in telophase II, primary phragmoplasts were organized between sister nuclei, and secondary phragmoplasts formed between non-sister nuclei. Cell plates were initiated by centrifugal expansion of phragmoplasts and cytoplasmic infurrowing started at the junctions between the microsporocyte wall and the expanded phragmoplasts. However, a secondary phragmoplast was absent in some microsporocytes. These observations suggest that the occurrence of 2n pollen may result from the partial failure of cytokinesis caused by the absence of secondary phragmoplasts in this hybrid.
Clonal differences in fertility (expressed as the number of female and male strobili) were determined for three consecutive years (2002-2004) in a clonal seed orchard of sugi (Cryptomeria japonica D. Don) in Korea. Fertility varied among clones and among years producing three-year averages of 196 and 652 for female and male strobili per ramet, respectively. Correlation between female and male strobilus production was positive over the three years and statistically significant in 2003, a good flowering year. Based on the observed fertility variation, the status numbers (Ns, measure of genetic diversity) were calculated and varied from 25.6 to 31.7 among the three studied years. On average (pooled), relative status number was 86% of the census number (N). Variation in female fertility was higher than that in male fertility, and this variation was reflected on female and male parents’ status numbers. Pooled Ns estimated from the three years was higher than that for any single year, implying that genetic diversity would increase when seeds collected from different years are pooled.
Polyploid breeding has the potential to increase the economic secondary metabolites of Eucommia ulmoides. However, pollination with induced ploidy-mixed pollen has failed to produce polyploids (GAO, 2006). In this investigation, the morphological characterization and in vitro germination of heat-induced ploidy-mixed pollen of E. ulmoides were analysed to determine why there is no polyploid production. Heat-treated pollen grains were easily distinguished as large and small according to their length. The large pollen grains were significantly longer than both untreated and heat-treated small samples, suggesting that they were probably 2n pollen. Rather than the three germinal pores in small pollen, the large grains typically had four pores and, in some cases, shallow furrows, which might affect their germination. Although the maximal germination rates of the treated small and large pollen were not significantly different, the large pollen germinated tardily during the early stages of incubation. The small pollen maintained its growth during the incubation, but the tube growth of large pollen almost stopped after 24 h incubation. Both vegetative and generative nuclei in the large pollen moved into tubes later than in small pollen and the frequency of mitosis in generative nuclei of large pollen was low. Therefore, the tardy germination, poor tube growth, and weak activity of both vegetative and generative nuclei probably caused the poor competition of large pollen in certation. Finally, techniques to increase the competition of highploidy pollen and the prospect of 2n female gamete induction in the polyploid breeding program of E. ulmoides are discussed.
Triploid breeding plays an important role in cultivar improvement in the genus Populus L. A novel approach for triploid production with colchicine during embryo sac development was reported recently by Wang et al. (2010). In the present investigation, female catkins of Populus pseudo-simonii × P. nigra ‘Zheyin3#’ during embryo sac development were exposed to high temperature to assess the effectiveness of high temperature for induction of triploid production. In the progeny, 45 triploids were determined by both flow cytometric analysis and somatic chromosome counting. The period 66-72 h after pollination was the most suitable for high temperature-induced triploid production during embryo sac development in the ‘Zheyin3#’. Cytological analysis showed that the frequency of eight-nucleate embryo sacs rose at an increased rate during 66-78 h after pollination, which suggested that the third mitosis during embryo sac development could be the optimal stage for high temperature-induced triploid production. The highest frequency of triploid production was 40%, which was obtained in the 44°C for 2 h treatment 72 h after pollination. In view of both triploid number and production efficiency, treatments with 41°C for 4-6 h or with 44°C for 2 h during 66-72 h after pollination were both effective for triploid induction in ‘Zheyin3#’. Statistical analysis showed that the growth of triploids and diploids was not significantly different. However, highly significant differences were observed for all leaf characteristics. Finally, the significance of high temperature treatment in Populus triploid breeding programs is discussed.
Genetic gain and diversity, expressed by status number, of seed crops from a clonal seed orchard of Pinus thunbergii were estimated considering selection, fertility variation and pollen contamination, and compared for different management alternatives (selective harvest, genetic thinning and combination of both options). Management variables included the proportion of clones left after selective harvest and/or genetic thinning. The impact on genetic gain and diversity of seed crops was quantified as a function of the quantity and quality of gene flow from outside the seed orchard. Genetic gain varied with the proportion of selected or thinned clones. Genetic thinning by means of truncation selection of clones resulted in a large decrease in status number, which was accompanied by greater genetic gain than achieved by selective harvest alone. As expected, gene flow from outside the seed orchard greatly increased status number of the seed crop at higher rates of pollen contamination under all management options. The formulae and results of the present study could be used for identifying favorable selection intensity and alternatives for orchard management.