The Sexual Reproduction And Mating System Of Robinia Pseudoacacia L.(Fabaceae)

Robinia pseudoacacia L. is a forest legume that is highly valued as a popular and important ecological and honey species, for use as lumber, fuel, bee keeping, and land reclamation. However, artificial hybridization in R. pseudoacacia was proved to be a rather difficult and delicate procedure. In order to advance the artificial breeding of R. pseudoacacia, it is urgent and vital important to study the sexual reproduction and mating system of R. pseudoacacia systematically and thoroughly.As such, the following researches were conducted by using controlled pollination, fluorescence microscopy observation and SSR paternity analysis. Firstly, the flowering biology was studied to detailed information about the artificial pollination biology of R. pseudoacacia. Secondly, to further understand the ovule maturation patterns, the effect of ovule position on the probability of seed maturation and abortion and their influence on fitness of seedlings in open pollination were detected. Meanwhile, the selection mechanism was investigated. Thirdly, to further detect the breeding system of R. pseudoacacia, pollen donor composition during three early phases of reproduction and gene flow was revealed by SSR paternity analysis and artificial pollination. Finally, the sexual selection and possible mechanism in R. pseudoacacia were discussed. Given these understanding, suggestions on hybridization genetic improvement in R. pseudoacacia were given. It provided theoretical and technical support for us to conduct the hybridization breeding in R. pseudoacacia.The main results and conclusions are as follows:1) The pattern of seed production in R. pseudoacacia was non-random, but shows a selective ovule (seed) maturation and abortion pattern which depends on the ovule position. Ovules close to the distal positions tend to be maturated more often than those close to basal positions during the ovule development. In contrast, ovules in basal positions were more likely to be aborted in ovule development than ovules in distal positions, although the differences existed between years and populations. Fluorescence microscopy observation results suggested that the ovules that lie close to the distal positions were more likely to be fertilized than those in the most basal positions. SSR paternity analysis results showed that the outcrossing rates were higher in distal position than that in basal position. The above results suggested that seed maturation and abortion might be the results of the gametophyte competition which shows the differences in the ability of developing seeds to gather maternal resources. Hence, ovule position exerts an influence on offspring fitness.2) We found that progenies resulting from cross-pollination treatments showed significant high fitness compared with progenies resulting from self-pollination, resulting in high levels of selfing depression. Selfing depression was not uniformly manifested, however, over the three stages. Selfing depression was greatest between fertilization and seed maturation stage (δ=0.5419), seedling emergence (0.3654) stage take second place. No significant differences in seedling growth were observed between selfed and corssed progenies. The selfing depression (δ) in10week and20week height growth merely0.0232and0.0873, respectively. Cumulative selfing depression (δ) across all three stages averaged0.7452. SSR paternity analysis results showed that there was an overall decrease in self-fertilized progeny across each life stage (P=0.042). Highest outcrossing rate was observed in seedling stage (95.83%), followed by the mature seed stage (91.25%), finially the abortion stage (76.25%).3) The breeding system of R. pseudoacacia is self-compatible, and may relay on a mixed-mating system to ensure reproductive success, however, the early viability selection conceals the truth of mixed-mating system and inbreeding depression, resulting in an apparently outcrossed population that does not differ significantly from Hardy-Weinberg expectations.4) The physical separation of stigmatal and antheral surfaces, protogynous flowering habit, the relatively slow germination and pollen tube growth, gametophyte competition as well as the inbreeding depression encourages outcrossing in R. pseudoacacia.5) The results suggest that microgametophytic competition and inbreeding depression might contribute to the mechanisms that ultimately determine the production of offspring in R. pseudoacacia.6) Considering the low fruit set via manual emasculation pollination and features of encouraging outcrossing in R. pseudoacacia, it may be a possible way to obtain hybridprogeny through mixed competition pollination without emasculation, then determined the seedlings by using paternity analysis.