Study On The Reproductive Biological Characteristics Of Taraxacum Tianschanicum

Apomixis is the reproductive process by which an organism produces offspring without the fusion of male and female gametes(fertilization).Both sexual and unisexual individuals were found in the same population of Taraxacum tianschanicum.This study was conducted by controlled pollination test,field observation and indoor research.The reproductive patterns of two types of individuals,the number of chromosomes and the level of ploidy,the morphological characteristics and seeds quality,the megasporogenesis,microsporogenesis and the differentiation of female and male gametophytes were studied.The main results of this paper are as follows:(1)The experimental results of controlled pollination show that the way of reproduction of T.tianschanicum is facultative apomixis reproduction;The chromosome number of the apomixis individuals and sexual individuals are 16.The relative DNA content in leaves and seeds of individuals with both forms of sexual expression was examined by flow cytometry.The results that the DNA content was found to be identical,indicatied the same ploidy level,this show are both diploid plants;The sex expression of the T.tianschanicum was found to be malleable,which expressed as a unisexual individualsin the spring,27.91%of the individuals whose sexual expression is unisexual will change into sexual individuals in the autumn,thus individuals with sexual reproduction,apomixis and monoecious sexual expression in the same population.(2)There were differences in sexual expression between sexual individuals and apomixis individuals.The number of ray floret,stomatal opening,stomatal index,fertile seed,thousand kernel weight and achene length of the apomixis individuals were significantly higher than those of the sexual individuals(P<0.01),but there was no significant difference in seed germination ability(P>0.05).The results showed that the apomixis individuals better adapted to drought,lack of male function by production of more seeds,which increase their reproductive fitness.(3)The differentiation of microsporocyte of the apomixis individuals were not significantly different from those of the sexual individuals.After the occurrence period of tetrad,the male gametophytes of the sexual individuals were divided into single-core,two-core and three-core pollen period in order,while the apomixis individuals entered the male gametophyte abortion period;Compared with megasporogenesis and differentiation of female gametophytes in sexual individuals,the megaspore mother cells of the apomixis individuals without having passed through the tetrad,the chalazal megaspore of the diad develops into the embryo sac,and the egg cells developed into cardiac embryos without fertilization.The abortion mode of male gametophyte in apomixis individuals may be caused by the failure of decomposition of callose or abnormal decomposition of tapetum;and the appearance of facultative apomixis characteristics may be the evolutionary adaptation strategy of the plant to the changing climate.The results of embryological observation further confirmed the facultative apomixis reproduction characteristics of T.tianschanicum.