Reproductive Ecology Of Taxus Yunnanensis

Taxus yunnanensis Cheng et L. K. Fu, distributing in north Myanmar, north Bhutan and southwest China, is famous for its high content of taxol which is an effective anti-cancer drug, and it becomes a research focus due to its endangered state. Endangered mechanism, however, is unclear till now, thus proper protection measures can not be conducted, and usage of this natural resource is limited. In order to investigate the endangered mechanism, this paper explored reproductive process, pollination mechanism and sexual determination and express in T. yunanens. This study can also contribute to plantation and systematics of T. yunnanensis. Main results are as follow.First, development of male reproductive system. Development of the pollen cone began in late June. Microsporogenesis occurred in September and November, and microgametogenesis occurred in December and April in the next years. The first meiotic division of the microspore mother cell formed two hemispherical cells. The second meiotic division formed isobilateral, T-shaped or tetrahedron tetrads of microspores. The mature microspores were uninucleate, non-saccate, and with numerous orbicules. During microgametogenesis in vitro, the first, second and third mitotic divisions were observed on the 6th, 20th and 40th days, respectively, and cell walls were formed on the first and second divisions. Spermatogenesis in vivo, the generative nucleus divided in one hemisphere of generative cell and then two equal-sized sperm were formed.Second, development of female reproductive system, development and germination of embryo. Ovule development began in August, and rectangle megasporocyte began to occur in December, and linear tetrads of megaspores were formed in meiosis. In most occasions, only the chalazal megaspore was functional. Then the functional megaspore nucleus began to divide and form a free-nuclear megagametophyte. Megagametophyte got matured when the walls were formed. Ten or more archegoniums occurred at the edge of megagametophyte, and the egg was found after two mitosises. One of two sperm get into an archegonium and fused with egg. The nucleus of fertilized egg began to divide, and the walls were formed after the third division. The eggs without fertilization divided two times and then disappeared. Pollination, with a rate of 44.7%, was a bottleneck in the ovule development. Simple and cleaved polyembryony were found frequently in the embryo development. In vitro, embryos from the newly matured seeds germinated in 2 weeks, but the germination rate declined as seeds were stored in sands. The Length and structure of embryos were invariable as seeds were stored in sands. It took about 14 years for seedlings get matured in sex.Third, the anthesis the movement of pollination drops. Microspores were released during the middle of November and the middle of December, and 50% microspores kept alive after been stored in room temperature for 42 days. Mature ovules were present from late December to mid-February in the next year, and the pollination drop were found until mid-March. During pollination, the pollination drops presented for a shorter period, and never appeared once ovules were pollinated. Without being pollinated, the pollination drops presented for a longer period, and it appeared again one more times after disappearance. As alive microspores, the died microspores could be hydrated, and the ovules no longer secret pollination drop once were pollinated.Fourth, sexual determination and express. Cytologic studies shew that the karyotype of male was 2n=24=24m, and the karyotype of female was 2n=24=23m+1sm,so it was speculated that sex determination in T. yunnanensis was ZZ/ZW. The results of sex expression shew that most individuals were unisexual, the ratio of unisexual individuals was 92.5%, 92% and 94.2% in 2005, 2006, 2007, respectively. In bisexual individuals, most stems was unisexual, the ratio of unisexual stems was 93.6% and 88.7% in 2006 and 2007, respectively. On bisexual stems, most reproductive organs were abnormal. Between years,the gender of most individuals were invariable, and 10% individuals transformed its gender from 2005 to 2006, and 7.6% individuals transformed its gender from 2006 to 2007. Males and females did not transform to each other directly, but males could transform to females by a medium, bisexual state, and females could also transform to males by a medium, unidentified state. Other gender could transform each other directly. In bisexual individuals, most stems were invariable, and 69.4% female stems, 89.4% male stems and 66.7% bisexual stems kept invariable from 2006 to 2007, and all stem gender could transform to each other directly.Fifth, the distribution of pollen cones and seeds. Spacial distribution of seed and pollen cones shew that there was no obvious difference of the relative concentration of seed and pollen cones in horizontal directions, but the difference in vertical height was significant. Down from the top, relative concentration of pollen cones declined, while relative concentration of seeds was the highest in the middle, and the lowest on the base. Difference of seeds and pollen cones'production between years was significant. The seed production of 50 females in 2006 was negatively related to that in 2007, R=-0.46. The pollen cone production of 26 males in 2006 was positively related to that in 2007, R=0.86. The seed production of 174 females was negatively related to the production of new twigs, R=-0.47, and was negatively related to the length of longest new twigs, R=-0.29.It can be seen from the results that long life history, uneconomical reproduction, unsynchronizated development between male reproductive system and female reproductive system and ineffective pollination are the possible causes for the endangered state of Taxus yunnanensis. Light and sexual ratio and match are principal factors in developing plantation. In Taxus, monoecious tends to evolve from dioecious, and Taxus yunnanensis is at primary stage in sexual evolution.