Embryology And Genetic Diversity Of Larix Chinensis Beissn.

Larix chinensis Beissn., an endemic species to China, belongs to the genus Larix Mill and only distributes on several peaks of Qinling Mountains in Shaanxi Province, with the altitude ranging from 2600 to 3500m. L. chinensis usually forms small populations and the individuals are runtishly in their discontinuous habitats, the natural populations grow very slowly and regenerate poorly. What is worse, L. chinensis has an extraordinarily low seed-setting rate and most seeds are atrophic. It has been listed among Grade II Endangered and Protected Plant Species.Through the systematic observation on the formation of male and female gametophytes and the embryonic development, this paper has given a clear picture of L. chinensis' embryonic development process and revealed its reproductive obstacle. Embryologic evidences are also provided for the determination of its systematic position according to the traits of its reproductive biology. Meanwhile, the genetic structure and variation of L. chinensis' natural populations have been illustrated by means of SSR molecular markers. Through all these research work, this paper reveals the possible causes for its endangered situation and provides a helpful reference for the prediction of L chinensis' genetic destiny and its biological conservation.The results of embryological research reveal that: (1) the male cone of L. chinensis differentiates during early July. The microsporangium wall of L chinensis consists of 5-6 cellular layers, namely the epidermis. the endothecium. the 2-3 middle layers and the tapetum. The tapetum belongs to the periplasmodial type. The sporogenous cell occurs in late July and develops into the microspore mother cell in early August. In early August, the latter starts its meiosis and enters into its diplotene phase in early October and later goes through dormancy in this form. The dormancy ends in late March the next year, and its meiosis continues. In middle April tetrads form and microspores are released between late April and early May. After four times' continuous mitoses, the microspore develop into the mature pollen (malegametophyte) which consists of five cells and begins shedding in middle May. (2) the female strobilus of L. chinensis Beissn. starts its differentiation in middle and late July;Megasporocyte forms during early and middle September and enters into dormancy in middle October; Between late April and early May the following year, the megasporocyte ends dormancy and begins its meiosis. Linear tetrads form around 10 May, and mature egg cells form and start the fertilization takes in early July. Simple polyembryony and rosette embryo occur frequently in L. chinensis. The mature embryo with 5~6 cotyledons forms in middle September. It takes 14 months from differentiation of female strobilus to formation of mature embryo. (3) the development of the microspore mother cell is nonsynchronous. Degeneration occurs in part of microspore mother cells during the developmental process, resulting in the formation of a large cavity in the microsporangium. Besides, the cells of tapetum undergo abnormal inflation and hyperplasia during the meiosis I of the microspore mother cell, which will probably cause abortion of the microspore mother cell. Some ovules undergo abnormal development in the early embryonic stage, with their megagametophytes becoming semitransparent or atrophic. All these phenomena may be responsible for the sexual reproductive degeneration and low seed-bearing. (4) the microspore mother cell of the L chinensis goes through dormancy in the diplotene stage and the mature pollen consists of five cells; Megasporocytes form during middle and late September, and begins its meiosis between April and May the following year after an approximate 7-month period of winter dormancy. L. chinensis has two to three archegonia per ovule, and each archegonium has two neck cells. Furthermore, during the early embryonic development, simple polyembryony and rosette embryos occur in L. chinensis. These embryological traits support the conclusion that the...