五唇兰大孢子发生的超微结构观察

Ultrastructural Observations on Megasporogenesis in Doritis pulcherrima (Orchidaceae)

五唇兰(DoritispulcherrimaLindl.)大孢子母细胞在减数分裂前呈长圆形,细胞核偏向珠孔端,细胞呈现极性分化。大孢子母细胞第一次减数分裂形成二分体。随后,二分体珠孔端的一个细胞退化,合点端的一个细胞体积增大成为功能大孢子。功能大孢子进行第二次减数分裂,形成二核胚囊。这一过程属于双孢子葱型胚囊的大孢子发生类型。珠孔端的二分体细胞(大孢子)在退化过程中质膜保持完好,液泡数量增多,染色质高度凝集,具有细胞程序死亡的部分特征。功能大孢子的细胞器和染色质分布均匀。功能大孢子合点端的细胞壁上有发达的胞间连丝,二核胚囊期胞间连丝消失。在功能大孢子靠近合点端一侧有吞噬结构,其内含有结构模糊的细胞器。

Megasporogenesis in Doritis pulcherrima collected from Hainan Province was observed undertransmission electron microscope. The megasporocyte is elliptical before the initiation of meiosis with its nucleus migrating toward the micropylar end and polarization being seen. The megasporocyte undergoes the first meiotic division, giving rise to two dyad cells. After meiosis Ⅰ, the chalazal dyad cell expands in size and differentiatesinto functional megaspore at the expense of the micropylar dyad cell. The functional megaspore undergoes thesecond meiotic division resulting in the formation of a binucleate embryo sac. The developmental pattern of the megasporogenesis in Doritis pulcherrima is of bisporic Allium type. Some characters of the programmed cell death such as integrated plasma membrane, increasing amount of vacuoles, and highly compressed chromatin appears during the degeneration of the micropylar dyad cell, while the organelles distribute uniformity and the chromatin diffuses in the functional megaspore. Some plasmodesmata occur on the chalazal cell wall of the functionalmegaspore but they disappear at the stage of binucleate embryo sac. Phagosomes with degenerated organelles are visible in functional megaspore at chalazal side.