Abstract
The formation of integuments, megasporogenesis and megagametogenesis in Dendrobium catenatum, an economically important orchid, are observed. After pollination, mitotic cell divisions of the placental epidermis result in the formation of a branching system of outgrowths. The tip of each branch consists of an archesporial cell derived from the differentiation of the terminal subepidermal nucellar cell. It differentiates directly into a megasporocyte. The first division of the meiosis of the megasporocyte produces a dyad approximately equal in size, in which the micropylar cell promptly degenerates. The second meiotic division of the remaining dyad cell results in the formation of two megaspores of unequal size. The larger chalazal cell becomes functional and eventually develops into a mature megagametophyte. The development of the megagametophyte conforms to the Monosporic Polygonum type. The final arrangement of the mature embryo sac conforms to a sevencelled/ eight-nucleate structure. The mature ovule is bitegmic, tenuinucellate and has an anatropous orientation. In the present study, we also discuss the differences between three main types of embryo sac development and the improvement of section techniques.
Highlights
Meiotic division allows for the random assortment of parental genes, whereas the union of parental genes upon a successful fertilization event enables the offspring to better adapt to the changing environment (Lee & Yeung 2012)
In the embryo sac development of Fritillaria ussuriensis Maxim (Li & Shen 1990) and Rudbeckia bicolor Nutt. (Musiał et al 2012), meiosis of the megasporocyte yields four megaspore nuclei which co-exist in the coenocyte and all participate in the constitution of the embryo sac [see Maheshwari (1937) and Hu (2005) for a review]
According to the number of megaspore nuclei which participate in the constitution of the embryo sac, three main types of embryo sacs are recognized, i.e. the Monosporic Polygonum type, the Bisporic Allium type and the Tetrasporic Fritillaria type
Summary
Meiotic division allows for the random assortment of parental genes, whereas the union of parental genes upon a successful fertilization event enables the offspring to better adapt to the changing environment (Lee & Yeung 2012). In angiosperms (flowering plants), meiosis of the megasporocyte (the megaspore mother cell) and subsequent mitoses (megagametogenesis) give rise to a female gametophyte (megagametophyte, embryo sac) in which double fertilization, a phenomenon which characterizes angiosperms (Ao 2013), occurs. These two successive cell division events are critical for ensuring successful sexual reproduction (Ao et al 2016). Meiosis of the megasporocyte usually produces four independent megaspores. According to the number of megaspore nuclei which participate in the constitution of the embryo sac, three main types of embryo sacs are recognized, i.e. the Monosporic Polygonum type, the Bisporic Allium type and the Tetrasporic Fritillaria type
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