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A mature embryo sac in a flowering plant may possess 7-cells and 8 nuclei. Explain with
help of labeled diagram.
uanto drou the dinarame of different stages of embryo develonment in a
Answers
Answer:
Explanation:
Agave tequilana is an angiosperm species that belongs to the family Asparagaceae (formerly Agavaceae). Even though there is information regarding to some aspects related to the megagametogenesis of A. tequilana, this is the first report describing the complete process of megasporogenesis, megagametogenesis, the early embryo and endosperm development process in detail. The objective of this work was to study and characterize all the above processes and the distinctive morphological changes of the micropylar and chalazal extremes after fertilization in this species. The agave plant material for the present study was collected from commercial plantations in the state of Jalisco, Mexico. Ovules and immature seeds, previously fixed in FAA and kept in ethanol 70%, were stained based on a tissue clarification technique by using a Mayer’s-Hematoxylin solution. The tissue clarification technique was successfully used for the characterization of the megasporogenesis, megagametogenesis, mature embryo sac formation, the early embryo and endosperm development processes by studying intact cells. The embryo sac of A. tequilana was confirmed to be of the monosporic Polygonum-type and an helobial endosperm formation. Also, the time-lapse of the developmental processes studied was recorded.
Electronic supplementary material
The online version of this article (doi:10.1186/2193-1801-3-575) contains supplementary material, which is available to authorized users.
Keywords: Agavaceae, Chalazal haustorium, Helobial endosperm, Hypostase, Megagametogenesis, Megasporogenesis, Polygonum-type
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Background
One of the main characteristics of Angiosperms is that they possess seeds enclosed inside a fruit derived from the ovary of flowers (Li and Ma 2002). Another outstanding characteristic of angiosperms is that they present alternation of generations in their life cycle (as in many other plants), that is divided in two phases: one dominant diploid phase, which is called sporophytic, and one haploid phase known as gametophytic (Haig 1990; Rodríguez-Garay et al. 2000; Fan et al. 2008; Ma and Sundaresan 2010). The main function of the gametophyte phase is the production of haploid gametes whether they are male or female (Reiser and Fischer 1993; Yadegari and Drews 2004; Fan et al. 2008).
The female gametophyte, also named megagametophyte or embryo sac, is developed within the carpel, which consists of three elements: the stigma, the style and the ovary, which can contain one or several ovules (Gutiérrez-Mora et al. 2012). In each ovule meiosis of the megaspore mother cell produces four haploid cells called megaspores. In the monosporic pattern, three of these megaspores degenerate while the closest cell to the chalazal region remains viable and gives rise to a single functional megaspore.
During the megagametogenesis process, the functional megaspore passes through one or more mitotic divisions without cytokinesis forming a multinucleate coenocyte. Latter, cell walls are formed around the nuclei resulting in a mature embryo sac (Rabiger and Drews 2013). The embryo sacs may present a diversity of developmental pathways, however, the most common is the monosporic Polygonum-type, in which the functional megaspore passes through three mitotic divisions producing a seven celled embryo sac (Chasan and Walbot 1993; Li and Ma 2002; Maheshwari 1937) consisting of three antipodal cells, one central cell formed by two polar nuclei, two synergid cells, and the egg cell (Dresselhaus 2006; Kägi and Groß-Hardt 2007; Yang et al. 2010).
Some studies have characterized the female gametophyte of different species belonging to the Asparagaceae family formerly Agavaceae (APG III 2009), where the majority of them have been described as Monosporic Polygonum-type. Among these species, Yucca rupicola (Watkins 1937); Y. aloifolia (Wolf 1940); Y. filamentosa (Reed 1903); Agave lechuguilla (Grove 1941), A. virginica (Regen 1941), Hesperocallis undulata, Leucocrinum montanum (Cave 1948) and Comospermum yedoense (Rudall 1999) are found. However, Piven et al. (2001) reported the embryo sac development of Agave fourcroydes and A. angustifolia as bisporic Allium-type.
Only one study could be found that was centered on the Agave tequilana female gametophyte development where Escobar-Guzmán et al. (2008) reported that the megagametophyte is originated from the megaspore that is located closest to the chalazal region, forming an embryo sac of seven cells (Polygonum-type). However, these authors did not report or describe the whole mitotic division process that gives rise to this sac with seven cells, nor the early embryogenesis. Even though there is information regarding to some aspects related to the megagametogenesis of A. tequilana, there is no published information describing the embryo and endosperm development in this species and the distinctive morphological changes of the embryo sac after fertilization.