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12.5: Sexual Reproduction in Angiosperms

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    The lifecycle of angiosperms follows the alternation of generations explained previously. The haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms.

    Male Gametophyte: The Pollen Grain

    The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive organs, development of pollen takes place in a structure known as the microsporangium (Figure 1). The microsporangia, which are usually bi-lobed, are pollen sacs in which the microspores develop into pollen grains. These are found in the anther, which is at the end of the stamen—the long filament that supports the anther.

    Illustration A shows cross section of an anther, which has four lobes each containing a pollen sac, or microsporangium. Inside the pollen sac is a layer called the tapetum, and within this ring are the microspore mother cells. As the microsporangium matures, two pollen sacs merge and an opening forms between them so that the pollen can be released. Micrographs in part B show pollen sacs with a visible opening between them.
    Figure 1. Shown is (a) a cross section of an anther at two developmental stages. The immature anther (top) contains four microsporangia, or pollen sacs. Each microsporangium contains hundreds of microspore mother cells that will each give rise to four pollen grains. The tapetum supports the development and maturation of the pollen grains. Upon maturation of the pollen (bottom), the pollen sac walls split open and the pollen grains (male gametophytes) are released. (b) In these scanning electron micrographs, pollen sacs are ready to burst, releasing their grains. (credit b: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

    Within the microsporangium, the microspore mother cell divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain (Figure 2). An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.

    Illustration shows the formation of pollen from a microspore mother cell. The mother cell undergoes meiosis to form a tetrad of cells, which separate to form the pollen grains. The pollen grains undergo mitosis without cytokinesis, resulting in four mature pollen grains with two nuclei each. One is called the generative nucleus, and the other is called the pollen tube nucleus. Two projective layers form around the mature pollen grain, the inner intine and the outer exine. Micrograph shows a pollen grain, which looks like puffed wheat.
    Figure 2. Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube cell. The pollen grain has two coverings: an inner layer (intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains. (credit “pollen micrograph”: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

    Female Gametophyte: The Embryo Sac

    Illustration depicts the embryo sac of an angiosperm, which is egg-shaped. The narrow end, called the micropylar end, has an opening that allows pollen to enter. The other end is called the chalazal end. Three cells called antipodals are at the chalazal end. The egg cell and two other cells called synergids are at the micropylar end. Two polar nuclei are inside the central cell in the middle of the embryo sac.
    Figure 3. As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell.

    While the details may vary between species, the overall development of the female gametophyte has two distinct phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium—an area of tissue in the ovules—undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of the nuclei—the polar nuclei—move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells (Figure 3). The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.

    A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.

    Practice Question

    An embryo sac is missing the synergids. What specific impact would you expect this to have on fertilization?

    1. The pollen tube will be unable to form.
    2. The pollen tube will form but will not be guided toward the egg.
    3. Fertilization will not occur because the synergid is the egg.
    4. Fertilization will occur but the embryo will not be able to grow.

    [reveal-answer q=”673553″]Show Answer[/reveal-answer]
    [hidden-answer a=”673553″]Statement b should be expected.[/hidden-answer]

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