Microspores areland plantspores that develop into malegametophytes, whereasmegaspores develop into female gametophytes.[1] The male gametophyte gives rise to sperm cells, which are used for fertilization of an egg cell to form azygote. Megaspores are structures that are part of thealternation of generations in many seedless vascularcryptogams, allgymnosperms and allangiosperms. Plants withheterosporous life cycles using microspores and megaspores arose independently in several plant groups during theDevonian period.[2] Microspores arehaploid, and are produced fromdiploid microsporocytes bymeiosis.[3]
The microspore has three different types of wall layers. The outer layer is called theperispore, the next is theexospore, and the inner layer is theendospore. The perispore is the thickest of the three layers while the exospore and endospore are relatively equal in width.[4]
In heterosporous seedless vascular plants, modified leaves calledmicrosporophylls bearmicrosporangia containing many microsporocytes that undergomeiosis, each producing four microspores. Each microspore may develop into a male gametophyte consisting of a somewhat sphericalantheridium within the microspore wall. Either 128 or 256 sperm cells with flagella are produced in each antheridium.[3] The only heterosporousferns are aquatic or semi-aquatic, including the generaMarsilea,Regnellidium,Pilularia,Salvinia, andAzolla. Heterospory also occurs in thelycopods in the spikemoss genusSelaginella and in the quillwort genusIsoëtes.
Types of seedless vascular plants:
Inseed plants the microspores develop intopollen grains each containing a reduced, multicellular male gametophyte.[5] The megaspores, in turn, develop into reduced female gametophytes that produce egg cells that, once fertilized, develop into seeds. Pollen cones ormicrostrobili usually develop toward the tips of the lower branches in clusters up to 50 or more. The microsporangia of gymnosperms develop in pairs toward the bases of the scales, which are therefore calledmicrosporophylls. Each of the microsporocytes in the microsporangia undergoes meiosis, producing four haploid microspores. These develop into pollen grains, each consisting of four cells and, in conifers, a pair of external air sacs. The air sacs give the pollen grains added buoyancy that helps with wind dispersal.[3]
Types of Gymnosperms:
As theanther of a flowering plant develops, four patches of tissue differentiate from the main mass of cells. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis producing a quartet of microspores. Four chambers (pollen sacs) lined with nutritivetapetal cells are visible by the time the microspores are produced. After meiosis, the haploid microspores undergo several changes:
These steps occur in sequence and when complete, the microspores have become pollen grains.[3]
Although it is not the usual route of a microspore, this process is the most effective way of yielding haploid and double haploid plants through the use of male sex hormones.[6] Under certain stressors such as heat or starvation, plants select for microspore embryogenesis. It was found that over 250 different species of angiosperms responded this way.[6] In theanther, after a microspore undergoesmicrosporogenesis, it can deviate towards embryogenesis and become star-like microspores. The microspore can then go one of four ways: Become an embryogenic microspore, undergocallogenesis toorganogenesis (haploid/double haploid plant), become a pollen-like structure or die.[6]
Microspore embryogenesis is used in biotechnology to produce double haploid plants, which are immediately fixed as homozygous for each locus in only one generation. The haploid microspore is stressed to trigger the embryogenesis pathway and the resulting haploid embryo either doubles its genome spontaneously or with the help of chromosome doubling agents. Without this double haploid technology, conventional breeding methods would take several generations of selection to produce a homozygous line.[7]
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