Gametogenesis is the process of gamete formation, which includes micro‐ and megagametogenesis. Gametogenesis initiates after specialized cells in the sporophyte undergo meiosis, and subsequent mitotic divisions yield the gametophytic phase of the plant life cycle. In higher plants, microgametogenesis occurs in the anther, producing tricellular pollen with two sperm cells within a vegetative cell. Megagametogenesis occurs in the ovule, producing an embryo sac. The male gametes, the two sperm cells, and the female gametes, the egg and central cell, fuse to yield the zygote and the endosperm, respectively. Both micro‐ and megagametogenesis are under strict genetic control. Studies of gametophytic mutants have identified genes important for gametogenesis. Furthermore, high‐throughput expression profiling techniques have helped identify gene regulatory networks that operate during gametogenesis.

Key concepts:

  • Plant life cycles alternate between sporophyte and gametophyte generations.

  • Meiosis produces spores which then undergo gametogenesis.

  • Higher plants produce two types of gametes, sperm cells in microgametophytes and egg cell and central cell in megagametophytes.

  • The male and female gametes fuse and give rise to the zygote and to the endosperm.

  • Identification of gametophytic mutants showed that gametogenesis is under strict genetic control.

  • Expression profiling studies provided further insight into the gene regulation of gametogenesis and identified new gametophytic genes.

Keywords: pollen; embryo sac; egg; sperm; central cell

Figure 1.

Schematic representation of the events during micro‐ and megagametogenesis. (a) In male gametogenesis (microgametogenesis), the microspore mother cell undergoes meiosis to give rise to a tetrad with four haploid microspores. Uninucleate microspores are released when callase β1,3‐glucanase, is secreted from the surrounding tapetal cells. Each uninucleate microspore enlarges and undergoes an asymmetric division to yield a bicellular pollen grain. The generative cell in the bicellular pollen grain undergoes mitosis to yield two sperm cells. (b) Female gametogenesis (megagametogenesis) starts with two meiotic divisions of a diploid megasporocyte, which gives rise to four megaspores. Only one of the four megaspores survives and continues gametogenesis with three mitotic divisions. The megagametophyte, now with eight nuclei, undergoes cellularization, cell differentiation and nuclear fusion to give rise to the seven‐celled embryo sac. AP, antipodal cells; EC, egg cell; GC, generative cells; PN, polar nuclei; SY, synergids and VC, vegetative cell.

Figure 2.

Mature pollen grains (male gametophytes). The DNA in the nuclei of the cells is stained with a fluorescent dye. (a) Pine pollen grain has air bladders on either side that facilitate its dispersal by wind, whereas the central chamber contains two cells; the body cell will form the pollen tube and the generative cell will divide into two nonmotile sperm cells. (b) Brassica pollen grain, with two intensely stained sperm cell nuclei and a more diffusely stained vegetative nucleus. (c) Wheat pollen grain, with two elongated sperm cells and a more diffusely stained vegetative nucleus. (d) Tomato pollen grain, with a diffusely stained vegetative nucleus and a more intensely stained generative cell nucleus. The generative cell will divide into two sperm cells during pollen tube growth.



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Further Reading

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Yang WC and Sundaresan V (2000) Genetics of gametophyte biogenesis in Arabidopsis. Current Opinion in Plant Biology 3: 53–57.

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Guan, Yuefeng, Boavida, Leonor, and McCormick, Sheila(Jan 2010) Gametogenesis. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002037.pub2]