Reproduction, Sexual | Research & Encyclopedia Articles

Sexual reproduction is a fundamental process in plants that involves the production of egg and sperm followed by their fusion to form a zygote, which then divides and eventually develops into a new plant. Sexual reproductionin flowering plants involves four sequential processes: sporogenesis, game-togenesis, pollination, and fertilization, all of which occur within the reproductive organs (the anthers and ovules) of the flower. Anthers are the site of (male) pollen formation, and ovules are the site of (female) egg formation.

Sporogenesis, or spore formation, begins with the differentiation of specialized spore mother cells within the anthers and ovules. The spore mother cells are unique because they undergo meiosis, a division that reduces the chromosome number by one-half, or from diploid to haploid. The haploid spores produced by meiosis in the anthers are called microspores (small spores), while those in the ovules are called megaspores (big spores).

During male gametogenesis, each microspore divides twice to produce a pollen grain, or mature male gametophyte, that consists of only three cells: two sperm cells and one vegetative cell. Female gametogenesis is slightly more complex. Of the four haploid megaspores formed following meiosis of the female spore mother cells, one typically divides four times to produce an eight-nucleate, seven-celled embryo sac, or mature female gametophyte. One of these cells becomes the egg.

Following gametogenesis, the sperm within the pollen grain must somehow reach the egg, which is buried within the ovary of the flower, before fertilization can occur. Flowering plants have evolved numerous adaptations that aid in the transfer of pollen to the tip of the pistil, or stigma. This process is referred to as pollination, and can be mediated by wind, insects, bats, or rodents. Once the pollen reaches the stigma, which is often sticky or hairy to trap the pollen grain, the pollen grain swells and germinates. It then sends a tip-growing tube through the style of the pistil to the egg. The vegetative cell of the pollen aids in tube growth. Once the tip of the pollen tube reaches the egg, it discharges the two sperm cells. One sperm cell fuses with the egg to form the diploid zygote, while the other sperm cell fuseswith two nuclei that reside very close to the egg cell within the embryo sac. The triploid cell that results from this second fertilization event divides to form triploid endosperm, which is starchy material stored in the seed and provides nutrition for the developing embryo. Coconut milk and cornstarch are familiar examples of endosperm.

Although not all plants produce flowers or seeds, all land plants do form gametophytes of various shapes and sizes. In many lower plants, such as mosses and ferns, the haploid spores are shed from their parent and can remain dormant for many years. Once in a favorable environment, the spores germinate and divide to form a multicellular gametophyte that develops independently of the parent plant. Each gametophyte produces motile sperm and nonmotile egg cells. Until it develops a root system, the young embryo remains attached to and dependent upon the gametophyte. Because the gametophytes of these plants lack water-conducting tissues and require water for the sperm to swim to the egg, they can only be found in places that are damp for at least part of the year. While the fern gametophyte is small (0.25 inches), the moss gametophyte is the lush green carpet we think of as the moss plant.

Most flowering plants produce "perfect" or hermaphroditic flowers with both male and female parts and can readily self-fertilize. One consequence of self-fertilization is inbreeding, which can have negative effects on off-spring because they have a high probability of being homozygous for lethal recessive mutations. To avoid self-fertilization, flowering plants have evolved a number of adaptations or modifications to promote out-crossing, or mating between two individuals. Among these are genetic incompatibility, temporal (time-related) separation of pollen and egg maturation, as well as physical separation of the sexes into different flowers or individuals. Monoecious ("one house") plants, such as maize (corn), produce unisexual male flowers or female flowers, but both types are present on the same plant. Dioecious ("two house") plants, such as holly, produce unisexual male or female flowers on different plants. In some dioecious species, the sex of the individual is determined by sex chromosomes, while in other species, the sex of the flower is determined normally, and can be manipulated by applying plant growth hormones. Monoecious and dioecious species are thought to have evolved from species that produced perfect flowers.

Scientists have only recently begun to study and identify the genes that are involved in the evolution of reproductive structures in plants by studying the evolution of maize. The domestication of modern maize (Zea mays spp. mays) from its wild progenitor species, teosinte (Zea mays ssp. parviglumis) began approximately ten thousand years ago. During this period, agriculturists selected for traits, such as the monoecious condition, that affect the reproductive structures of this plant. What is known at this point is that the large differences one observes between maize and teosinte are attributed to differences in a very small number of genetic loci. Once all of these loci have been cloned, scientists will be able to understand at the molecular level how reproductive characteristics evolve.

Many plants propagate themselves readily by asexual reproduction. Cattails (Typha latifolia), for example, vegetatively multiply by underground stems to form large stands of genetically identical individuals. Why do such plants expend great amounts of energy to produce the floral structures necessary for sexual reproduction when they can successfully reproduce without sexual reproduction? Scientists believe that sexual reproduction is widespread among living organisms because the advantages it provides to the species outweigh the disadvantages. The key to understanding these advantages has to do with the genetic processes that occur during meiosis and fertilization. During meiosis, homologous pairs of chromosomes (each chromosome of a pair previously contributed by each parent) pair with each other and recombine, or exchange genetic material. The resulting haploid cell or gamete contains only one of each chromosome pair, yet each chromosome has a mixture of genetic material from both parents. This mixing of genetic information during sexual reproduction results in offspring that are genetically and morphologically different (compare the appearance of genetically identical twins to nonidentical siblings). These differences allow natural selection and adaption to changing conditions. Sexual reproduction thus serves two purposes: in many cases, it is necessary to propagate the species and in all cases is needed to maintain genetic diversity within a species. The long-term consequence of a species that lacks genetic diversity between its members is extinction.

Breeding Systems; Corn; Flowers; Pollination; Reproduction, Alternation of Generations And; Reproduction, Fertilization And; Reproduction, Sexual; Seeds.

Banks, J. "Gemetophyte Development in Ferns." Annual Review of Plant Physiology and Plant Molecular Biology 50 (1999): 163-86.

Doebley, J., and A. Stec. "Inheritance of the Morphological Differences Between Maize and Teosinte: Comparison of Results for Two F2 Populations." Genetics 134 (1993): 559-70.

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