Life Cycles of Sexually Reproducing Organisms
In sexual reproduction, the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends upon the organism. The process of meiosis, the division of the contents of the nucleus that divides the chromosomes among gametes, reduces the chromosome number by half, while fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in eukaryotic organisms: diploid-dominant, haploid-dominant, and alternation of generations.
Diploid-Dominant Life Cycle
In the diploid-dominant life cycle, the multicellular diploid stage is the most obvious life stage, as occurs with most animals, including humans. Nearly all animals employ a diploid-dominant life cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads (e.g. testes and ovaries). Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state .
Diploid-Dominant Life Cycle
In animals, sexually-reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.
Haploid-Dominant Life Cycle
Within haploid-dominant life cycles, the multicellular haploid stage is the most obvious life stage. Most fungi and algae employ a life cycle type in which the "body" of the organism, the ecologically important part of the life cycle, is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals, designated the (+) and (−) mating types, join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although haploid like the "parents," these spores contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are conducive, the spores form multicellular haploid structures by many rounds of mitosis .
Haploid-Dominant Life Cycle
Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis.
Alternation of Generations
The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes because the organism that produces the gametes is already a haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes .
Alternation of Generations
Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte.
Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte organism is the free-living plant, while the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living; however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.
Sexual reproduction takes many forms in multicellular organisms. However, at some point in each type of life cycle, meiosis produces haploid cells that will fuse with the haploid cell of another organism. The mechanisms of variation (crossover, random assortment of homologous chromosomes, and random fertilization) are present in all versions of sexual reproduction. The fact that nearly every multicellular organism on earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations, although there are other possible benefits as well.