Simpler organisms, such as unicellular ones, reproduce by asexual reproduction, in which offspring are produced by a single individual. The offspring have the same genotype as the parent. Examples of asexual reproduction range from binary fission in bacteria to vegetative propagation of plants by cuttings or the budding of the hydra, a jellyfishlike animal.
In sexual reproduction, offspring are produced by combining genetic material from two distinct individuals. The offspring are genetically different from either parent by virtue of having a combination of the genes from each. Sexual reproduction evolved as a strategy to greatly increase variation within a population. Recombination during meiosis can combine pieces of two genes from alleles from different parents to create novel genes. This facilitates adaptation to changing conditions. When environmental conditions change, it is likely that some subset of the population will have a genotype that is more appropriate than the majority of the population to the new conditions. This minority will tend to flourish, establishing a new majority that is better adapted. To put it succinctly: Sex produces variation without mutation. This is the advantage that sexual reproduction confers on a species.
In diploid organisms that reproduce sexually, the germ cell line alternates between hap-loid and diploid with each generation. The transformations from one to the other are performed by the processes of meiosis and fertilization. Meiosis converts diploid cells into haploid cells, which become the gametes. In fertilization, two gametes unite to create the diploid zygote cell. Around this basic plan, there are three variations (Figure 6.10).
In zygotic meiosis, the haploid cells produced by meiosis are called spores. Spores are haploid cells that can undergo mitosis, producing a multicellular haploid organism. [In contrast, gametes are haploid cells that must fuse (in the process of fertilization) to form diploid cells, which in turn divide mitotically to form a multicellular diploid organism.] Spores reproduce to form a haploid multicellular individual (or many unicellular ones). These individuals eventually produce gametes, which through fertilization produce a diploid zygote. The zygote then immediately undergoes meiosis. The zygote is the only diploid form in organisms with this type of life cycle. Organisms with this life cycle include the fungi and some algae, such as Chlamydomonas.
Gametic meiosis is a life cycle wherein meiosis produces haploid gametes almost directly. Subsequent fusion of two gametes in fertilization results in a diploid zygote, as in zygotic meiosis. However, in gametic meiosis the zygote now forms a multicellular individual. Thus, the diploid form is the only multicellular form, and it dominates the life cycle. This life cycle is characteristic of most animals, some protists, and at least one alga. In animals, the female gametophyte is the egg, and the male gametophyte is the sperm.
In sporic meiosis, multicellular forms appear after both meiosis and fertilization. The haploid organism is called a gametophyte; the diploid form is a sporophyte. This plan is typical of all plants and many algae. Sometimes the gametophyte looks like the
sporophyte (e.g., certain brown algae). In most plants, they are distinct. In simpler plants, such as mosses, the gametophyte is larger than the sporophyte and is self-sufficient nutritionally. In the higher vascular plants the sporophyte is dominant and the gametophyte depends on the sporophyte for nutrition. In the most extreme example, in flowering plants the female gametophyte has only seven cells and the male gametophyte (pollen) has only three.
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