In Chapter 26, we saw how Bacteria and Archaea generate genetic diversity by horizontal gene transfer. Horizontal gene transfer has been documented in eukaryotic species, but it is relatively uncommon. How then do eukaryotes generate and maintain genetic diversity within populations? The answer is sex.

Sexual reproduction involves meiosis and the formation of gametes, and the subsequent fusion of gametes during fertilization (Chapters 11 and 42). Sex promotes genetic variation in two simple ways. First, meiotic cell division results in gametes or spores that are genetically unique. Each gamete has a combination of alleles different from the other gametes and from the parental cell as a result of recombination and independent assortment. Second, in fertilization, new combinations of genes are brought together by the fusion of gametes. Interestingly, a few eukaryotic groups have lost the capacity for sexual reproduction. The best-studied of these eukaryotes are tiny animals called bdelloid rotifers. The genetic diversity of these organisms is actually high, maintained by high rates of horizontal gene transfer.

Meiotic cell division results in cells with one set of chromosomes. Such cells are haploid. Sexual fusion brings two haploid (1n) cells together to produce a diploid (2n) cell that has two sets of chromosomes. The life cycle of sexually reproducing eukaryotes, then, necessarily alternates between haploid and diploid states.

Many single-celled eukaryotes normally exist in the haploid stage and reproduce asexually by mitotic cell division (Fig. 27.3a). The green alga Chlamydomonas is one such organism. Under the right conditions, however—typically, starvation or other environmental stress—two cells fuse, forming a diploid cell, or zygote. The zygote formed by these single-celled eukaryotes commonly functions as a resting cell. It covers itself with a protective wall and then lies dormant until environmental conditions improve. In time, further signals from the environment induce meiotic cell division, resulting in four genetically distinct haploid cells that emerge from their protective coating to complete the life cycle.

As shown in Fig. 27.3b, some single-celled eukaryotes normally exist as diploid cells. An example is provided by the diatoms, single-celled eukaryotes commonly found in lakes, soils, and the oceans. Diatom cells are mostly diploid and reproduce asexually by mitotic cell division to make more diploid cells. Because their mineralized skeletons constrain growth, diatoms become smaller with each asexual division. Once a critical size is reached, meiotic cell division is triggered, producing haploid gametes that fuse to regenerate the diploid state as a round, thick-walled cell. This cell eventually germinates to form an actively growing, skeletonized cell. In diatoms, then, short-lived gametes constitute the only haploid phase of the life cycle.

The two life cycles just introduced, of Chlamydomonas and diatoms, are similar in many ways. In both, haploid cells fuse to form diploid cells, and diploid cells undergo meiotic cell division to generate haploid cells. Both life cycles also commonly include cells capable of persisting in a protected form when the environment becomes stressful. Why some single-celled eukaryotes usually occur as haploid cells and others usually occur as diploid cells remains unknown—a good question for continuing research.

Sexual reproduction has never been observed in some eukaryotes, but most species appear to be capable of sex, even if they reproduce asexually most of the time. Variations on the eukaryotic life cycle can be complex, especially in parasitic microorganisms that have multiple animal hosts. All, however, have the same fundamental components as the two life cycles described here.

In the following chapters, we discuss multicellular organisms in detail. Here we note only that animals, plants, and other complex multicellular organisms have life cycles with the same features as those just discussed. The big difference is that in animals, the zygote divides many times to form a multicellular diploid body before a small subset of cells within the body undergoes meiotic cell division to form haploid gametes (eggs and sperm). During fertilization, the egg and sperm combine sexually to form a zygote (Fig. 27.3c). In animals, as in diatoms, the only haploid phase of the life cycle is the gamete. As we will see in Chapter 32, plants have two multicellular phases in their life cycle, one haploid and one diploid (Fig. 27.3d). Like single-celled eukaryotes, many plants and animals can reproduce asexually. Humans and other mammals are unusual in that we cannot reproduce asexually.

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