In Multicellular Animals and Most Plants, the Diploid Phase Is Complex and Long, the Haploid Simple and Fleeting

Cells proliferate by mitotic division. In most organisms that reproduce sexually, this proliferation occurs during the diploid phase. Some primitive organisms, such as fission yeasts, are exceptional in that the haploid cells proliferate mitotically and the diploid cells, once formed, proceed directly to meiosis. A less extreme exception occurs in plants, where mitotic cell divisions occur in both the haploid and the diploid phases. In all but the most primitive plants, such as mosses and ferns, however, the haploid phase is very brief and simple, while the diploid phase is extended into a long period of development and proliferation. For almost all multicellular animals, including vertebrates, practically the whole of the life cycle is spent in the diploid state: the haploid cells exist only briefly, do not divide at all, and are highly specialized for sexual fusion (Figure 20-3).

Figure 20-3

Haploid and diploid cells in the life cycle of higher and some lower eucaryotes. The haploid cells are shown in red and the diploid cells in blue. Cells in higher eucaryotic organisms usually proliferate in the diploid phase to form a multicellular organism; (more...)

Haploid cells that are specialized for sexual fusion are called gametes. Typically, two types of gametes are formed: one is large and nonmotile and is referred to as the egg (or ovum); the other is small and motile and is referred to as the sperm (or spermatozoon) (Figure 20-4). During the diploid phase that follows the fusion of gametes, the cells proliferate and diversify to form a complex multicellular organism. In most animals, a useful distinction can be drawn between the cells of the germ line, from which the next generation of gametes will be derived, and the somatic cells, which form the rest of the body and ultimately leave no progeny. In a sense, the somatic cells exist only to help the cells of the germ line (the germ cells) survive and propagate.

Figure 20-4

Scanning electron micrograph of a clam egg with sperm bound to its surface. Although many sperm are bound to the egg, only one will fertilize it, as we discuss later. (Courtesy of David Epel.)

Sexual Reproduction Gives a Competitive Advantage to Organisms in an Unpredictably Variable Environment

The machinery of sexual reproduction is elaborate, and the resources spent on it are large (Figure 20-5). What benefits does it bring, and why did it evolve? Through genetic recombination, sexual individuals produce unpredictably dissimilar offspring, whose haphazard genotypes are at least as likely to represent a change for the worse as a change for the better. Why, then, should sexual individuals have a competitive advantage over individuals that breed true, by an asexual process? This problem continues to perplex population geneticists, but the general conclusion seems to be that the reshuffling of genes in sexual reproduction helps a species to survive in an unpredictably variable environment. If a parent produces many offspring with a wide variety of gene combinations, there is a better chance that at least one of the offspring will have the assortment of features necessary for survival. Sexual reproduction also allows the many deleterious mutations that accumulate randomly to be eliminated, while permitting the rare advantageous mutations that arise in separate individuals to be combined in a single individual.

Figure 20-5

A peacock displaying his elaborate tail. This extravagant plumage serves solely to attract females for the purpose of sexual reproduction. (© Cyril Laubscher.)

Whatever the benefits of sex may be, it is striking that practically all complex present-day organisms have evolved largely through generations of sexual, rather than asexual, reproduction. Asexual organisms, although plentiful, seem mostly to have remained simple and primitive.

We now turn to the cellular mechanisms of sex, beginning with the events of meiosis, which segregates the chromosomes into new sets, as the diploid cells in the germ line divide to produce haploid gametes. We then focus our discussion on mammals. We consider the diploid cells of the germ line that give rise to the gametes and how the sex of a mammal is determined. Finally, we discuss the nature of the gametes themselves, as well as the process of fertilization, in which two gametes fuse to form a zygote, which develops into a new diploid organism.

Summary

Sexual reproduction has been favored by evolution probably because the random recombination of genetic information improves the chances of producing at least some offspring that will survive in an unpredictably variable environment. The sexual reproductive cycle involves an alternation of diploid and haploid states: diploid cells divide by meiosis to form haploid cells, and the haploid cells from two individuals fuse in pairs at fertilization to form new diploid cells. In the process, genomes are mixed and recombined to produce individuals that inherit novel assortments of genes. Most of the life cycle of higher plants and animals is spent in the diploid phase; only a small proportion of the diploid cells (those in the germ line) undergo meiosis to produce haploid cells (the gametes), and the haploid phase is very brief.