Unit Four. The Evolution and Diversity of Life
17. Protists: Advent of the Eukaryotes
In the previous section, we mentioned some of the structural differences between prokaryotes and eukaryotes.
But one of the most profoundly important characteristics of eukaryotes is the capacity for sexual reproduction. Indeed, many types of protists undergo sexual reproduction. In sexual reproduction, two different parents contribute gametes to form the offspring. Gametes are usually formed by meiosis, discussed in chapter 9. In most eukaryotes, the gametes are haploid (have a single copy of each chromosome), and the offspring produced by their fusion are diploid (have two copies of each chromosome). In this section, we examine sexual reproduction among the eukaryotes and how it evolved.
Life Without Sex
To fully understand sexual reproduction, we must first examine asexual reproduction among the eukaryotes. Consider, for example, a sponge. A sponge can reproduce by simply fragmenting its body, a process called budding. Each small portion grows and gives rise to a new sponge. This is an example of asexual reproduction, reproduction without forming gametes. In asexual reproduction, the offspring are genetically identical to the parent, barring mutation. The majority of protists reproduce asexually most of the time. Some protists such as the green algae exhibit a true sexual cycle, but only transiently. Asexual reproduction in a protist called Paramecium is shown in figure 17.3a. The single cell duplicates its DNA, grows larger, and then splits in two. The fusion of two haploid cells to create a diploid zygote, the essential act of sexual reproduction, occurs only under stress. Paramecium is again shown in figure 17.3b but now undergoing sexual reproduction. In this case, the cell is not splitting in half; rather two cells are coming into close contact. In a process called conjugation, they exchange genetic information in their haploid nuclei.
Figure 17.3. Reproduction among paramecia.
(a) When Paramecium reproduces asexually, a mature individual divides, and two genetically identical individuals result. (b) In sexual reproduction, two mature cells fuse in a process called conjugation (x100) and exchange haploid nuclei.
The development of an adult from an unfertilized egg is a form of asexual reproduction called parthenogenesis. Parthenogenesis is a common form of reproduction among insects. In bees, for example, fertilized eggs develop into females, while unfertilized eggs become males. Some lizards, fishes, and amphibians reproduce by parthenogenesis; an unfertilized egg undergoes mitosis without cytokinesis to produce a diploid cell, which then undergoes development as if it had been produced by sexual union of two gametes.
Many plants and marine fishes undergo a form of sexual reproduction that does not involve partners. In selffertilization, one individual provides both male and female gametes. Mendel’s peas, discussed in chapter 10, produced their F2 generations by “selfing.” Why isn’t this asexual reproduction (after all, there is only one parent)? This is considered to be sexual rather than asexual reproduction because the offspring are not genetically identical to the parent. During the production of the gametes by meiosis, considerable genetic reassortment occurs—that is why Mendel’s F2 plants were not all the same!
If reproduction without sex is so common among eukaryotes today, it is a fair question to ask why sex occurs at all. Evolution is the result of changes that occur at the level of individual survival and reproduction, and it is not immediately obvious what advantage is gained by the progeny of an individual that engages in sexual reproduction. Indeed, the segregation of chromosomes that occurs in meiosis tends to disrupt advantageous combinations of genes more often than it assembles new, better-adapted ones. Because all the progeny could maintain a parent’s successful gene combinations if the parent employed asexual reproduction, the widespread use of sexual reproduction among eukaryotes raises a puzzle: Where is the benefit from sex that promoted the evolution of sexual reproduction?
How Sex Evolved
In attempting to answer this question, biologists have looked more carefully at where sex first evolved—among the protists. Why do many protists form a diploid cell in response to stress? Biologists think this occurs because only in a diploid cell can certain kinds of chromosome damage be repaired effectively, particularly double-strand breaks in DNA. Such breaks are induced, for example, by desiccation—drying out. The early stages of meiosis, in which the two copies of each chromosome line up and pair with each other, seems to have evolved originally as a mechanism for repairing doublestrand damage to DNA by using the undamaged version of the chromosome as a template to guide the fixing of the damaged one. In yeasts, mutations that inactivate the system that repairs double-strand breaks of the chromosomes also prevent crossing over. Thus, it seems likely that sexual reproduction and the close association between pairs of chromosomes that occurs during meiosis first evolved as mechanisms to repair chromosomal damage by using the second copy of the chromosome as a template.
Why Sex Is Important
One of the most important evolutionary innovations of eukaryotes was the invention of sex. Sexual reproduction provides a powerful means of shuffling genes, quickly generating different combinations of genes among individuals. Genetic diversity is the raw material for evolution. In many cases the pace of evolution appears to be geared to the level of genetic variation available for selection to act upon—the greater the genetic diversity, the more rapid the evolutionary pace. Programs for selecting larger domestic cattle and sheep, for example, proceed rapidly at first but then slow as all of the existing genetic combinations are exhausted; further progress must then await the generation of new gene combinations. The genetic recombination produced by sexual reproduction has had an enormous evolutionary impact because of its ability to rapidly generate extensive genetic diversity.
Sexual Life Cycles
Many protists are haploid all their lives, but with few exceptions, animals and plants are diploid at some stage of their lives. That is, the body cells of most animals and plants have two sets of chromosomes, one from the male and one from the female parent. The production of haploid gametes by meiosis, followed by the union of two gametes in sexual reproduction, is called the sexual life cycle.
Eukaryotes are characterized by three major types of sexual life cycles (figure 17.4):
1. In the simplest of these, found in many algae, the zygote formed by the fusion of gametes is the only diploid cell. This sort of life cycle, which you can see in figure 17.4a, is said to represent zygotic meiosis, because in algae the zygote undergoes meiosis. Haploid cells occupy the major portion of the life cycle, as indicated by the larger yellow box; the diploid zygote undergoes meiosis immediately after it is formed.
2. In most animals, the gametes are the only haploid cells. They exhibit gametic meiosis, because in animals meiosis produces the gametes. Here the diploid cells occupy the major portion of the life cycle, as indicated by the larger blue box in figure 17.4b.
3. Plants exhibit sporic meiosis, because in plants the spore-forming cells undergo meiosis. In plants there is a regular alternation of generations between a haploid phase (the yellow boxed area in figure 17.4c) and a diploid phase (the blue boxed area in figure 17.4c). The diploid phase produces spores that give rise to the haploid phase, and the haploid phase produces gametes that fuse to give rise to the diploid phase.
Figure 17.4. Three types of eukaryotic life cycles.
(a) Zygotic meiosis, a life cycle found in most protists. (b) Gametic meiosis, a life cycle typical of animals. (c) Sporic meiosis, a life cycle found in plants.
The genesis of sex, then, involved meiosis and fertilization with the participation of two parents. We have previously said that bacteria lack true sexual reproduction, although in some groups, two bacteria do pair up in conjugation and exchange parts of their genome. The evolution of true sexual reproduction among the protists has no doubt contributed importantly to their tremendous diversification and adaptation to an extraordinary range of ways of life, as we shall see in section 17.3.
Key Learning Outcome 17.2. Sex evolved among eukaryotes as a mechanism to repair chromosomal damage, but its importance is as a means of generating diversity.