Concept 20.3: Protists Reproduce Sexually and Asexually

Although most protists engage in both asexual and sexual reproduction, sexual reproduction has yet to be observed in some groups. In some protists, as in all prokaryotes, the acts of sex and reproduction are not directly linked.

Several asexual reproductive processes have been observed among the protists:

• The equal splitting of one cell into two by mitosis followed by cytokinesis
• The splitting of one cell into multiple (i.e., more than two) cells
• The outgrowth of a new cell from the surface of an old one (known as budding)
• The formation of specialized cells (spores) that are capable of developing into new individuals (known as sporulation)

Asexual reproduction results in offspring that are genetically nearly identical to their parents (they only differ by new mutations that may arise during DNA replication). Such asexually reproduced groups of nearly identical organisms are known as clonal lineages.

Sexual reproduction among the protists takes various forms. In some protists, as in animals, the gametes are the only haploid cells. In others, the zygote is the only diploid cell. In still others, both diploid and haploid cells undergo mitosis, giving rise to alternating multicellular diploid and haploid life stages.

Some protists have reproduction without sex and sex without reproduction

As we noted in Concept 20.2, members of the genus Paramecium are ciliates, which commonly have two types of nuclei in a single cell (one macronucleus and from one to several micronuclei; see Figure 20.6). The micronuclei, which are typical eukaryotic nuclei, are essential for genetic recombination. Each macronucleus contains many copies of the genetic information, packaged in units containing very few genes each. The macronuclear DNA is transcribed and translated to regulate the life of the cell.

When paramecia reproduce asexually, all of the nuclei are copied before the cell divides. Paramecia also have an elaborate sexual behavior called conjugation, in which two individuals line up tightly against each other and fuse in the oral groove region of the body. Nuclear material is extensively reorganized and exchanged over the next several hours (FIGURE 20.18). Each cell ends up with two haploid micronuclei, one of its own and one from the other cell, which fuse to form a new diploid micronucleus. A new macronucleus develops from the micronucleus through a series of dramatic chromosomal rearrangements. The exchange of nuclei is fully reciprocal: each of the two paramecia gives and receives an equal amount of DNA. The two organisms then separate and go their own ways, each equipped with new combinations of alleles.

Figure 20.18: Paramecia Achieve Genetic Recombination by Conjugating The exchange of micronuclei by two conjugating Paramecium individuals results in genetic recombination. After conjugation, the cells separate and continue their lives as two individuals.

Conjugation in Paramecium is a sexual process of genetic recombination, but it is not a reproductive process. Two cells begin the process, and two cells are there at the end, so no new cells are created. As a rule, each asexual clone of paramecia must periodically conjugate. Experiments have shown that if some species are not permitted to conjugate, the clones can live through no more than approximately 350 cell divisions before they die out.

Some protist life cycles feature alternation of generations

Alternation of generations is a type of life cycle found in many multicellular protists, all land plants, and some fungi. A multicellular, diploid, spore-producing organism gives rise to a multicellular, haploid, gamete-producing organism. When two haploid gametes fuse, a diploid organism is produced. The haploid organism, the diploid organism, or both may also reproduce asexually.

The two alternating (spore-producing and gamete-producing) generations differ genetically (one has diploid cells, the other haploid cells), but they may or may not differ morphologically. In heteromorphic alternation of generations, the two generations differ morphologically, but in isomorphic alternation of generations, they do not. Examples of both heteromorphic and isomorphic alternation of generations are found among the brown algae.

The gamete-producing generation does not produce gametes by meiosis because the gamete-producing organism is already haploid. Instead, specialized cells of the diploid sporeproducing organism, called sporocytes, divide meiotically to produce four haploid spores. The spores may eventually germinate and divide mitotically to produce the multicellular haploid generation, which then produces gametes by mitosis and cytokinesis.

Gametes, unlike spores, can produce new organisms only by fusing with other gametes. The fusion of two gametes produces a diploid zygote, which then undergoes mitotic divisions to produce a diploid organism. The diploid organism’s sporocytes then undergo meiosis and produce haploid spores, starting the cycle anew.