Learning Objectives
By the end of this section, you will be able to:
- Explain that meiosis and sexual reproduction are evolved traits
- Identify variation among offspring as a potential evolutionary advantage to sexual reproduction
- Describe the three different life-cycle types among sexual multicellular organisms and their commonalities
On the surface, creating offspring that are genetic clones of the parent appears to be a better system. If the parent is successful, offspring with the same traits would be successful. There is the obvious benefit to an organism that can produce offspring whenever circumstances are favorable. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In asexual reproduction populations, every individual is capable of reproduction. So, in theory, an asexual population could grow twice as fast.
Multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexuality so common? There are several possible explanations. One reason is that the variation that sexual reproduction creates is very important to the survival and reproduction of the population. On average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population.
Evolution Connection
The Red Queen Hypothesis
It is not in dispute that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. But why, even in the face of fairly stable conditions, does sexual reproduction persist when it is more difficult and costly for individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973.[1] The concept was named in reference to the Red Queen’s race in Lewis Carroll’s book, Through the Looking-Glass.
All species co-evolve with other organisms. Predators evolve with their prey, and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a co-evolving species to maintain its own share of the resources is to also continually improve its fitness. As one species gains an advantage, this increases selection on the other species. They must also develop an advantage or they will be outcompeted. No single species progresses too far ahead. Genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.”
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms:
(1) diploid-dominant – the multicellular diploid stage is the most obvious life stage, as with most animals including humans
(2) haploid-dominant – the multicellular haploid stage is the most obvious life stage, as with all fungi and some algae
(3) alternation of generations – the two stages are apparent to different degrees depending on the group, as with plants and some algae
Diploid-Dominant Life Cycle
Nearly all animals employ a diploid-dominant life-cycle strategy. The only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads, the testes and ovaries. Germ cells are capable of mitosis to carry on the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 1).
![This illustration shows the life cycle of animals. Through meiosis, adult males produce haploid (1n) sperm, and adult females produce haploid eggs. Upon fertilization, a diploid (2n) zygote forms, which, through mitosis and cell division, grows into an adult.](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/198/2016/11/28182406/Figure_11_02_01-1024x687.jpg)
Figure 1. In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.
Haploid-Dominant Life Cycle
Most fungi and algae employ a life-cycle type in which the “body” of the organism is haploid. The haploid cells making up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although haploid like the “parents,” these spores contain a new genetic combination from two parents. If conditions are questionable for success, the spores can remain dormant for a period of time. Eventually, when conditions are favorable, the spores form multicellular haploid structures by many rounds of mitosis (Figure 2).
Art Connection
![This illustration shows the life cycle of fungi. In fungi, the diploid (2n) zygospore undergoes meiosis to form haploid (1n) spores. Mitosis of the spores occurs to form hyphae. Hyphae can undergo asexual reproduction to form more spores, or they form plus and minus mating types that undergo nuclear fusion to form a zygospore.](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/198/2016/11/28182442/Figure_11_02_02-1024x707.jpg)
Figure 2. Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. (credit “zygomycota” micrograph: modification of work by “Fanaberka”/Wikimedia Commons)
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
Alternation of Generations
The third life-cycle type, employed by some algae and all plants, is a blend of the two others. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes, producing gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case. The organism that produces the gametes is already a haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes (Figure 3).
![This illustration shows the life cycle of fern plants. The diploid (2n) zygote undergoes mitosis to produce the sphorophyte, which is the familiar, leafy plant. Sporangia form on the underside of the leaves of the sphorophyte. Sporangia undergo meiosis to form haploid (1n) spores. The spores germinate and undergo mitosis to form a multicellular, leafy gametophyte. The gametophyte produces eggs and sperm. Upon fertilization, the egg and sperm form a diploid zygote.](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/198/2016/11/28182518/Figure_11_02_03-1024x584.jpg)
Figure 3. Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. The diploid plant is called a sporophyte, producing haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes that produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte. (credit “fern”: modification of work by Cory Zanker; credit “sporangia”: modification of work by “Obsidian Soul”/Wikimedia Commons; credit “gametophyte and sporophyte”: modification of work by “Vlmastra”/Wikimedia Commons)
Sexual reproduction takes many forms in multicellular organisms. However, at some point in each type of life cycle, meiosis produces haploid cells that will fuse with the haploid cell of another organism. The mechanisms of variation, including crossing over, random assortment of homologous chromosomes, and random fertilization, are present in all versions of sexual reproduction. Nearly every multicellular organism on Earth employs sexual reproduction. This is strong evidence for the benefits of producing offspring with unique gene combinations.
Section Summary
Nearly all eukaryotes undergo sexual reproduction. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, containing half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. The ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and the alternation of generations, demonstrated by plants and other algae.
Additional Self Check Questions
- If a mutation occurs so that a fungus is no longer able to produce mating type, will it still be able to reproduce?
- List the three main types of life cycles in multicellular organisms and give an example of an organism that employs each.
Answers
- Yes, it will be able to reproduce asexually.
- The three main types of life cycles are: diploid-dominant, haploid-dominant, and alternation of generations.
Glossary
alternation of generations: life-cycle type in which the diploid and haploid stages alternate
diploid-dominant: life-cycle type in which the multicellular diploid stage is prevalent
haploid-dominant: life-cycle type in which the multicellular haploid stage is prevalent
gametophyte: a multicellular haploid life-cycle stage that produces gametes
germ cells: specialized cell line that produces gametes, such as eggs or sperm
life cycle: the sequence of events in the development of an organism and the production of cells that produce offspring
sporophyte: a multicellular diploid life-cycle stage that produces haploid spores by meiosis
- Leigh Van Valen, “A New Evolutionary Law,” Evolutionary Theory 1 (1973): 1–30 ↵