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.

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).

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

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).

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.