Life histories ofsexually reproducing organisms Sexually reproducing organisms typically go through a life cycle that includes the fusion of gametes to begin a diploid (2n) stage of life, followed by meiosis to begin a haploid (n) stage of life, followed by . . . the fusion of gametes . . . etc. A single organism goes through the cycle only once, and each succeeding generation also goes through the cycle once. This alternation of phases caused by the fusion of gametes (= syngamy) followed by meiosis is sometimes called the sexual cycle. [by R.C. Clark, Eastern Kentucky University, 2007)
The sexual cycle, in general Here is a general outline of the sexual cycle. Notice that syngamy and meiosis are both necessary to complete the cycle. All organisms that produce gametes go through this cycle. (diploid) (haploid)
Animal pattern of the sexual cycle The details of the sexual cycle vary in different organisms. This is the typical pattern for animals. The reason it seems so familiar is that we are animals, so it is what we do! Notice that in the animal sexual cycle the haploid phase consists only of the gametes. The sex organs of animals are called gonads, and the gametes are produced directly by meiosis.
Plant pattern of the sexual cycle This is the typical sexual cycle pattern for plants. Notice that the basic cycle is the same. In plants, There also are some close parallels with the animal pattern – for instance, the product of syngamy is still called a zygote, and it develops into an immature stage called the embryo. However, there are some fundamental differences between the plant pattern and the animal pattern. In plants, meiosis occurs in sporangia. The products of meiosis are one-celled structures called spores (technically, they are meiospores). The spores typically grow into a multicellular haploid structure called a gametophyte. The gametophyte develops special structures called gametangia (singular = gametangium) The gametangia produce gametes by mitosis. So, gametangia are the sex organs of plants. Obviously, since gametophytes, gametangia and gametes are haploid, this means that haploid cells can and do divide by mitosis – this is a fact that biology instructors usually forget to mention to their students!
Other variations on the sexual cycle There are millions (probably even billions, if you could count the prokaryotes) of different kinds of organisms on Earth. The sexual cycle patterns presented here are generalities. Organisms have many systems of genetic recombination that do not fit the patterns. For instance, bacteria can easily exchange DNA fragments with “unrelated” other bacteria. Some algae (especially brown algae and diatoms) have life cycles very similar to the animal sexual cycle pattern. Some fungi are not known to have gametes; instead, genetic recombination occurs through a phenomenon called parasexuality. The complex fungi typically have haploid (n), diploid (2n) and n+n stages in their life cycles. That is a topic we will cover later. The most basic points to remember are that the life cycles of plants and animals are different. You should know what the differences are, what the structures in a typical plant life history are, where they come from, and what they do. Asexual reproduction also is common in organisms. The mechanisms of asexual reproduction are extremely varied.
The function of sex It is logical to assume that any widespread characteristic of life must be advantageous to life. If a process or structure were detrimental, it would be selected against, and therefore would be rare. Therefore, since sex is so widespread, we must assume that sex gives organisms some type of advantage. The advantage that sex gives organisms is that sex produces and maintains genetically variable populations. Genetically variable populations are more likely to survive extreme environmental conditions. This does not mean that an entire population will survive an environmental “selection event”, but genetic variability makes it much more likely that at least some members of a population will survive an environment crunch.