Biology as Poetry: Ecology

Bacteriophage Ecology Group

Sexual Reproduction

Process of meiosis that is followed, after varying intervals, by fertilization, and which is then followed, again after varying intervals, by meiosis.

The above definition of sexual reproduction is somewhat vague since sexual cycles can vary somewhat between different types of organisms. First, for something to be sexual reproduction it must genuinely give rise somewhat directly to reproduction, that is, rather than sexual processes that do not lead directly to reproduction (e.g., bacterial sex) or sexual processes that are simply associated with reproduction rather than directly leading to reproduction (i.e., virus sex).

Second, even given direct ties between sex and subsequent reproduction, there still exists a great deal of diversity in approaches to sexual reproduction. Furthermore, that diversity exists even ignoring the obvious anatomical and behavioral differences between how different species participate in matings.

In animals, and as familiar to us, sexual reproduction follows the path of meiosis → gametes → fertilization → mitosis → meiosis (thus completing the cycle). The mitosis part literally is us, that is, our bodies are the product of the mitotic division as indicated in this progression.

In a number of organisms the bodies are haploid rather than the commonly observed diploid of animals. In this case the following progression instead occurs: meiosis → spores → mitosis → gametes → fertilization → meiosis. In particular, as fertilization is what restores the diploid state but is followed directly by meiosis, almost all of what takes place in the life cycle of these organisms takes place with haploid individuals. Such life cycles are predominantly seen with fungi (though with a lot more complicating detail than has been presented).

Lastly there are plants, which display something known as alternation of generations. Here the progression is meiosis → spores → mitosis → gametes → fertilization → mitosis → meiosis. Note how as with fungi the meiosis step gives rise to spores rather than gametes. Unlike fungi, however, there is mitosis following fertilization, that is, plants alternate between haploid and diploid individuals (plant bodies), a.k.a., alternation of generations. Various multicellular algae also display alternation of generations.

To reiterate, all of these variations on sexual reproduction involve meiosis, gametes, fertilization, and also mitosis. In addition, for organisms that exist as multicellular haploid individuals, spores also are present in this life cycle. The haploid organisms that germinate from these spores, though, give rise to gametes essentially by asexual rather than sexual means.

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