Sex, Lies, and ... Meiosis

Continuing my theme on "10 Things Every Biology Student Should Know," what is meiosis and, considering it may be a boring topic for most, why is it of interest from a Darwinian view of life? That's where we're going. In the process, we'll talk about sex and lies, no doubt.

Many living things lead sexual life styles. Although there are several different types of sexual modes of operation, as those in some plants and fungi, we will focus on sexual life cycles in animals, including humans ("We're all sexual people, with so much love to give" - Marvin Gaye). Homo sapiens and other vertebrates are physiologically made up of many, many millions of somatic cells. These comprise most of the body and are not passed directly to offspring. Another set of cells, termed the germline, is comprised of all gamete cells and gamete-producing cells from gamete to zygote and back (Fig. 1).

Figure 1. This diagram reveals the "dead end" nature of somatic cell lines in relation to the theoretically eternal germline (reference).

After fertilization, some of our cells go on to specialize in gamete (sperm or egg) generation. These are located in the male testis and female ovary. This is not a new concept. However, as Fig. 1 points out, it is important students of biology realize somatic cells are never passed on from one generation to the next under normal conditions (e.g., barring mutation of somatic cells). That's why your liver cells are not inherited from your parents. For the empirical materialist, this is evidence for the idea of the selfish gene, which Richard Dawkins set forth in his book by the same title. Some philosophers use this fact and others about somatic and germ cell lines to make arguments with important implications for not only how we view the world, what we can learn from these facts, but also about applications of this knowledge in medical studies; for example, stem cell research.

So, we have sex, which drives much of the advertising industry and is a major player in people's behavior and a major influence upon American culture (music, clothing, tv shows, language, etc.), which tends to direct the rest of the world's societies for good or bad. The fact of sex does not set us far apart from the many other animals with sexual life cycles. So, we are like animals in this respect. But I'm not going to discuss that claim. A major question that all human beings grapple with is, "Why do we have sex?" In response to this important issue, I am, however, going to make the claim that sexual reproduction is likely to have been conserved for a variety of reasons which only make sense in light of evolution. There are many many reasons--perhaps you can name several from your own experience, even--why sex has negative effects, why sexual life cycles aren't perfect systems.

"What are the negative aspects of sex?" you say. "Why would he say sexual reproduction is a bad way to operate, in the loose sense? What's wrong with the human reproductive system?" Well, among other things, sex consumes a large amount of time, resources, chemical energy, and so on. Also, our plumbing seems a bit off down below, especially for the males of our species, who have all their genetalia dangling out in the environment (necessary because body heat negatively effects sperm production, unnecessary because of other more obvious problems including physical damage). Add to these difficulties associated with obtaining mates (e.g., competition), and it seems sex is an awfully troublesome means of getting the reproduction job done. So what is the evolutionary significance of sex? The answer lies in the chromosomal basis of inheritance and the mechanisms responsible for genetic variation related to meiosis and other genetic processes. I am going to discuss reasons why evolution has selected this modus operandi anyway and will be detailing the way our bodies generate and pass gametes from one generation to the next, with an emphasis on the contrast between mitosis and meiosis.

Meiosis

I have discussed the cell cycle and mitosis at length. Recall, mitosis is the body's means of increasing the number of cells in a living organism and in part this is due to the fact that increasing the surface area of cell connections and maintaining small cell size increases the speed and efficiency of cell signaling, nutrient processing, and cell packaging and shipping of cellular products, et cetera. Mitosis starts with a single diploid cell and generates two copies of the original starting material (roughly exact copies). Our somatic cells undergo lots of mitotic divisions (some of them, others to a lesser extent/rate). So, for every action, there is an equal and opposite reaction. Meiosis is our bodies counter to constant division, because it is the sole means of reducing the body's ploidy number; meiosis reduces chromosome number by generating four haploid cells from a single diploid cell.

Figure 2. Mitosis vs. Meiosis.

Mitosis involves prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis in sequential order. Meiosis involves the same steps, but carries out the phases twofold with a modification of theme. Every time meiosis happens, it happens again for the same daughter cells, generating four haploid cells (Fig. 2, right side).

The main difference between mitosis and meiosis (which is just mitosis X2), is that during metaphase (metaphase I) homologous chromosome pairs line up in tetrads and experience synapsis and crossing over at the center of the cell. In contrast, individual sets of duplicated chromosomes line up during mitotic metaphase. Also, sister chromatids remain connected to one another in meiosis during anaphase I, while during mitosis these are the objects being pulled apart toward different cell poles on microtubule spindles. For the most part, that's it (of course, there are some details we've glossed over). If that's where your high school biology teacher or professor stopped talking or you stopped listening, or if you missed that day of biology class, let me explain the coolest part of this whole idea of chromosome splitting and cell duplication and gamete production.

Darwin and Meiosis

Darwin recognized the importance of heritable variation in nature. He studied natural and artificial phenomena. Then from a diverse set of facts, he contrived a consilient explanation for how this variation is used as raw material for descent with modification, or evolution. However, we are all subject to the ignorances of our times, and Darwin is not immune to this funny little fact about the human condition. There were many things his generation never came to know that we have discovered in the nearly 150 years since his death, a time of rapid scientific and technological progress. For our purposes, the most important of these were the genetic, molecular and chemical mechanisms of inheritance described by geneticists based on the work of that famous monk, Gregor Mendel.

The biodiversity around and within us could not be possible without mechanisms generating diversity, the chief of which is molecular mutation, which comes in many different forms. Aside from mutation, however, other powerful forces shape the genetic variation found in the natural world. Several of these processes have their origin in the statistical ramifications of the physical acts during meiosis, hence resulting from sexual life cycles. Three of these would be independent chromosomal assortment, chiasmata, and randomization of fertilization.

Not only has meiosis aided in the generation of genetic variability in nature, but also it has an evolutionary history of its own. For example, because of genetic differences, human meiosis is more advanced than forms of division found in some insects (Bogdanov 2003). Meiosis and sexual reproduction are thought to have evolved early in evolutionary time in unicellular organisms, and are controlled by hundreds of genes (Bogdanov 2003).

So, you've been no doubt wondering all this time where the "lies" in the title come in, right? Simple. The lie is that meiosis, life cycles, and chromosomal evolution and associated gene expression, regulation, and cellular processes are "boring." That's the lie. In fact, these represent exciting areas of research, even disease research, which are uncovering more and more about an integral process that evolved in some of our most ancient sexual ancestors. Think about that the next time you put on Marvin Gaye or Barry White! ~ JB

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
References

Bogdanov, Y. F. 2003. Variation and evolution of meiosis. Russian Journal of Genetics 39:363-381