Be prepared to read the un-sexiest lesson about sex that you have ever seen! Though the biohistory of sex is not erotic in the least, it is fascinating in its own right. This lesson challenges our preconceived notions of sexual reproduction. If you are studying this for the first time, there are probably some big surprises in store.
The first big surprise is how early it came onto the scene. Like so many major evolutionary developments, sexual reproduction did not happen all at once. It was not invented by your generation or your parents’ generation. It was not first practiced by humans, or even animals. The first creatures to reproduce sexually were single-celled!
It takes two words to say, “sexual reproduction.” That’s because sex and reproduction are not the same thing. Sex is the exchange of DNA between organisms. Reproduction is the creation of new life from old life. From the beginning, there was reproduction without sex and (next big surprise) sex without reproduction. These functions came together in eukaryotic cells, as protists were evolving 1 – 2 BYA.
Recall that even bacteria are able to literally have sex. It is often called horizontal gene transfer or recombination. Genetic material may pass from one bacterium to another. In early life, this could have happened when cellular membranes were poorly formed and DNA could occasionally escape from one cell into another. Later in bacterial history, membranes were more impermeable, but pieces of DNA could still be transmitted by viruses. Genes can also be transferred when one cell eats another or when two cells fuse. We have discussed the transfer of DNA from mitochondria to host cells in endosymbiosis. Such exchanges of DNA contributed greatly to the variety of life as cells evolved over the gigayears.
Meanwhile, of course, cells were routinely splitting apart to create copies of themselves. This method of asexual reproduction is called binary fission. To be successful, each daughter cell must contain a full copy of the mother cell’s DNA (in asexual reproduction, female is the default gender). Bacteria and archaea developed structures to facilitate binary fission, as well as chemical pathways to regulate the timing and make sure it went smoothly. These archaic mechanisms were adopted and improved upon by eukaryotes.
In asexual protist cells, reproduction evolved from binary fission to mitosis. This was a more complicated process because eukaryotes are more complex cells, but used essentially the same principles. Leading up to mitosis, each chromosome is replicated. (I call the cell “pregnant” at this point). Then the mother cell pinches off into two new daughter cells, with one full set of chromosomes in each cell. Each daughter is genetically identical to the parent, and half the size. The daughters then grow to full size and perpetuate the cycle. Mitosis is still practiced by most cells in your body for growth and healing. A skin cell does not need to have sex to reproduce. It just makes a copy of itself. In the illustrations, each letter represents one copy of a gene.
Somewhere along the way, protists evolved a similar but crucially different mode of reproduction called meiosis. Meiosis was the critical breakthrough that combined elements of sex (recombination) with reproduction (mitosis). In overly simplified terms, meiosis is like two phases of mitosis one right after the other, without giving the chromosomes time to copy themselves before the second phase. The first-generation adult cell is called a germ cell. The end result is four sex cells, either sperms or eggs. The next illustration is a cartoon example of meiosis step-by-step for germ cells that have only two genes each.
For the sake of simplicity, we’ll assign the mother and father germ cells the same genome, a and A. 1 First, the genes are replicated, so now each parent cell momentarily has four genes: a, a, A, and A. Each pregnant germ cell then divides into two “germlets” or “germettes” (depending on sex), each with two genes. 2 Each germlet gets both copies of one gene, so it is not identical to the germ cell. Before the chromosomes have another chance to replicate, each germlet immediately divides again. This results in four sex cells, each with only one gene. Each sex cell has only half of the parent’s genome. Two each have one copy of gene a, and the other two each have one copy of gene A. The sex cells are called haploid for this reason, a term related to the word “half.” The parent germ was a diploid cell, a term related to the word “two.” 3
To complete the life cycle, a sperm and an egg need to get together and “have sex,” or combine their genes. If a a egg cell had sex with a A sperm cell, they would together form a diploid aA offspring, identical to each parent. But there are other possibilities. Depending on which sex cells combined, the result could be an aa cell or an AA cell. These would be new varieties of cell! This is one illustration of how sex can result in offspring that are not identical to the parents.
In real life, meiosis is more complex and subtle than the pedagogical example above. A real cell has multiple chromosomes, each of which has multiple genes. Furthermore, recombination can occur already in meiosis phase I, within the germ cell, before it even splits into germlets. This is called crossing over, and it can be fairly well illustrated with another small-scale example. (Follow along with the next figure). Suppose now that the parent cell has chromosome M with genes a and b, and chromosome P with genes A and B. (I am using the names M and P to foreshadow Maternal / Paternal). Each chromosome is copied. The parent cell’s genome is now momentarily a tetrad, meaning four related chromosomes:
Chromosome M = a, b
Chromosome M′ = a, b (the prime mark indicates a copy)
Chromosome P = A, B
Chromosome P′ = A, B
Now suppose that chromosomes M and P′ cross over by swapping b/B genes (any two chromosomes from this tetrad can cross over). The result:
Chromosome M = a, B
Chromosome M′ = a, b
Chromosome P = A, B
Chromosome P′ = A, b
The pregnant germ cell now splits into two germlet cells, and then into four sex cells. Each sex cell has one of the four chromosomes.
If two of these simple cartoon organisms had sex, there would be dozens of possible genomes for the offspring. Again, real-world cells have much more variety because they typically have tens of tetrads and tens of thousands of genes. A parent germ cell can yield an astronomical number of genetically distinct haploid sex cells.
The next major step was the differentiation between male and female sex cells. Some sex cells, eggs, evolved to become especially large. A large egg is great, because it provides food for the developing offspring. On the other hand, eggs are sluggish and few in number. Eggs were simply not good at finding each other. This forced other sex cells to specialize as numerous, mobile sperm, to make fertilization easier. 2 Since sperm were very numerous, they were necessarily competitive in chasing the egg. The stereotypical mating-game gender roles evolved before our ancestors even had bodies!
Sexual reproduction may seem like a straightforward fact of life, but it is actually one of biology’s greatest unanswered questions. Our third “big surprise” is that biologists do not yet understand why sex was so helpful for reproduction. To understand why this is a difficult question, consider a colony of cells that can reproduce either sexually or asexually. (There are several such species of protists, plants, and even some animals. When meiosis was new, this was probably commonplace.) If two asexual cells each reproduced by binary fission, they would immediately double their family size to four cells. If, instead, they had sex to create one offspring at a time, they would need two rounds of reproduction to double their numbers. After just 20 rounds, the asexual cells would outnumber the sexuals 1,000 to one – and that’s assuming that the eggs and sperms beat all odds to find each other. Sexual reproduction is beginning to look very inefficient! It is still not entirely known how sexual cells survived competition with their asexual sisters.
Clues might be found by studying the cells of today that have sexual / asexual split personalities. Such cells tend to reproduce asexually when times are good, and then switch to sexual mode when subjected to stress like food shortage. Environmental stress can damage or destroy chromosomes. By sharing chromosomes, sex cells may be able to protect each other against this damage. 3 Another possible factor is diversity. There may have been occasions when environmental disasters killed most asexual clones in a colony, whereas sex produced many different varieties of offspring, some of which were better equipped to weather the storm. 4
Whatever the advantages were, it is clear that sex proved highly successful. Today, most eukaryotes and virtually all animals reproduce sexually. We can thank our ancestral cells of a billion years ago for getting that party started!
- Picture from http://www.pdimages.com/03709.html , which claims the image as public domain (accessed 8/10/19). ↩
- Michael Bulmer and Geoff A. Parker, “The evolution of anisogamy: a game-theoretic approach,” Proc. R. Soc. Lond. B: 269:2381-2388 (11/22/2002), http://rspb.royalsocietypublishing.org/content/269/1507/2381 (accessed and saved 8/04/19). ↩
- Harris Bernstein et al., “Genetic damage, mutation, and the evolution of sex”, Science 229 (4719): 1277–81 (Oct., 1985) https://science.sciencemag.org/content/229/4719/1277 (accessed and saved 8/04/19). ↩
- S.P. Otto and A.C. Gerstein, (August 2006). “Why have sex? The population genetics of sex and recombination”, Biochem. Soc. Trans.34 (4): 519–22 (7/21/2006), http://www.biochemsoctrans.org/content/34/4/519 (accessed and saved 8/04/19). ↩
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