Sex as a major transition

Let us see how one of the transitions stands up to these challenges. Sex technically refers to a special type of cell cycle (Figure 2.2), not, as is more normally used, copulation. Understanding the evolution of sex therefore means thinking hard about how cells replicate and divide and why this might change. Since the way cells do that is normally taken for granted, it is useful to be prepared for the unexpected in the paragraphs that follow.

Sex undoubtedly evolved in eukaryotes from a clonal ancestral state. A normal (mitotic) cell cycle is comparatively simple: some time into its life, each chromosome copies itself, and then the cell divides into two. In a sexual (meiotic) life cycle, new diploid offspring are born by fusion of two haploid gametes (syngamy). The gametes that fuse are normally very different in form and behaviour (anisogamy), one small and motile (sperm), the other large and immobile (egg). A number of mitotic cell cycles may then follow (development in multicellular organisms). Then some homologous chromosomes swap bits of DNA (recombination), in a process known as 'crossing over'because of the appearance of the process under the microscope.

Pre-meiotic doubling

Fig. 2.2 A sexual life cycle. The dark lines are chromatids of a single homologous pair of chromosomes, drawn to indicate whether the cell is haploid or diploid, and whether the chromatids have replicated or not. The circles are cells.

They then copy themselves and undergo two cell divisions to give rise to four haploid cells. In some organisms these also undergo mitotic divisions before syngamy (Figure 2.2).

Which of these steps came first, according to Maynard Smith and Szathmary (1995)? One of the surviving ancient protist lineages, Barbulanympha,which lives inside the guts of insects, has a cycle that involves endomitosis (gain of diploid state by copying of the haploid chromosomes) instead of syngamy. This led Cleveland (1947) to suggest that the first stage might have been the acquisition of a life cycle that alternated between a diploid stage acquired via endomitosis, and a haploid stage via a single one-step reduction division. Next, according to Maynard Smith and Szathmary (1995), endomitosis would be replaced by syngamy. This would leave an otherwise normal one-step meiosis as seen in many sporozoans (the group to which the malaria parasite belongs). Crossing over and chromosome doubling followed, giving a two-step meiosis, and finally anisogamy (Figure 2.3). Let us see how we can account for one of those steps.

The vast majority of work on the evolution of sex has addressed the advantage of crossing over, or recombination. There are two processes that might have selected for its evolution. The first is that recombination can lower the genetic load if mutations act synergistically (having two is more than twice as bad as having one) (Kondrashov 1988). Imagine a distribution of deleterious mutations at equilibrium in a clonal population. Most organisms have a few, and a few have many (Figure 2.4). Now imagine that recombination occurs. The mutations are redistributed among the population, and once, after selection has acted and equilibrium is achieved, there are fewer mutations

Ancestral eukaryote mitosis?

Endomitosis plus one-step meiosis

Endomitosis plus one-step meiosis

Premeiotic doubling

Anisogamy

Crossing over?

Multicellular metakaryotes

Fig. 2.3 Possible sequence of steps in the origin of sex, with intermediate states represented by some extant organisms, after Maynard Smith and Szathmary (1995).

Crossing over?

Premeiotic doubling

Anisogamy

Multicellular metakaryotes

Fig. 2.3 Possible sequence of steps in the origin of sex, with intermediate states represented by some extant organisms, after Maynard Smith and Szathmary (1995).

Fig. 2.4 The genetic load in sexual and asexual populations. Recombination can reduce the load of synergistic mutations (1). In small asexual populations, the genetic load of slightly deleterious mutations can increase, ratchet-like (2).

(Figure2.4). This is because recombination in each generation throws together unlucky individuals with many such mutations, which suffer more severely than their fellows with only a few because of their synergistic effects. The death of these individuals purges the population somewhat of the mutations. This process can work even in an infinite population, and only requires the presence of synergistic mutations. There is presently little factual evidence for the syn-ergistic effects of mutations, but theoretically it is a reasonable expectation (Szathmary 1993).According to metabolic theory,mutations affecting a metabolic cycle should affect mainly the concentration of chemical intermediates, not that of end-products. If it is maximal end-product production that is important, as is likely in small organisms whose fitness depends on fast growth, then mutations are not synergistic. If it is some optimal balance of intermediates that is important, as in large long-lived organisms, then mutations will be synergistic and recombination will be favoured. Rather nicely, the frequency of recombination varies markedly across species, and is most frequent in large, long-lived organisms (Bell and Burt 1987), where we would most expect the effects of mutations to be synergistic.

The second possible process that might have favoured recombination is selection for change (directional selection) on polygenic traits (traits controlled at several loci) (Maynard Smith 1979). Hamilton (1980) most famously adhered to this hypothesis to explain not only the origin of sex but also more specifically its maintenance. He regarded co-evolutionary arms races (see Chapter 11) between hosts and parasites as a likely and widespread source of such directional selection. This idea has become colloquially known as the Red Queen theory (Van Valen 1973) after the Lewis Carol character in 'Through the Looking Glass'who had to run as fast as she could to stay in the same place.

The early protists were certainly not immune from such co-evolutionary forces, though the selective pressure is much greater on long-lived macroscopic organisms with long generation times relative to their parasites,where the traits under selection for change are those involved with defence and resistance. Nicely but at the same time frustratingly, this also fits well with the observation that long-lived organisms have higher rates of recombination. The frustration is that both Hamilton's and Kondroshov's hypotheses make the same prediction about the frequency of recombination relative to size and lifespan, so the observation fits but does nothing to narrow the range of plausible hypotheses. Rather more fortunately, there is independent evidence for the Red Queen hypothesis, which we will examine later in relation to the maintenance of sex.

Another step in the origin of sex is worth mentioning here. In many single celled organisms, such as the single celled alga, Chlamydomonas reinhardii, familiar in many school biology classrooms, the gametes that fuse to form a diploid alga are of identical size (isogamy). In most sexual species, however, one gamete of the pair (the egg) is larger and specialized to carry the organelles. Cosmides and Tooby (1981) and later Hurst and Hamilton (1992) argued that such specialization has evolved to prevent conflict between organelles from different parents. Many organelles, such as mitochondria and chloroplasts, contain their own DNA, (in the latter cases they were originally independent prokaryotic organisms). Such replicating entities should presumably be selected in the short term to produce copies of themselves at the expense of competing entities, and this is likely to be detrimental to the eukaryote cell as a whole. The potential problem is apparently real, for even in isogamous protists, uniparental inheritance of the organelles occurs, and in Chlamydomonas is apparently controlled by nuclear genes (central control, analogous to a police force in human society). Hurst and Hamilton argue that uniparental inheritance is possible only if there are two mating types and no more, for otherwise there is the danger of offspring lacking organelles entirely. Thus, conflict between organelles has, they claim, led to the origin of two (not three nor some other number) sexes. In fact, some protists exchange genetic information without cytoplasmic exchange (a process known as conjugation). In these cases there is no possibility of organelle competition, and multiple 'sex' or 'incompatability' types are known.

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