Читать книгу Parasitology - Alan Gunn - Страница 33

Sex has fascinated biologists (amongst others) for generations. From a logical point of view, sexual reproduction does not make sense because of what is referred to as the two‐fold cost of sex. Firstly, the males, who usually constitute in the region of 50% of a population, serve only to inseminate the females and do not reproduce themselves. Furthermore, a lot of time and effort is often employed in searching for a mate and mating can itself be an energetically expensive and potentially dangerous process. By contrast, in an asexually reproducing organism 100% of the population can reproduce. Consequently, an asexually reproducing population is theoretically able to grow faster and respond to changes in the environment (e.g., increased food supply) faster than one that reproduces sexually. The other ‘cost’ of sexual reproduction is that the gametes are haploid and the process of recombination at meiosis means that an individual can only pass on 50% of its genes to each of its offspring. Consequently, useful genes and gene combinations could be lost in the process of generating new genetic variants. Despite these problems, and several others, most organisms undertake sexual reproduction and therefore it must have some major advantage(s)

There are several theories why so many organisms reproduce sexually (Burke and Bonduriansky 2017). One of the most popular is that of Hamilton et al. (1990) who suggest that sexual reproduction arose as a mechanism by which organisms can limit the problems of parasitic infections. Parasites can potentially reproduce faster than their hosts, and therefore, they will evolve to overcome the most common combination of host resistance alleles. Therefore, hosts with rarer resistance alleles will then be at a competitive advantage and ultimately one of these will become the most common resistance allele combination in the host population. The arms race will continue ad infinitum with the parasites adapting to the most common resistance allele combination and the host generating new allele combinations. The process of recombination ensures that (provided the initial gene pool is sufficiently diverse) there will be a constant supply of novel resistance alleles. Furthermore, a resistance allele combination to which parasites have adapted need not be lost from the population because it may prove useful again in the future. By contrast, in an asexually reproducing organism the offspring will have the same resistance allele combinations as their parents, and once parasites have overcome these, then the whole population is vulnerable to infection.

If sexual reproduction arose as means of reducing the depredations of parasites, then one would expect it to be common where parasites are abundant and challenge frequent. By contrast, asexual reproduction should be favoured where parasites are absent, or the level of challenge is low. Although there are several instances of exactly this in the literature, they remain remarkably few. The best‐known example is that of the snail Potamopyrgus antipodarum that originated in New Zealand and has since spread to many parts of the world. It exists as sexually reproducing populations, asexually reproducing populations, and mixed sexually and asexually reproducing populations. Positive correlations have been described between the extent of parasitism by parasitic flatworms and the frequency of sexual reproduction. Sexual reproduction is rare where flatworm parasite challenge is low, and conversely, it is common where the parasite challenge is high (Lively and Jokela 2002). Another commonly cited example is that of certain minnow populations living in Mexico (Lively 1996). These minnows exist as both asexually reproducing and sexually reproducing populations, but those reproducing sexually tend to have lower parasite burdens (except where inbreeding has resulted in reduced genetic diversity). Most multicellular parasites reproduce sexually themselves, although some combine it with asexually reproducing larval stages, such as schistosomes and the tapeworm Echinococcus granulosus. Even some parasitic protozoa, such as the trypanosomes, exhibit something akin to sexual reproduction. This suggests that even endoparasites living in protected environments such as the gut or bloodstream of another animal remain vulnerable to infections. However, although there is experimental evidence that parasitism influences the evolution and maintenance of sexual reproduction (Auld et al. 2016), there are almost certainly many other factors involved. For example, sexual reproduction may help protect against transmissible cancer cells (Thomas et al. 2019).