Clonal reproduction in parasitic organisms - role of
The subpopulation of parasites in one host very frequently corresponds to a clone of genetically identical individuals. There is minimal variability within the clone and thus the effectiveness of individual selection that would favor selfish individuals is also minimal. If an individual in the subpopulation mutates towards a greater rate of reproduction, its progeny are at an advantage only in the first host; following transmission to another host, they yield a clone of identical individuals, so that any selectional advantage of the particular mutation disappears. To the contrary, the disadvantage connected with the deviation from optimum virulence is manifested and the entire daughter subpopulation finally produces less infectious stages than the subpopulation of unmutated individuals.
The advantage of the limited effectiveness of individual selection within an infrapopulation is apparently a cause of the fact that asexual reproduction is very frequently encountered in parasitic organisms, for example parthenogenesis and polyembryony (division of the embryo into several or even many embryos) which occurs in taxonomic groups in which wild species reproduce mainly sexually. Parasites sexually produce practically only the invasive stages, which leave the host for the outside environment. Whenever the progeny could compete with one another or with the parent individuals, they are produced by asexual reproduction, where new genotypes are formed only by the slow route of new mutations, and not by the rapid process of genetic recombination.
It is mostly stated that the reason for the emergence of secondary forms of asexual reproduction lies in the necessity of ensuring reproduction even when only a single parasite is capable of entering the host. However, this is inadequate to explain the repeated formation of asexual reproduction. The same goal could be achieved in a far simpler way by the emergence of hermaphroditism, the production of a microgamete and a macrogamete in a single individual. Although sexual reproduction is the commonest means of reproduction in nonparasitic eukaryotic organisms and thus it can be assumed that it is evolutionarily very advantageous, a great many parasitic species apparently are not capable of sexual reproduction or are capable of reproducing in their life cycle only in one of their hosts, i.e. in the definitive host. It is very probable that the selection pressure that, through interspecies selection, has led in parasitic species to the formation of this otherwise unexplainable “sexual abstinence” consisted in the necessity of limiting individual selection in favor of group selection.