Manipulation hypothesis of origin of sex
Intuition suggests that only structures and mechanisms that represent some kind of selection advantage for their carriers can emerge in evolution (an advantage for the individual, for the population or for the particular species). However, this impression is erroneous. There are a number of situations in which organisms exhibit properties that are clearly detrimental for their carriers, providing an advantage for someone else at his expense. This fact forms the basis for hypotheses about the emergence of sexual reproduction as a manifestation of a selfish gene or as a manifestation of a parasite (Hickey 1993; Bell 1993).
The idea that one of the most obvious (and most pleasant) patterns of behaviour of contemporary organisms, sexual reproduction, could be a manifestation of the activity of a selfish gene or even a parasite is somewhat surprising. However, the unusualness of an idea is not an argument for or against the validity of a scientific hypothesis.s
During the evolution of any genome, genes could most probably be formed that would cause that their carriers exchanged genetic material with other organisms. While other genes were capable of spreading in the population only vertically in asexually reproducing organisms, i.e. from parents to offspring, through DNA replication, these mutated genes could also spread horizontally, i.e. from one individual to another (Hickey 1993; Bell 1993). One of the ways consists in transfer of a cytoplasmatic genetic agent, plasmid or virus with the relevant gene at the moment of physical contact of two cells. For example, during ciliate conjugation during exchange of nuclei between protozoa, the transfer of an infectious agent can occur very easily. A further type of horizontal spreading of a gene programming sexual reproduction is based on transfer of the gene from one chromosome to another within a single zygote by, for example gene conversion accompanying crossing-overs. The ability to force cells to reproduce sexually and thus create a precondition for horizontal spreading in the population is certainly advantageous for such a gene. It is thus probable that genes with this ability will be evolutionarily very successful and will be readily fixed by natural selection. It simultaneously makes no difference to the gene whether the actual process of sexual reproduction is or is not selectionally advantageous for the particular organism. Genes act selfishly; those that program their carriers so that they are themselves spread most effectively in the population are successful in evolution.
The F-plasmid of bacteria can serve as a prototype for such emergence of a sexual process. This plasmid contains genes encoding the formation of a sex pilus, through which bacteria containing an F-plasmid attach to another bacterial cell and the genes ensure transfer of a copy of the F-plasmid by this pilus to the cells of the recipient. Because the plasmid is capable of integrating into the bacterial DNA or is capable of integrating part of this DNA into itself, it can cause the transfer of bacterial genes during its transfer from cell to cell. Thus, it can be considered to be an adaptive structure that facilitates sexual reproduction for the bacterial cell. Similarly, however, the F-plasmid can be considered to be an infectious agent, a kind of bacteriophage, that has learned not to damage its host cell and that “doesn’t want anything else” than its own reproduction and spreading in the bacterial population. Integration into the bacterial DNA and transfer of bacterial genes thus can constitute only secondary improvement of the effectiveness of the process of plasmid reproduction. A plasmid that integrates into itself, for example, a gene for resistance to antibiotics (R-plasmid) or that is capable of being useful for the bacterial cell in some other way is, of course, at a substantial selection advantage compared to the original F-plasmid.