Mating system
A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to animals, the term describes which males and females mate under which circumstances. Recognised systems include monogamy, polygamy (which includes polygyny, polyandry, and polygynandry), and promiscuity, all of which lead to different mate choice outcomes and thus these systems affect how sexual selection works in the species which practice them. In plants, the term refers to the degree and circumstances of outcrossing. In human sociobiology, the terms have been extended to encompass the formation of relationships such as marriage.
In microorganisms[edit]
Bacteria[edit]
Mating in bacteria involves transfer of DNA from one cell to another and incorporation of the transferred DNA into the recipient bacteria's genome by homologous recombination. Transfer of DNA between bacterial cells can occur in three main ways. First, a bacterium can take up exogenous DNA released into the intervening medium from another bacterium by a process called transformation. DNA can also be transferred from one bacterium to another by the process of transduction, which is mediated by an infecting virus (bacteriophage). The third method of DNA transfer is conjugation, in which a plasmid mediates transfer through direct cell contact between cells.
Transformation, unlike transduction or conjugation, depends on numerous bacterial gene products that specifically interact to perform this complex process,[23] and thus transformation is clearly a bacterial adaptation for DNA transfer. In order for a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state termed natural competence. In Bacillus subtilis about 40 genes are required for the development of competence and DNA uptake.[24] The length of DNA transferred during B. subtilis transformation can be as much as a third and up to the whole chromosome.[25][26] Transformation appears to be common among bacterial species, and at least 60 species are known to have the natural ability to become competent for transformation.[27] The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells.[28]
Archaea[edit]
In several species of archaea, mating is mediated by formation of cellular aggregates.
Halobacterium volcanii, an extreme halophilic archaeon, forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another in either direction.[29]
When the hyperthermophilic archaea Sulfolobus solfataricus[30] and Sulfolobus acidocaldarius[31] are exposed to the DNA damaging agents UV irradiation, bleomycin or mitomycin C, species-specific cellular aggregation is induced. Aggregation in S. solfataricus could not be induced by other physical stressors, such as pH or temperature shift,[30] suggesting that aggregation is induced specifically by DNA damage. Ajon et al.[31] showed that UV-induced cellular aggregation mediates chromosomal marker exchange with high frequency in S. acidocaldarius. Recombination rates exceeded those of uninduced cultures by up to three orders of magnitude. Frols et al.[30] and Ajon et al.[31] hypothesized that cellular aggregation enhances species-specific DNA transfer between Sulfolobus cells in order to provide increased repair of damaged DNA by means of homologous recombination. This response appears to be a primitive form of sexual interaction similar to the more well-studied bacterial transformation systems that are also associated with species specific DNA transfer between cells leading to homologous recombinational repair of DNA damage.
Protists[edit]
Protists are a large group of diverse eukaryotic microorganisms, mainly unicellular animals and plants, that do not form tissues. Eukaryotes emerged in evolution more than 1.5 billion years ago.[32] The earliest eukaryotes were likely protists. Mating and sexual reproduction are widespread among extant eukaryotes. Based on a phylogenetic analysis, Dacks and Roger[33] proposed that facultative sex was present in the common ancestor of all eukaryotes.
However, to many biologists it seemed unlikely until recently, that mating and sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that mating and sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree. However, several of these protists are now known to be capable of, or to recently have had, the capability for meiosis and hence mating. To cite one example, the common intestinal parasite Giardia intestinalis was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. intestinalis was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.[34] These results suggested that G. intestinalis is capable of meiosis and thus mating and sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of mating and sexual reproduction, was also found in G. intestinalis.[35] Other protists for which evidence of mating and sexual reproduction has recently been described are parasitic protozoa of the genus Leishmania,[36] Trichomonas vaginalis,[37] and acanthamoeba.[38]
Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.
Viruses[edit]
Both animal viruses and bacterial viruses (bacteriophage) are able to undergo mating. When a cell is mixedly infected by two genetically marked viruses, recombinant virus progeny are often observed indicating that mating interaction had occurred at the DNA level. Another manifestation of mating between viral genomes is multiplicity reactivation (MR). MR is the process by which at least two virus genomes, each containing inactivating genome damage, interact with each other in an infected cell to form viable progeny viruses. The genes required for MR in bacteriophage T4 are largely the same as the genes required for allelic recombination.[39] Examples of MR in animal viruses are described in the articles Herpes simplex virus, Influenza A virus, Adenoviridae, Simian virus 40, Vaccinia virus, and Reoviridae.