Editing RNA world (section)

== Origin of sexual reproduction ==
{{Further|Evolution of sexual reproduction}}

Eigen ''et al''.<ref>{{cite journal | vauthors = Eigen M, Gardiner W, Schuster P, Winkler-Oswatitsch R | title = The origin of genetic information | journal = Scientific American | volume = 244 | issue = 4 | pages = 88–92, 96, et passim | date = April 1981 | pmid = 6164094 | doi = 10.1038/scientificamerican0481-88 | bibcode = 1981SciAm.244a..88H }}</ref> and Woese<ref>Woese CR (1983). The primary lines of descent and the universal ancestor. Chapter in {{cite book | last = Bendall | first = D. S. | name-list-style = vanc | title = Evolution from molecules to men | publisher = Cambridge University Press | location = Cambridge, UK | year = 1983 | isbn = 978-0-521-28933-7 | url-access = registration | url = https://archive.org/details/evolutionfrommol0000unse }} pp. 209-233.</ref> proposed that the genomes of early [[protocell]]s were composed of single-stranded RNA, and that individual genes corresponded to separate RNA segments, rather than being linked end-to-end as in present-day DNA genomes. A protocell that was haploid (one copy of each RNA gene) would be vulnerable to damage, since a single lesion in any RNA segment would be potentially lethal to the protocell (e.g., by blocking replication or inhibiting the function of an essential gene).

Vulnerability to damage could be reduced by maintaining two or more copies of each RNA segment in each protocell, i.e., by maintaining diploidy or polyploidy. Genome redundancy would allow a damaged RNA segment to be replaced by an additional replication of its homolog. However, for such a simple organism, the proportion of available resources tied up in the genetic material would be a large fraction of the total resource budget. Under limited resource conditions, the protocell reproductive rate would likely be inversely related to ploidy number. The protocell's fitness would be reduced by the costs of redundancy. Consequently, coping with damaged RNA genes while minimizing the costs of redundancy would likely have been a fundamental problem for early protocells.

A cost-benefit analysis was carried out in which the costs of maintaining redundancy were balanced against the costs of genome damage.<ref name=Bernstein84>{{cite journal | vauthors = Bernstein H, Byerly HC, Hopf FA, Michod RE | title = Origin of sex | journal = Journal of Theoretical Biology | volume = 110 | issue = 3 | pages = 323–351 | date = October 1984 | pmid = 6209512 | doi = 10.1016/S0022-5193(84)80178-2 | bibcode = 1984JThBi.110..323B }}</ref> This analysis led to the conclusion that, under a wide range of circumstances, the selected strategy would be for each protocell to be haploid, but to periodically fuse with another haploid protocell to form a transient diploid. The retention of the haploid state maximizes the growth rate. The periodic fusions permit mutual reactivation of otherwise lethally damaged protocells. If at least one damage-free copy of each RNA gene is present in the transient diploid, viable progeny can be formed. For two, rather than one, viable daughter cells to be produced would require an extra replication of the intact RNA gene homologous to any RNA gene that had been damaged prior to the division of the fused protocell. The cycle of haploid reproduction, with occasional fusion to a transient diploid state, followed by splitting to the haploid state, can be considered to be the sexual cycle in its most primitive form.<ref name=Bernstein84 /><ref>{{cite book | last1 = Bernstein | first1 = Carol | last2 = Bernstein | first2 = Harris | name-list-style = vanc | title = Aging, sex, and DNA repair |publisher=Academic Press |location=Boston |year=1991 |isbn=978-0-12-092860-6 }} see pgs. 293-297</ref> In the absence of this sexual cycle, haploid protocells with damage in an essential RNA gene would simply die.

This model for the early sexual cycle is hypothetical, but it is very similar to the known sexual behavior of the segmented RNA viruses, which are among the simplest organisms known. [[Orthomyxoviridae|Influenza virus]], whose genome consists of 8 physically separated single-stranded RNA segments,<ref>{{cite journal | vauthors = Lamb RA, Choppin PW | title = The gene structure and replication of influenza virus | journal = Annual Review of Biochemistry | volume = 52 | pages = 467–506 | year = 1983 | pmid = 6351727 | doi = 10.1146/annurev.bi.52.070183.002343 }}</ref> is an example of this type of virus. In segmented RNA viruses, "mating" can occur when a host cell is infected by at least two virus particles. If these viruses each contain an RNA segment with a lethal damage, multiple infection can lead to reactivation providing that at least one undamaged copy of each virus gene is present in the infected cell. This phenomenon is known as "multiplicity reactivation". Multiplicity reactivation has been reported to occur in influenza virus infections after induction of RNA damage by [[Ultraviolet germicidal irradiation|UV-irradiation]],<ref>{{cite journal | vauthors = Barry RD | title = The multiplication of influenza virus. II. Multiplicity reactivation of ultraviolet irradiated virus | journal = Virology | volume = 14 | issue = 4 | pages = 398–405 | date = August 1961 | pmid = 13687359 | doi = 10.1016/0042-6822(61)90330-0 | hdl-access = free | hdl = 1885/109240 }}</ref> and ionizing radiation.<ref>{{cite journal | vauthors = Gilker JC, Pavilanis V, Ghys R | title = Multiplicity reactivation in gamma irradiated influenza viruses | journal = Nature | volume = 214 | issue = 5094 | pages = 1235–1237 | date = June 1967 | pmid = 6066111 | doi = 10.1038/2141235a0 | s2cid = 4200194 | bibcode = 1967Natur.214.1235G }}</ref>