Sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid).[1] This is typical in animals, though the number of chromosome sets and how that number changes in sexual reproduction varies, especially among plants, fungi, and other eukaryotes.[2][3]
Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants.[4][5] Sexual reproduction also occurs in some unicellular eukaryotes.[2][6] Sexual reproduction does not occur in prokaryotes, unicellular organisms without cell nuclei, such as bacteria and archaea. However, some processes in bacteria, including bacterial conjugation, transformation and transduction, may be considered analogous to sexual reproduction in that they incorporate new genetic information.[7] Some proteins and other features that are key for sexual reproduction may have arisen in bacteria, but sexual reproduction is believed to have developed in an ancient eukaryotic ancestor.[8]
In eukaryotes, diploid precursor cells divide to produce haploid cells in a process called meiosis. In meiosis, DNA is replicated to produce a total of four copies of each chromosome. This is followed by two cell divisions to generate haploid gametes. After the DNA is replicated in meiosis, the homologous chromosomes pair up so that their DNA sequences are aligned with each other. During this period before cell divisions, genetic information is exchanged between homologous chromosomes in genetic recombination. Homologous chromosomes contain highly similar but not identical information, and by exchanging similar but not identical regions, genetic recombination increases genetic diversity among future generations.[9]
During sexual reproduction, two haploid gametes combine into one diploid cell known as a zygote in a process called fertilization. The nuclei from the gametes fuse, and each gamete contributes half of the genetic material of the zygote. Multiple cell divisions by mitosis (without change in the number of chromosomes) then develop into a multicellular diploid phase or generation. In plants, the diploid phase, known as the sporophyte, produces spores by meiosis. These spores then germinate and divide by mitosis to form a haploid multicellular phase, the gametophyte, which produces gametes directly by mitosis. This type of life cycle, involving alternation between two multicellular phases, the sexual haploid gametophyte and asexual diploid sporophyte, is known as alternation of generations.
The evolution of sexual reproduction is considered paradoxical,[10] because asexual reproduction should be able to outperform it as every young organism created can bear its own young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation.[11] This 50% cost is a fitness disadvantage of sexual reproduction.[12] The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. However, one definite advantage of sexual reproduction is that it increases genetic diversity and impedes the accumulation of harmful genetic mutations.[13][9]
Sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates for sexual reproduction.[14][15] It has been described as "a powerful evolutionary force that does not exist in asexual populations".[16]
Bacteria and archaea[edit]
Three distinct processes in prokaryotes are regarded as similar to eukaryotic sex: bacterial transformation, which involves the incorporation of foreign DNA into the bacterial chromosome; bacterial conjugation, which is a transfer of plasmid DNA between bacteria, but the plasmids are rarely incorporated into the bacterial chromosome; and gene transfer and genetic exchange in archaea.
Bacterial transformation involves the recombination of genetic material and its function is mainly associated with DNA repair. Bacterial transformation is a complex process encoded by numerous bacterial genes, and is a bacterial adaptation for DNA transfer.[20][21] This process occurs naturally in at least 40 bacterial species.[49] For a bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as competence (see Natural competence). Sexual reproduction in early single-celled eukaryotes may have evolved from bacterial transformation,[22] or from a similar process in archaea (see below).
On the other hand, bacterial conjugation is a type of direct transfer of DNA between two bacteria mediated by an external appendage called the conjugation pilus.[50] Bacterial conjugation is controlled by plasmid genes that are adapted for spreading copies of the plasmid between bacteria. The infrequent integration of a plasmid into a host bacterial chromosome, and the subsequent transfer of a part of the host chromosome to another cell do not appear to be bacterial adaptations.[20][51]
Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange[52][53] Ajon et al.[53] hypothesized that this cellular aggregation enhances species-specific DNA repair by homologous recombination. DNA transfer in Sulfolobus may be an early form of sexual interaction similar to the more well-studied bacterial transformation systems that also involve species-specific DNA transfer leading to homologous recombinational repair of DNA damage.