Sexual reproduction
At its core, sexual reproduction is a biological symphony of two individuals contributing genetic material to create a new organism, a process fundamental to nearly all complex life on Earth. It's a grand strategy for survival, constantly shuffling the genetic deck to ensure adaptability and resilience across generations. But how does this intricate dance of DNA actually unfold, and what makes it such a powerful engine of evolution? Sexual reproduction is defined by the fusion of two haploid gametes into a diploid zygote, a process that inherently increases genetic diversity. Despite appearing less efficient than asexual reproduction, the "two-fold cost of sex" is outweighed by long-term evolutionary benefits like adaptation and DNA repair. Life cycles vary dramatically across kingdoms, from animal fertilization to the unique 'alternation of generations' in plants, yet all converge on the same goal: genetic mixing.
AI Summary
At its core, sexual reproduction is a biological symphony of two individuals contributing genetic material to create a new organism, a process fundamental to nearly all complex life on Earth. It's a grand strategy for survival, constantly shuffling the genetic deck to ensure adaptability and resilience across generations. But how does this intricate dance of DNA actually unfold, and what makes it such a powerful engine of evolution?
- Sexual reproduction is defined by the fusion of two haploid gametes into a diploid zygote, a process that inherently increases genetic diversity.
- Despite appearing less efficient than asexual reproduction, the "two-fold cost of sex" is outweighed by long-term evolutionary benefits like adaptation and DNA repair.
- Life cycles vary dramatically across kingdoms, from animal fertilization to the unique 'alternation of generations' in plants, yet all converge on the same goal: genetic mixing.
The Foundation of Life's Diversity
Sexual reproduction is a cornerstone of biological diversity, weaving together genetic information from two parents to create offspring that are genetically unique. This complex process involves specialized reproductive cells, called gametes, each carrying a single set of chromosomes. When these gametes unite, they form a zygote with a full, double set of chromosomes—the blueprint for a new organism.
A Universal Strategy
While its specific mechanisms can vary wildly, sexual reproduction is the prevailing life cycle for most multicellular eukaryotes — that's animals, plants, and fungi. Even some single-celled eukaryotes engage in it. This widespread adoption across such diverse life forms speaks volumes about its evolutionary advantages.
The Dance of Chromosomes: Meiosis
Central to sexual reproduction is a special type of cell division called meiosis. Imagine a diploid 'parent' cell, containing two sets of chromosomes, preparing to create haploid 'child' cells, each with just one set. Meiosis orchestrates this reduction, ensuring that when two gametes eventually fuse, the offspring ends up with the correct number of chromosomes.
Before the cell divides, its DNA is meticulously replicated, resulting in four copies of each chromosome. Then, through two rounds of cell division, these copies are precisely distributed, leading to the creation of four haploid gametes. Each gamete is genetically distinct, ready to contribute its unique half to the next generation.
Genetic Shuffling: Crossing Over
One of the most remarkable events during meiosis is genetic recombination, also known as 'crossing over.' As homologous chromosomes — the pairs inherited from each parent — line up, they physically exchange segments of their DNA. Think of it as shuffling a deck of cards, creating entirely new combinations of genes on each chromosome.
This exchange dramatically increases genetic diversity. Since homologous chromosomes carry similar, but not identical, genetic information, mixing and matching these segments ensures that no two gametes are exactly alike. This innovative process is a powerful engine for evolution, constantly generating novel genetic combinations for natural selection to act upon.
The Moment of Union: Fertilization
Following meiosis, two haploid gametes — say, a sperm and an egg — come together in a process called fertilization. Their nuclei fuse, combining their single sets of chromosomes to form a single, diploid cell: the zygote. This zygote now possesses a complete genetic blueprint, half from each parent.
From this single-celled zygote, a new multicellular organism begins to develop through repeated cell divisions via mitosis. Mitosis simply copies cells, maintaining the diploid chromosome number as the organism grows and differentiates into its various tissues and organs. The journey from a single cell to a complex organism is truly astounding.
The Paradox of Sex: Why Pay the Price?
Biologists have long pondered a fascinating paradox: why is sexual reproduction so prevalent when asexual reproduction seems far more efficient? Asexual organisms can produce offspring on their own, essentially cloning themselves, meaning every individual can reproduce. This gives asexual populations an inherent capacity to grow much faster.
This 'two-fold cost of sex' refers to the fact that sexual reproduction typically requires two parents, and each parent only passes on 50% of its genes to its offspring. It's like paying double the price for the same product. Yet, despite this apparent disadvantage, sexual reproduction thrives across the tree of life.
The Hidden Advantages
The answer lies in the long-term benefits of genetic diversity. Sexual reproduction acts as a powerful shield against harmful genetic mutations by constantly reshuffling genes, making it harder for detrimental traits to accumulate. It also allows populations to adapt much more rapidly to changing environments, much like having a diverse toolkit for unexpected challenges.
This constant genetic innovation is a significant evolutionary advantage, ensuring that at least some offspring will be well-suited to new conditions, even if the environment shifts dramatically. It’s a strategy that prioritizes adaptability and long-term survival over short-term reproductive efficiency.
Evolutionary Origins: An Ancient Story
The earliest fossil evidence of sexual reproduction in eukaryotes dates back an astonishing 1.05 billion years, found in the Stenian period. This suggests that the fundamental mechanisms for genetic exchange are incredibly ancient, predating the diversification of most complex life forms we see today.
Scientists propose several interwoven reasons for its evolutionary maintenance: it helps repair damaged DNA, increases the rate of adaptation, reduces competition by creating diverse offspring, and masks deleterious mutations. These combined forces make sexual reproduction a robust strategy, especially for larger populations which tend to reap its benefits more quickly.
The Force of Sexual Selection
Beyond natural selection, sexual reproduction introduces another powerful evolutionary force: sexual selection. This is where individuals don't just survive; they must also secure a mate to pass on their genes. Some individuals out-reproduce others not because they are better at surviving, but because they are better at attracting reproductive partners.
This often leads to spectacular, and sometimes seemingly impractical, traits. Think of a peacock's elaborate tail, a lion's magnificent mane, or a deer's imposing antlers. These features often don't help with everyday survival, but they are crucial for attracting a mate, particularly for males who often compete for female choice. This competition can even lead to 'Fisherian runaway' — a positive feedback loop where traits become increasingly exaggerated.
Sexual selection drives what we call sexual dimorphism — distinct differences in appearance or behavior between males and females of the same species. These can range from body size and strength to elaborate ornamentation and courtship rituals, all shaped by the relentless pressure to find a reproductive partner.
Reproduction Across Kingdoms
Animals: Diverse Strategies
Animals showcase a incredible array of reproductive strategies. In placental mammals, fertilization is internal, with sperm meeting eggs inside the female's body. Offspring are born as developed juveniles, reaching sexual maturity later in life. Most female mammals are only fertile during specific periods of their estrous cycle, making timing crucial for successful reproduction.
The Aquatic World of Fish
Most fish species practice external fertilization: females release eggs, and males then fertilize them in the water. However, some fish have adopted internal fertilization, either releasing developing eggs or giving birth to live young. Remarkable examples include ovoviviparous species like guppies, where eggs hatch inside the female, and viviparous species where the female nourishes the developing offspring internally.
Some fish are even hermaphroditic, possessing both male and female organs. They can be simultaneous hermaphrodites, acting as both sexes at once, or serial, changing sex during their lifetime. In rare cases, self-fertilization can occur. One fascinating species, Poecilia formosa, uses a unique form of parthenogenesis called gynogenesis: its eggs need sperm from another species to stimulate development, but the sperm doesn't actually fertilize the egg, resulting in clones of the female.
Reptilian Rites
Reptiles generally reproduce sexually, with all activity centered around a single opening called the cloaca. Males often possess specialized copulatory organs; turtles and crocodiles have a single penis, while snakes and lizards sport a pair of 'hemipenes.' Uniquely, Tuataras lack these organs, relying on a simple cloacal kiss for sperm transfer. Most reptiles lay amniotic eggs, encased in protective shells.
Arthropods: Ancient & Varied
Arthropods, including insects and crustaceans, predominantly rely on sexual reproduction, though some can resort to parthenogenesis (asexual cloning) when conditions are right. Terrestrial arthropods universally use internal fertilization, often employing ingenious methods like spermatophores — waterproof packets of sperm — which males transfer to females, sometimes after elaborate courtship displays.
Plants: Alternating Generations
Plants have a life cycle distinct from animals, characterized by an 'alternation of generations.' This means they switch between two multicellular phases: a diploid sporophyte and a haploid gametophyte. The sporophyte produces haploid spores through meiosis, which then grow into a gametophyte that produces gametes through mitosis.
Flowering Plants: The Apex of Pollination
Flowering plants, or angiosperms, dominate terrestrial ecosystems thanks to their sophisticated reproductive structures: flowers. Pollen grains, containing male gametes, must land on the stigma of a female reproductive structure. This triggers a remarkable process called 'double fertilization,' where one sperm fertilizes the egg to form an embryo, and another fertilizes a central cell to form the nutrient-rich endosperm.
The oldest evidence of this intricate process dates back 100 million years, found in fossilized amber, showing tiny pollen tubes penetrating a flower's stigma. After fertilization, the ovule develops into a seed, and the ovary matures into a fruit, often designed to aid in seed dispersal. Whether by self-pollination or cross-pollination, the goal is always the same: to create new, genetically diverse seeds.
Ferns and Bryophytes: Water's Role
For ferns and bryophytes (mosses, liverworts), water is essential for sexual reproduction. Their motile sperm, equipped with flagella, must swim through a film of water to reach the egg cells. Ferns have distinct sporophyte (the familiar leafy fern) and gametophyte (small, heart-shaped) phases. Bryophytes, on the other hand, have a dominant gametophyte that is the most visible part of the plant, with the sporophyte growing parasitically upon it.
Fungi: Spores for Survival
Fungi employ diverse methods of sexual reproduction, often culminating in the production of hardy resting spores. These spores are crucial for surviving harsh conditions and for dispersal. Their sexual cycle involves three key phases: plasmogamy, where the cytoplasm of two parent cells fuses; karyogamy, where their nuclei then fuse; and finally, meiosis, which generates new haploid spores.
For many fungi, like the Dikaryons (Ascomycota and Basidiomycota), the adaptive benefit of sexual reproduction appears to be closely linked to DNA repair. The recombination that occurs during meiosis provides a mechanism to fix genetic damage caused by environmental stresses, highlighting yet another vital role for this ancient process.
Prokaryotic 'Sex': Genetic Exchange
While prokaryotes — bacteria and archaea — lack true sexual reproduction with gametes and meiosis, they possess fascinating analogous processes that achieve genetic mixing. These mechanisms allow them to incorporate new genetic information, bolstering their adaptability and survival in constantly changing microbial worlds.
Bacterial transformation involves a bacterium taking up and integrating foreign DNA directly from its environment. Conjugation, often called 'bacterial sex,' is the direct transfer of plasmid DNA between bacteria via a specialized pilus. Lastly, archaea exhibit similar genetic exchange, with some species inducing cellular aggregation to facilitate DNA transfer and repair. These ancient forms of genetic sharing may even represent the evolutionary precursors to eukaryotic sexual reproduction.
Article
Sexual reproduction
In the first stage of sexual reproduction, meiosis, the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During fertilisation, haploid gametes come together to form a diploid zygote, and the original number of chromosomes is restored.
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). 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.
In placental mammals, sperm cells exit the penis through the male urethra and enter the vagina during copulation, while egg cells enter the uterus through the oviduct. Other vertebrates of both sexes possess a cloaca for the release of sperm or egg cells.
Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. Sexual reproduction also occurs in some unicellular eukaryotes. 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. 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.
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.
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, 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. This 50% cost is a fitness disadvantage of sexual reproduction. 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.
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. It has been described as "a powerful evolutionary force that does not exist in asexual populations".
Evolution
Sexual reproduction
The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1.05 billion years old.
Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, dealing with competition, DNA repair, masking deleterious mutations, and reducing genetic variation on the genomic level. All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes.
However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms.
Sexual reproduction allows these species to exhibit characteristics that depend on the specific environment that they inhabit, and the particular survival strategies that they employ.
Sexual selection
Sexual reproduction
In order to reproduce sexually, both females and males need to find a mate. Generally in animals mate choice is made by females while males compete to be chosen. This can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a Fisherian runaway. Thus sexual reproduction, as a form of natural selection, has an effect on evolution. Sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species. Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits. However, sexual selection is only implied over an extended period of time leading to sexual dimorphism.
Animals have different ways of going about sexual selection. One common example is with male peacocks fanning out their wings in order to show all their colors and attract a female mate. Lions with bigger and fuller manes are more likely to attract a female mate. Male deer with larger antlers are more likely to gain a female mate. These are just few of many examples in nature that show how sexual selection would be used in nature when females are choosing a mate.
Animals
Insects
An Australian emperor dragonfly laying eggs, guarded by a male
Insect species make up more than two-thirds of all extant animal species. Most insect species reproduce sexually, though some species are facultatively parthenogenetic. Many insect species have sexual dimorphism, while in others the sexes look nearly identical. Typically they have two sexes with males producing spermatozoa and females ova. The ova develop into eggs that have a covering called the chorion, which forms before internal fertilization. Insects have very diverse mating and reproductive strategies most often resulting in the male depositing a spermatophore within the female, which she stores until she is ready for egg fertilization. After fertilization, and the formation of a zygote, and varying degrees of development, in many species the eggs are deposited outside the female; while in others, they develop further within the female and the young are born live.
Mammals
There are three extant kinds of mammals: monotremes, placentals and marsupials, all with internal fertilization. In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although not reproductively functional. After several months or years, depending on the species, the sex organs develop further to maturity and the animal becomes sexually mature. Most female mammals are only fertile during certain periods during their estrous cycle, at which point they are ready to mate. For most mammals, males and females exchange sexual partners throughout their adult lives.
Fish
The vast majority of fish species lay eggs that are then fertilized by the male. Some species lay their eggs on a substrate like a rock or on plants, while others scatter their eggs and the eggs are fertilized as they drift or sink in the water column.
Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fish that have live-bearing offspring include the guppy and mollies or Poecilia. Fishes that give birth to live young can be ovoviviparous, where the eggs are fertilized within the female and the eggs simply hatch within the female body, or in seahorses, the male carries the developing young within a pouch, and gives birth to live young. Fishes can also be viviparous, where the female supplies nourishment to the internally growing offspring. Some fish are hermaphrodites, where a single fish is both male and female and can produce eggs and sperm. In hermaphroditic fish, some are male and female at the same time while in other fish they are serially hermaphroditic; starting as one sex and changing to the other. In at least one hermaphroditic species, self-fertilization occurs when the eggs and sperm are released together. Internal self-fertilization may occur in some other species. One fish species does not reproduce by sexual reproduction but uses sex to produce offspring; Poecilia formosa is a unisex species that uses a form of parthenogenesis called gynogenesis, where unfertilized eggs develop into embryos that produce female offspring. Poecilia formosa mate with males of other fish species that use internal fertilization, the sperm does not fertilize the eggs but stimulates the growth of the eggs which develops into embryos.
Reptiles
Reptiles generally reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs through the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis, while squamates, including snakes and lizards, possess a pair of hemipenes, only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm. Most reptiles lay amniotic eggs covered with leathery or calcareous shells.
Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother.
Plants
Sexual reproduction
Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis. Male gametes are called sperm, and female gametes are called eggs or ova. In animals, fertilization of the ovum by a sperm results in the formation of a diploid zygote that develops by repeated mitotic divisions into a diploid adult. Plants have two multicellular life-cycle phases, resulting in an alternation of generations. Plant zygotes germinate and divide repeatedly by mitosis to produce a diploid multicellular organism known as the sporophyte. The mature sporophyte produces haploid spores by meiosis that germinate and divide by mitosis to form a multicellular gametophyte phase that produces gametes at maturity. The gametophytes of different groups of plants vary in size. Mosses and other pteridophytic plants may have gametophytes consisting of several million cells, while angiosperms have as few as three cells in each pollen grain.
Flowering plants
Flowers contain the sexual organs of flowering plants.
Flowering plants are the dominant plant form on land and they reproduce either sexually or asexually. Often their most distinctive feature is their reproductive organs, commonly called flowers. The anther produces pollen grains which contain the male gametophytes that produce sperm nuclei. For pollination to occur, pollen grains must attach to the stigma of the female reproductive structure (carpel), where the female gametophytes are located within ovules enclose within the ovary. After the pollen tube grows through the carpel's style, the sex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.
In 2013, flowers dating from the Cretaceous (100 million years before present) were found encased in amber, the oldest evidence of sexual reproduction in a flowering plant. Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. The pollen was sticky, suggesting it was carried by insects.
Ferns
Ferns produce large diploid sporophytes with rhizomes, roots and leaves. Fertile leaves produce sporangia that contain haploid spores. The spores are released and germinate to produce small, thin gametophytes that are typically heart shaped and green in color. The gametophyte prothalli, produce motile sperm in the antheridia and egg cells in archegonia on the same or different plants. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the archegonia where they fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of different thallus. After fertilization, a zygote is formed which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of generations.
Bryophytes
The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominant gametophyte form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antheridia (male) and archegonia (female) by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. The zygote divides by mitotic division and grows into a multicellular, diploid sporophyte. The sporophyte produces spore capsules (sporangia), which are connected by stalks (setae) to the archegonia. The spore capsules produce spores by meiosis and when ripe the capsules burst open to release the spores. Bryophytes show considerable variation in their reproductive structures and the above is a basic outline. Also in some species each plant is one sex (dioicous) while other species produce both sexes on the same plant (monoicous).
Fungi
Sexual reproduction
Puffballs emitting spores
Fungi are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production of resting spores that are used to survive inclement times and to spread. There are typically three phases in the sexual reproduction of fungi: plasmogamy, karyogamy and meiosis. The cytoplasm of two parent cells fuse during plasmogamy and the nuclei fuse during karyogamy. New haploid gametes are formed during meiosis and develop into spores. The adaptive basis for the maintenance of sexual reproduction in the Ascomycota and Basidiomycota (dikaryon) fungi was reviewed by Wallen and Perlin. They concluded that the most plausible reason for maintaining this capability is the benefit of repairing DNA damage, caused by a variety of stresses, through recombination that occurs during meiosis.
Bacteria and archaea
Sexual reproduction
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. This process occurs naturally in at least 40 bacterial species. 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, 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. 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.
Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange Ajon et al. 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.