Sexual reproduction
Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants.
[2][8] 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.
[11] During sexual reproduction, two haploid gametes combine into one diploid cell known as a zygote in a process called fertilization.
Multiple cell divisions by mitosis (without change in the number of chromosomes) then develop into a multicellular diploid phase or generation.
[19][20] 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,[21] dealing with competition, DNA repair, masking deleterious mutations, and reducing genetic variation on the genomic level.
[26] 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.
[31] Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem,[32] that appears to have remained unsettled.
Opiliones (harvestmen), millipedes, and some crustaceans use modified appendages such as gonopods or penises to transfer the sperm directly to the female.
However, most male terrestrial arthropods produce spermatophores, waterproof packets of sperm, which the females take into their bodies.
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.
In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although not reproductively functional.
Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring.
[44] 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.
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.
Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis.
Plant zygotes germinate and divide repeatedly by mitosis to produce a diploid multicellular organism known as the sporophyte.
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 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.
They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction.
The outcome of sexual reproduction most often is the production of resting spores that are used to survive inclement times and to spread.
The adaptive basis for the maintenance of sexual reproduction in the Ascomycota and Basidiomycota (dikaryon) fungi was reviewed by Wallen and Perlin.
[50] 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.
Sexual reproduction in early single-celled eukaryotes may have evolved from bacterial transformation,[24] or from a similar process in archaea (see below).
[22][53] Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange[54][55] Ajon et al.[55] hypothesized that this cellular aggregation enhances species-specific DNA repair by homologous recombination.
