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
"fertilization", haploid gametes come together to form a diploid
zygote and the original number of chromosomes is restored.
Sexual
reproduction is a form of reproduction where
two morphologicallydistinct types of specialized
reproductive cells called gametes fuse together, involving
a female's large ovum (or egg) and a male's smaller sperm. Each
gamete contains half the number of chromosomes of normal cells. They
are created by a specialized type of cell division, which only occurs
in eukaryoticcells, known as meiosis. The two gametes fuse
during fertilization to produceDNA replication and the creation
of a single-celled zygote which includes genetic material from both
gametes. In a process called genetic recombination, genetic material (DNA)
joins up so that homologous chromosome sequences are aligned with
each other, and this is followed by exchange of genetic information. Two rounds
of cell division then produce four daughter cells with half the number of
chromosomes from each original parent cell, and the same number of chromosomes
as both parents, though self-fertilization can occur. For instance,
in human reproduction each human cell contains 46 chromosomes, 23
pairs, except gamete cells, which only contain 23 chromosomes, so the child
will have 23 chromosomes from each parent genetically recombined into 23 pairs.
Cell division initiates the development of a new
individual organism in multicellular
organisms, including animals and plants, for the vast majority
of whom this is the primary method of reproduction. A species is defined
as a taxonomic rank. A species is often defined as the largest group
of organisms where two hybrids are capable of
reproducing fertile offspring, typically using sexual reproduction,
although the species problem encompasses a series of difficult
related questions that often come up when biologists define the word
species.
The evolution
of sexual reproduction is a major puzzle because asexual
reproduction should be able to outcompete 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. One definite advantage of sexual
reproduction is that it prevents the accumulation of 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"
Prokaryotes reproduce
through asexual reproduction but may display processes similar to
sexual reproduction (mechanisms for lateral gene transfer such as bacterial conjugation, transformation and transduction), but they do not
lead to reproduction. In prokaryotes, the initial cell has additional or
transformed genetic material.
Evolution of sexual reproduction
The
first fossilized evidence of sexual reproduction in eukaryotes is
from the Stenian period, about 1 to 1.2 billion years ago.
Biologists
studying evolution propose several explanations for why sexual
reproduction developed and why it is maintained. These reasons include fighting
the accumulation of deleterious mutations, increasing rate
of adaptation to changing environments, dealing with competition, or
masking deleterious mutations. 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 benefits obtained through
sexual reproduction than do smaller population sizes.
Maintenance
of sexual reproduction has been explained by theories that work at
several levels of selection, though some of these models remain
controversial.
New 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
In order to
sexually reproduce both males and females 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.
Sex ratio
Apart from
some eusocial wasps, organisms which reproduce sexually have a
1:1 sex ratio of male and female births. The English statistician and
biologist Ronald Fisher outlined why this is so in what has come to
be known as Fisher's principle. This essentially says the following:
Suppose male
births are less common than female.
A newborn
male then has better mating prospects than a newborn female, and therefore can
expect to have more offspring.
Therefore
parents genetically disposed to produce males tend to have more than average
numbers of grandchildren born to them.
Therefore the
genes for male-producing tendencies spread, and male births become more common.
As the 1:1
sex ratio is approached, the advantage associated with producing males dies
away.
The same
reasoning holds if females are substituted for males throughout. Therefore 1:1
is the equilibrium ratio.
Insects
Insect and Reproduction and development
Australian Emperor laying egg, guarded by the
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 insects 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 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 are born live.
Birds
Mammalian reproduction
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. Individual male and female mammals meet and carry
out copulation.[citation needed] For most mammals, males and
females exchange sexual partners throughout their adult lives.
Fish reproductive anatomy
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 formosa
Plants
Plant reproduction
Animals typically
produce gametes directly by meiosis. Male gametes are called sperm, and female
gametes are called eggs or ova. In animals, fertilization follows immediately
after meiosis. Plants on the other hand have mitosis occurring in spores, which
are produced by meiosis. The spores germinate into the gametophyte phase. The
gametophytes of different groups of plants vary in size; angiosperms have as
few as three cells in pollen, and mosses and other so called primitive plants
may have several million cells. Plants have an alternation of
generations where the sporophyte phase is succeeded by the gametophyte
phase. The sporophyte phase produces spores within the sporangium by meiosis.
Flowering plants
Flowers are 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 distinguishing feature is their
reproductive organs, commonly called flowers.
The anther produces pollen grains which contain the
male gametophytes (sperm). For pollination to occur, pollen grains
must attach to the stigma of the female reproductive structure (carpel), where
the female gametophytes (ovules) are located inside 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.
Non flowering
plants like ferns, moss and liverworts use other means
of sexual reproduction.
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 mostly
produce large diploid sporophytes with rhizomes, roots and
leaves; and on fertile leaves called sporangium,spores are produced.
The spores are released and germinate to produce short, thin gametophytes that
are typically heart shaped, small and green in color. The gametophytes or thallus,
produce both motile sperm in the antheridia and egg cells in
separate archegonia. 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.
A zygote is formed after fertilization, which grows into a new
sporophytic plant. The condition of having separate sporephyte and gametophyte
plants is called alternation of generations. Other plants with similar
reproductive means include the Psilotum, Lycopodium, Selaginella and
Equisetum.
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 dominate form, which is a multicellular haploid body
with leaf-like structures that photosynthesize. Haploid gametes are
produced in antherida and archegonia by mitosis. The sperm released from the
antherida 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 sporophyte that is diploid. The
multicellular diploid sporophyte produces structures called spore
capsules, which are connected by seta to the archegonia. The spore
capsules produce spores by meiosis, when ripe the capsules burst open and the
spores are released. Bryophytes show considerable variation in their breeding
structures and the above is a basic outline. Also in some species each plant is
one sex while other species produce both sexes on the same plant.
Mating in fungi
Fungus
Reproduction
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.
Bacteria and archaea
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,[13] 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 through 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
TAGS :
COMMENTS