Sexual reproduction
The life cycle of sexually reproducing organisms
cycles through haploid and diploid stages.
Sexual reproduction is a process where organisms form
offspring that combine genetic traits from both parents.
Chromosomes are passed on from one parent to another in this
process. Each cell has half the chromosomes of the mother and
half of the father.[1]
Genetic traits are contained within the
deoxyribonucleic acid (DNA) of
chromosomes — by combining one of each type of chromosomes
from each parent, an organism is formed containing a doubled set
of chromosomes. This double-chromosome stage is called "diploid",
while the single-chromosome stage is "haploid".
Diploid organisms can, in turn, form haploid cells (gametes)
that randomly contain one of each of the chromosome pairs, via a
process called
meiosis.[2]
Meiosis also involves a stage of
chromosomal crossover, in which regions of DNA are exchanged
between matched types of chromosomes, to form a new pair of
mixed chromosomes.
Crossing over and
fertilization (the recombining of single sets of chromosomes
to make a new diploid) result in the new organism containing a
different set of genetic traits from either parent.
In many organisms, the haploid stage has been reduced to just
gametes specialized to recombine and form a new diploid
organism; in others, the gametes are capable of undergoing
cell division to produce
multicellular haploid organisms. In either case, gametes may
be externally similar, particularly in size (isogamy),
or may have
evolved an asymmetry such that the gametes are different in
size and other aspects (anisogamy).[3]
By convention, the larger gamete (called an ovum, or
egg cell) is considered female, while the smaller gamete
(called a spermatozoon, or
sperm
cell) is considered male. An individual that produces
exclusively large gametes is
female, and one that produces exclusively small gametes is
male.
An individual that produces both types of gametes is a
hermaphrodite; in some cases hermaphrodites are able to
self-fertilize and produce offspring on their own, without a
second organism.[4]
Animals
Animal sexual behavior includes
mating systems using
sexual conflict to a greater extent than simpler organisms.
Most sexually reproducing animals spend their lives as diploid
organisms, with the haploid stage reduced to single cell
gametes.[5]
The gametes of animals have male and female forms—spermatozoa
and
egg cells. These gametes combine to form embryos which
develop into a new organism.
The male gamete, a
spermatozoan (produced within a
testicle), is a small cell containing a single long
flagellum which propels it.[6]
Spermatozoa are extremely reduced cells, lacking many cellular
components that would be necessary for embryonic development.
They are specialized for motility, seeking out an egg cell and
fusing with it in a process called
fertilization.
Female gametes are
egg cells (produced within
ovaries), large immobile cells that contain the nutrients
and cellular components necessary for a developing embryo.[7]
Egg cells are often associated with other cells which support
the development of the embryo, forming an
egg. In mammals, the fertilized embryo instead develops
within the female, receiving nutrition directly from its mother.
Animals are usually mobile and seek out a partner of the
opposite sex for
mating. Animals which live in the water can mate using
external fertilization, where the eggs and sperm are
released into and combine within the surrounding water.[8]
Most animals that live outside of water, however, must transfer
sperm from male to female to achieve
internal fertilization.
In most birds, both excretion and reproduction is done
through a single posterior opening, called the
cloaca—male and female birds touch cloaca to transfer sperm,
a process called "cloacal kissing".[9]
In many other terrestrial animals, males use specialized sex
organs to assist the transport of sperm—these male sex organs
are called
intromittent organs. In humans and other mammals this male
organ is the
penis,
which enters the female reproductive tract (called the
vagina) to achieve
insemination—a process called
sexual intercourse. The penis contains a tube through which
semen
(a fluid containing sperm) travels. In female mammals the vagina
connects with the
uterus, an organ which directly supports the development of
a fertilized embryo within (a process called
gestation).
Humans
The biology of
human sexuality examines the influence of biological
factors, such as organic and neurological response,[10]
heredity,
hormones, and
sexual dysfunction;[11]
it examines the basic functions of reproduction and the physical
means to carry out
sexual intercourse. The biological perspective helps to
analyze the factors, and ultimately aids in understanding them
and using them to deal with sexual problems.
Plants
Flowers are the sexual organs of flowering plants,
usually containing both male and female parts.
Female (left) and male (right) cones are the sex organs
of pines and other conifers.
Like animals, plants have developed specialized male and
female gametes.[12]
Within most familiar plants, male gametes are contained within
hard coats, forming
pollen. The female gametes of plants are contained within
ovules; once fertilized by pollen these form
seeds
which, like eggs, contain the nutrients necessary for the
development of the embryonic plant.
Many plants have
flowers and these are the sexual organs of those plants.
Flowers are usually hermaphroditic, producing both male and
female gametes. The female parts, in the center of a flower, are
the
carpels—one or more of these may be merged to form a single
pistil. Within carpels are ovules which develop into seeds
after fertilization. The male parts of the flower are the
stamens: these long filamentous organs are arranged between
the pistil and the petals and produce pollen at their tips. When
a pollen grain lands upon the top of a carpel, the tissues of
the plant react to transport the grain down into the carpel to
merge with an ovule, eventually forming seeds.
In
pines and other
conifers the sex organs are
cones
and have male and female forms. The more familiar female cones
are typically more durable, containing ovules within them. Male
cones are smaller and produce pollen which is transported by
wind to land in female cones. As with flowers, seeds form within
the female cone after pollination.
Because plants are immobile, they depend upon passive methods
for transporting pollen grains to other plants. Many plants,
including conifers and grasses, produce lightweight pollen which
is carried by wind to neighboring plants. Other plants have
heavier, sticky pollen that is specialized for transportation by
insects. The plants attract these insects with
nectar-containing flowers. Insects transport the pollen as they
move to other flowers, which also contain female reproductive
organs, resulting in
pollination.
Mushrooms are produced as part of fungal sexual
reproduction.
Fungi
Most fungi reproduce sexually, having both a haploid and
diploid stage in their life cycles. These fungi are typically
isogamous, lacking male and female specialization: haploid
fungi grow into contact with each other and then fuse their
cells. In some of these cases the fusion is asymmetric, and the
cell which donates only a nucleus (and not accompanying cellular
material) could arguably be considered "male".[13]
Some fungi, including
baker's yeast, have
mating types that create a duality similar to male and
female roles. Yeast with the same mating type will not fuse with
each other to form diploid cells, only with yeast carrying the
other mating type.[14]
Fungi produce
mushrooms as part of their sexual reproduction. Within the
mushroom diploid cells are formed, later dividing into haploid
spores—the height of the mushroom aids the
dispersal of these sexually produced offspring.
Sex helps the spread of advantageous traits through
recombination. The diagrams compare evolution of
allele frequency in a sexual population (a) and an
asexual population (b). The vertical axis shows
frequency and the horizontal axis shows time. The
alleles a/A and b/B occur at random. The
advantageous combination AB arises rapidly with
recombination (a), but must arise independently in
(b).
Evolution
Sexual reproduction first appeared about a billion years ago,
evolved within ancestral single-celled eukaryotes.[15]
The reason for the initial evolution of sex, and the reason it
has survived to the present, are still matters of debate. Some
of the many plausible theories include: that sex creates
variation among offspring, sex helps in the spread of
advantageous traits, and that sex helps in the removal of
disadvantageous traits.
Sexual reproduction is a process specific to
eukaryotes, organisms whose cells contain a nucleus and
mitochondria. In addition to animals, plants, and fungi,
other eukaryotes (eg. the
malaria parasite) also engage in sexual reproduction. Some
bacteria use
conjugation to transfer genetic material between bacteria;
while not the same as sexual reproduction, this also results in
the mixture of genetic traits.
What is considered defining of sexual reproduction is the
difference between the gametes and the binary nature of
fertilization. Multiplicity of gamete types within a species
would still be considered a form of sexual reproduction.
However, no third gamete is known in multicellular animals.[16][17][18]
Sex determination
The most basic sexual system is one in which all organisms
are
hermaphrodites, producing both male and female gametes—this
is true of some animals (eg. snails) and the majority of
flowering plants.[19]
In many cases, however, specialization of sex has evolved such
that some organisms produce only male or only female gametes.
The biological cause for an organism developing into one sex or
the other is called
sex determination.
In the majority of species with sex specialization organisms
are either
male
(producing only male gametes) or
female (producing only female gametes). A few exceptions
exist—for example, in the roundworm
C. elegans the two sexes are hermaphrodite and male (a
system called
androdioecy).
Sometimes an organism's development is intermediate between
male and female, a condition called
intersex. Sometimes intersex individuals are called
"hermaphrodite" but, unlike biological hermaphrodites, intersex
individuals are unusual cases and are not typically fertile in
both male and female aspects.
Genetic
Like humans and other mammals, the common fruit fly
has an XY sex determination system.
In genetic
sex determination systems, an organism's sex is determined
by the genome it inherits. Genetic sex determines usually
depends on asymmetrically inherited sex chromosomes which carry
genetic features that influence
development; sex may be determined either by the presence of
a sex chromosome or by how many the organism has. Genetic sex
determination, because it is determined by chromosome
assortment, usually results in a 1:1 ratio of male and female
offspring.
Humans and other
mammals have an
XY sex determination system: the Y chromosome carries
factors responsible for triggering male development. The default
sex, in the absence of a Y chromosome, is female. Thus, XX
mammals are female and XY are male. XY sex determination is
found in other organisms, including the
common fruit fly and some plants.[19]
In some cases, including in the fruit fly, it is the number of X
chromosomes that determines sex rather than the presence of a Y
chromosome.
In
birds, which have a
ZW sex-determination system, the opposite is true: the W
chromosome carries factors responsible for female development,
and default development is male.[20]
In this case ZZ individuals are male and ZW are female. The
majority of butterflies and moths also have a ZW
sex-determination system. In both XY and ZW sex determination
systems the sex chromosome carrying the critical factors is
often significantly smaller, carrying little more than the genes
necessary for triggering the development of a given sex.[21]
Many
insects use a sex determination system based on the number
of sex chromosomes. This is called
XX/XO sex determination—the O indicates the absence of the
sex chromosome. All other chromosomes in these organisms are
diploid, but organisms may inherit one or two X chromosomes. In
field crickets, for example, insects with a single X
chromosome develop as male, while those with two develop as
female.[22]
In the nematode
C. elegans most worms are self-fertilizing XX
hermaphrodites, but occasionally abnormalities in chromosome
inheritance regularly give rise to individuals with only one X
chromosome—these XO individuals are fertile males (and half
their offspring are male).[23]
Other insects, including
honey bees and
ants,
use a
haplodiploid sex-determination system.[24]
In this case diploid individuals are generally female, and
haploid individuals (which develop from unfertilized eggs) are
male. This sex-determination system results in highly biased
sex ratios, as the sex of offspring is determined by
fertilization rather than the assortment of chromosomes during
meiosis.
Clownfish are initially male; the largest fish
in a group becomes female.
Nongenetic
For many species sex is not determined by inherited traits,
but instead by environmental factors experienced during
development or later in life. Many
reptiles have
temperature-dependent sex determination: the temperature
embryos experience during their development determines the sex
of the organism. In some
turtles, for example, males are produced at lower incubation
temperatures than females; this difference in critical
temperatures can be as little as 1-2°C.
Many
fish change sex over the course of their lifespan, a
phenomenon called
sequential hermaphroditism. In
clownfish, smaller fish are male, and the dominant and
largest fish in a group becomes female. In many
wrasses the opposite is true—most fish are initially female
and become male when they reach a certain size. Sequential
hermaphrodites may produce both types of gametes over the course
of their lifetime, but at any given point they are either female
or male.
In some
ferns
the default sex is hermaphrodite, but ferns which grow in soil
that has previously supported hermaphrodites are influenced by
residual hormones to instead develop as male.[25]
