Sex
Sex is the biological trait that determines whether a sexually reproducing organism produces male or female gametes.
[1][2][3][4][5] During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent.
The terms male and female typically do not apply in sexually undifferentiated species in which the individuals are isomorphic (look the same) and the gametes are isogamous (indistinguishable in size and shape), such as the green alga Ulva lactuca.
[11] Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to eukaryotes.
The eukaryote cell has a set of paired homologous chromosomes, one from each parent, and this double-chromosome stage is called "diploid".
During sexual reproduction, a diploid organism produces specialized haploid sex cells called gametes via meiosis,[12] each of which has a single set of chromosomes.
In animals, the haploid stage only occurs in the gametes, the sex cells that fuse to form a zygote that develops directly into a new diploid organism.
In either case, the gametes may be externally similar (isogamy) as in the green alga Ulva or may be different in size and other aspects (anisogamy).
Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.
The male gamete, a spermatozoon (produced in vertebrates within the testes), is a small cell containing a single long flagellum which propels it.
[20] Most animals that live outside of water, however, use internal fertilization, transferring sperm directly into the female to prevent the gametes from drying up.
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).
The male parts of the flower are the stamens: these consist of long filaments arranged between the pistil and the petals that produce pollen in anthers at their tips.
When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a pollen tube that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed.
The larger and longer-lived female cones are typically more durable, and contain ovules within them that develop into seeds after fertilization.
These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.
In some cases, the fusion is asymmetric, and the cell which donates only a nucleus (and no accompanying cellular material) could arguably be considered male.
[26] Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.
[43] Studies on green algae have provided genetic evidence for the evolutionary link between sexes and mating types.
[46] The most basic role of meiosis appears to be conservation of the integrity of the genome that is passed on to progeny by parents.
[49] Genetic variation, often produced as a byproduct of these processes, may provide long-term advantages in those sexual lineages that favor outcrossing.
[30] Within animals and other organisms that have genetic sex-determination systems, the determining factor may be the presence of a sex chromosome.
[50] Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring.
[54] XY sex determination is found in other organisms, including insects like the common fruit fly,[55] and some plants.
This system is found in most arachnids, insects such as silverfish (Apterygota), dragonflies (Paleoptera) and grasshoppers (Exopterygota), and some nematodes, crustaceans, and gastropods.
[64] For many species, sex is not determined by inherited traits, but instead by environmental factors such as temperature experienced during development or later in life.
[70] Diploid bees and ants are generally female, and haploid individuals (which develop from unfertilized eggs) are male.
[78] In many animals and some plants, individuals of male and female sex differ in size and appearance, a phenomenon called sexual dimorphism.
[81] Other examples demonstrate that it is the preference of females that drives sexual dimorphism, such as in the case of the stalk-eyed fly.
Mammal species with extreme sexual size dimorphism, such as elephant seals, tend to have highly polygynous mating systems, presumably due to selection for success in competition with other males.
