Some of the many plausible theories include: Conversely, there are also species in which there is no sexual specialization, and the same individuals both contain masculine and feminine reproductive organs, and they are called hermaphrodites. Genetic sex-determination 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. The plants attract these insects or larger animals such as humming birds and bats with nectar-containing flowers. Other plants have heavier, sticky pollen that is specialized for transportation by insects. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. This may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm—larger females are able to produce more eggs.
In pines and other conifers the sex organs are conifer cones and have male and female forms. Sexual reproduction is a process specific to eukaryotes , organisms whose cells contain a nucleus and mitochondria. In field crickets , for example, insects with a single X chromosome develop as male, while those with two develop as female. The ZW sex-determination system is shared by birds, some fish and some crustaceans. In many cases the male of a species is larger than the female. This process of cell division is called mitosis. 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". In clownfish , smaller fish are male, and the dominant and largest fish in a group becomes female. The "default sex," in the absence of a Y chromosome, is female-like. In the majority of species with sex specialization, organisms are either male producing only male gametes or female producing only female gametes. These fungi are typically isogamous , lacking male and female specialization: Genetic Like humans and other mammals, the common fruit fly has an XY sex-determination system. These gametes combine to form embryos which develop into a new organism. All other chromosomes in these organisms are diploid, but organisms may inherit one or two X chromosomes. In birds, which have a ZW sex-determination system , the opposite is true: The male gamete, a spermatozoon produced in vertebrates within the testes , is a small cell containing a single long flagellum which propels it. 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. As with flowers, seeds form within the female cone after pollination. Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. Female left and male right cones are the sex organs of pines and other conifers. Many species, both plants and animals, have sexual specialization, and their populations are divided into male and female individuals. However, no third gamete type is known in multicellular plants or animals. Isogamy and Anisogamy The life cycle of sexually reproducing organisms cycles through haploid and diploid stages Sexual reproduction in eukaryotes is a process whereby organisms form offspring that combine genetic traits from both parents. 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. Diploid organisms can, in turn, form haploid cells gametes that randomly contain one of each of the chromosome pairs, via meiosis. They are specialized for motility, seeking out an egg cell and fusing with it in a process called fertilization. Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms.
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