What Is An Organism Sexually

What Is An Organism Sexually

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William Harvey
Lazzaro Spallanzani
Christian Konrad Sprengel
Jan Evangelista Purkinje
Nathanael Pringsheim

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fertilization
sexual intercourse
zygote
gamete
pseudocopulation

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reproduction: Reproduction of organisms
In the sexual reproduction of all organisms except bacteria, there is one common feature: haploid, uninucleate gametes...

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sexual reproduction , the production of new organisms by the combination of genetic information of two individuals of different sexes. In most species the genetic information is carried on chromosomes in the nucleus of reproductive cells called gametes , which then fuse to form a diploid zygote . The zygote develops into a new individual. Sexual reproduction is the dominant form of reproduction in living beings.
Sexual reproduction allows for the reshuffling of genetic material, both within and between individuals of one generation, resulting in the potential for an extraordinary array of offspring, each with a genetic makeup different from that of its parents. In the sexual reproduction of all organisms except bacteria , there is one common feature: haploid, uninucleate gametes are produced that join in fertilization to form a diploid, uninucleate zygote. At some later stage in the life history of the organism, the chromosome number is again reduced by meiosis to form the next generation of gametes. The gametes may be equal in size (isogamy), or one may be slightly larger than the other (anisogamy); the majority of forms have a large egg and a minute sperm (oogamy). The sperm are usually motile and the egg passive, except in higher plants, in which the sperm nuclei are carried in pollen grains that attach to the stigma (a female structure) of the flower and send out germ tubes that grow down to the egg nucleus in the ovary . Some organisms, such as most flowering plants , earthworms , and tunicates , are bisexual (hermaphroditic, or monoecious)—i.e., both the male and female gametes are produced by the same individual. All other organisms, including some plants (e.g., holly and the ginkgo tree) and all vertebrates , are unisexual (dioecious): the male and female gametes are produced by separate individuals.
In animals, fertilization—that is, the uniting of the gametes—can be either internal or external. In external fertilization, the male releases sperm over eggs that have been released by a female. This form of fertilization is used by many invertebrates and by most fish and amphibians . In internal fertilization, the sperm unites with the egg within the body of an adult. In the vast majority of cases, fertilization takes place within the body of the female.
Sexual reproduction in plants also requires the uniting of male and female gametes. There is a much greater variety of structures and strategies used by plants. In general, flowers and cones are the locus of fertilization in plants.
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Biologists have been speculating on the reason why such a complicated process for reproduction, sex, became the most common mode for advanced organisms, particularly when asexual reproduction has so many advantages. It is easier, faster, uses a lot less energy, a mate is not required, and the result is an offspring which is fully matured, and can protect and care for itself. With sexual reproduction, finding a mate can be challenging. Once the risky business of impregnation and birth have taken place, protecting and caring for the baby remains difficult, leaving families open to attack from challengers and predators.
Single celled organisms such as bacteria reproduce asexually. Among complex organisms, many plants and even some animals do too. These include bananas, starfish, and even komodo dragons. Despite this, up to 99% of complex organisms reproduce sexually, at least some of the time. So it must convey some type of advantage.
Dr. Stuart Auld and colleagues at the University of Stirling in Scotland wanted to explore further. Auld is among the Faculty of Natural Sciences at the university. He said that this question is one of the oldest in evolutionary biology. What’s more, sex’s presence is pervasive in nature. "Sex explains the presence of the peacock's tail, the stag's antlers and the male bird of paradise's elaborate dance,” Auld said.
Organisms go through a lot to find a mate and reproduce sexually. How does it benefit them?
German evolutionary biologist August Weismann in 1886 proposed that sex was a way to hasten evolution. Beneficial mutations could be introduced quickly, while those which were harmful would be sloughed off. Sex also allows for different combinations of genes which can help organisms evolve rapidly to fit new situations. A theory, developed by Leigh Van Valen in the late 1980s, called the “Red Queen Hypothesis,” is now the prevailing one. This was taken from the character in Through the Looking Glass, more commonly known as Alice in Wonderland.
When Alice meets the Red Queen, she must take part in a bizarre chess game , where she runs as fast as she can in order to keep up with the other players. This constant running to maintain position is the theme the hypothesis adopts. Organisms react not only to the environment but each other. When one organism develops an adaptation that gives it an advantage, it affects its predator, and prey.
Lions for instance depend on the antelope population. Should antelope develop the ability to run faster through a rapid mutation, the lion population would come under pressure. Only when lions developed the ability to run faster or to pounce farther would a balance be struck. There exists a similar arms race between host organisms and their parasites. But since single cells organisms don’t live too long, pathogens must evolve rapidly or face extinction. Meanwhile, a host organism needs to evolve just as quickly to resist infection.
The water flea is one of those rare species which reproduces both sexually and asexually.
To hasten evolution, the right combination of genes is required. So the more combinations an organism has access to, the better its chances. Though a strong theory, it’s been difficult to test. After all, how do you compare those organisms who reproduce sexually to those who don’t? Auld and colleagues found a way.
Published in the journal Royal Society Proceedings B, researchers found that at the time when sexual reproduction came on the scene, “parasites adapted to infect the previous generations." Therefore, reproducing sexually meant seriously undermining the parasites’ ability to cause infection. Auld and colleagues selected the water flea, a bizarre creature which reproduces both sexually and asexually. Just a few others organisms do, such as yeast and the snail.
The water fleas used in this experiment were collected from the natural environment, as were their bacterial parasites. After a period, researchers gathered female water flea offspring who were produced either sexually or through cloning. Under controlled conditions, they exposed the offspring to the parasites. Those who reproduced sexually were twice as resistant to infection, researchers found. According to Dr. Auld, these findings suggest that, "The ever-present need to evade disease can explain why sex persists in the natural world in spite of the costs."
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Organisms That Reproduce Both Asexually & Sexually

Organisms typically reproduce through sexual or asexual reproduction. Explore the two types of reproduction, and discover organisms that reproduce both sexually and asexually.

Updated: 01/05/2022

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