More complicated chromosomal sex determining systems also exist. For example, some swordtail fish have three sex chromosomes in a population. The sex of some other species is not determined by chromosomes, but by some aspect of the environment.
Sex determination in alligators, some turtles, and tuataras, for example, is dependent on the temperature during the middle third of egg development. This is referred to as environmental sex determination, or more specifically, as temperature-dependent sex determination.
In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females, while in many other species of turtles, the reverse is true. In some crocodiles and some turtles, moderate temperatures produce males and both warm and cool temperatures produce females. Individuals of some species change their sex during their lives, switching from one to the other. The wrasses, a family of reef fishes, are all sequential hermaphrodites. Some of these species live in closely coordinated schools with a dominant male and a large number of smaller females.
If the male dies, a female increases in size, changes sex, and becomes the new dominant male. The fusion of a sperm and an egg is a process called fertilization. This can occur either inside internal fertilization or outside external fertilization the body of the female. Humans provide an example of the former, whereas frog reproduction is an example of the latter. External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water.
After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male s release sperm in the same area, at the same time.
The spawning may be triggered by environmental signals, such as water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans such as crabs and shrimp , mollusks such as oysters , squid, and echinoderms such as sea urchins and sea cucumbers.
Frogs, corals, mayflies, and mosquitoes also spawn Figure Internal fertilization occurs most often in terrestrial animals, although some aquatic animals also use this method. Internal fertilization may occur by the male directly depositing sperm in the female during mating.
It may also occur by the male depositing sperm in the environment, usually in a protective structure, which a female picks up to deposit the sperm in her reproductive tract. There are three ways that offspring are produced following internal fertilization.
This occurs in some bony fish, some reptiles, a few cartilaginous fish, some amphibians, a few mammals, and all birds. Most non-avian reptiles and insects produce leathery eggs, while birds and some turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard.
Chicken eggs are an example of a hard shell. The eggs of the egg-laying mammals such as the platypus and echidna are leathery. This process helps protect the eggs until hatching.
This occurs in some bony fish like the platyfish Xiphophorus maculatus, Figure In viviparity the young are born alive. They obtain their nourishment from the female and are born in varying states of maturity. This occurs in most mammals Figure Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the genetic material from two individuals is combined to produce genetically diverse offspring.
Asexual reproduction in animals occurs through fission, budding, fragmentation, and parthenogenesis. Sexual reproduction may involve fertilization inside the body or in the external environment. A species may have separate sexes or combined sexes; when the sexes are combined they may be expressed at different times in the life cycle.
The sex of an individual may be determined by various chromosomal systems or environmental factors such as temperature. Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can occur either outside the bodies or inside the female.
The method of fertilization varies among animals. Some species release the egg and sperm into the environment, some species retain the egg and receive the sperm into the female body and then expel the developing embryo covered with shell, while still other species retain the developing offspring throughout the gestation period. Learning Objectives By the end of this section, you will be able to: Describe advantages and disadvantages of asexual and sexual reproduction Discuss asexual reproduction methods Discuss sexual reproduction methods Discuss internal and external methods of fertilization.
Concept in Action View this video to see a hydra budding. Exercises In which group is parthenogenesis a normal event? Compared to separate sexes and assuming self-fertilizing is not possible, what might be one advantage and one disadvantage to hermaphroditism?
Answers B A A Temperatures can vary from year to year and an unusually cold or hot year might produce offspring all of one sex, making it hard for individuals to find mates. A possible advantage of hermaphroditism might be that anytime an individual of the same species is encountered a mating is possible, unlike separate sexes that must find an individual of the right sex to mate.
Also, every individual in a hermaphrodite population is able to produce offspring, which is not the case in populations with separate sexes. A disadvantage might be that hermaphrodite populations are less efficient because they do not specialize in one sex or another, which means a hermaphrodite does not produce as many offspring through eggs or sperm as do species with separate sexes.
Other answers are possible. Glossary asexual reproduction: a mechanism that produces offspring that are genetically identical to the parent. Next: There are a number of ways that animals reproduce asexually. Fission , also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms.
Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps Figure Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals.
Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult and breaks away from the main body, as illustrated in Figure Watch a video of a hydra budding.
Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow. For example, in many sea stars, asexual reproduction is accomplished by fragmentation. Figure Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams.
Fragmentation also occurs in annelid worms, turbellarians, and poriferans. Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximate size are formed.
Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water flees, rotifers, aphids, stick insects, some ants, wasps, and bees.
Bees use parthenogenesis to produce haploid males drones and diploid females workers. If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced. Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos.
Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males. Sexual reproduction is the combination of usually haploid reproductive cells from two individuals to form a third usually diploid unique offspring.
Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes—male and female—determined at conception. However, in the animal kingdom, there are many variations on this theme. Hermaphroditism occurs in animals where one individual has both male and female reproductive parts.
Invertebrates such as earthworms, slugs, tapeworms and snails, shown in Figure Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring.
Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams. Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. The presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.
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