What occurs when a form of asexual reproduction takes place where an organism is split into fragments?

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 24.5, are often hermaphroditic. 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.

Figure 24.5. Many snails are hermaphrodites. When two individuals mate, they can produce up to one hundred eggs each. (credit: Assaf Shtilman)

Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. 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.

Avian sex determination is dependent on the presence of Z and W chromosomes. Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use this system.

The sex of some species is not determined by genetics but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods 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. In some crocodiles, moderate temperatures produce males and both warm and cool temperatures produce females. In some species, sex is both genetic- and temperature-dependent.

Individuals of some species change their sex during their lives, alternating between male and female. If the individual is female first, it is termed protogyny or “first female,” if it is male first, its termed protandry or “first male.” Oysters, for example, are born male, grow, and become female and lay eggs; some oyster species change sex multiple times.

Summary

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 occurs through fission, budding, and fragmentation. Sexual reproduction may mean the joining of sperm and eggs within animals’ bodies or it may mean the release of sperm and eggs into the environment. An individual may be one sex, or both; it may start out as one sex and switch during its life, or it may stay male or female.

Which form of reproduction is thought to be best in a stable environment?
1. asexual
2. sexual
3. budding
4. parthenogenesis
Which form of reproduction can result from damage to the original animal?
1. asexual
2. fragmentation
3. budding
4. parthenogenesis
Which form of reproduction is useful to an animal with little mobility that reproduces sexually?
1. fission
2. budding
3. parthenogenesis
4. hermaphroditism
Genetically unique individuals are produced through ________.
1. sexual reproduction
2. parthenogenesis
3. budding
4. fragmentation
Why is sexual reproduction useful if only half the animals can produce offspring and two separate cells must be combined to form a third?
What determines which sex will result in offspring of birds and mammals?

Answers

A
B
D
A
Sexual reproduction produces a new combination of genes in the offspring that may better enable them to survive changes in the environment and assist in the survival of the species.
The presence of the W chromosome in birds determines femaleness and the presence of the Y chromosome in mammals determines maleness. The absence of those chromosomes and the homogeneity of the offspring (ZZ or XX) leads to the development of the other sex.

In asexual reproduction, one individual produces offspring that are genetically identical to itself. Reproduction is a marvelous culmination of individual transcendence in that organisms "transcend" time through the reproduction of offspring. In animal organisms, reproduction can occur by two primary processes: asexual reproduction and sexual reproduction.

Organisms produced by asexual reproduction are the product of mitosis. In this process, a single parent replicates body cells and divides into two individuals. Many invertebrates, including sea stars and sea anemones, reproduce in this manner. Common forms of asexual reproduction include: budding, gemmules, fragmentation, regeneration, binary fission, and parthenogenesis.

Many hydras reproduce asexually by producing buds in the body wall, which grow to be miniature adults and break away when they are mature. Ed Reschke/Photolibrary/Getty Images

Hydras exhibit a form of asexual reproduction called budding. In this form of asexual reproduction, an offspring grows out of the body of the parent, then breaks off into a new individual. In most instances, the budding is restricted to certain specialized areas. In some other limited cases, buds may come from any number of places on the body of the parent. The offspring typically remain attached to the parent until it is mature.

Progeny are budding on the body of a sponge in the Red Sea. Jeff Rotman Photography/Corbis Documentary/Getty Images

Sponges exhibit a form of asexual reproduction that relies on the production of gemmules or internal buds. In this form of asexual reproduction, a parent releases a specialized mass of cells that can develop into offspring. These gemmules are hardy and can be formed when the parent experiences harsh environmental conditions. The gemmules are less likely to become dehydrated and in some cases may be able to survive with a limited oxygen supply.

Planaria can reproduce asexually by fragmentation. They split into fragments, which develop into adult planaria. Ed Reschke/Photolibrary/Getty Images

Planarians exhibit a form of asexual reproduction known as fragmentation. In this type of reproduction, the body of the parent breaks into distinct pieces, each of which can produce an offspring. The detachment of the parts is intentional, and if thy are large enough, the detached parts will develop into new individuals.

Starfish are able to regrow missing limbs and produce new organisms through regeneration. Paul Kay/Oxford Scientific/Getty Images

Echinoderms exhibit a form of asexual reproduction known as regeneration. In this form of asexual reproduction, a new individual develops from a part of another. This typically happens when a part, like an arm, becomes detached from the parent's body. The separated piece can grow and develop into a completely new individual. Regeneration can be thought of as a modified form of fragmentation.

This paramecium is dividing by binary fission. Ed Reschke/Photolibrary/Getty Images

Paramecia and other protozoan protists, including amoebae and euglena, reproduce by binary fission. In this process, the parent cell duplicates its organelles and increases in size by mitosis. The cell then divides into two identical daughter cells. Binary fission is typically the most common form of reproduction in prokaryotic organisms such as bacteria and archaea.

This water flea (Daphnia longispina) can be seen with developing parthenogenetic or unfertilized eggs.

Roland Birke/Photolibrary/Getty Images

Parthenogenesis involves the development of an egg that has not been fertilized into an individual. Most organisms that reproduce through this method can also reproduce sexually. Animals like water fleas reproduce by parthenogenesis. Most kinds of wasps, bees, and ants (which have no sex chromosomes) also reproduce by parthenogenesis. Additionally, some reptiles and fish are capable of reproducing in this manner.

This sea star has lost an arm which can develop into a new sea star by the asexual process of fragmentation.

Karen Gowlett-Holmes/Oxford Scientific/Getty Images

Asexual reproduction can be very advantageous to certain higher animals and protists. Organisms that remain in one particular place and are unable to look for mates would need to reproduce asexually. Another advantage of asexual reproduction is that numerous offspring can be produced without "costing" the parent a great amount of energy or time. Environments that are stable and experience very little change are the best places for organisms that reproduce asexually.

One major disadvantage of this type of reproduction is the lack of genetic variation. All of the organisms are genetically identical and therefore share the same weaknesses. A gene mutation can persist in the population as it is continuously repeated in the identical offspring. Since organisms produced asexually grow best in a stable environment, negative changes in the environment can have deadly consequences for all individuals. Due to the high numbers of offspring that can be produced in a relatively short period of time, population explosions often occur in favorable environments. This extreme growth may lead to rapid depletion of resources and an exponential death rate in the population.

This is a colored scanning electron micrograph (SEM) of puffball fungus spores. These are the reproductive cells of the fungus. Credit: Steve Gschmeissner/Science Photo Library/Getty Images

Animals and protists are not the only organisms that reproduce asexually. Yeast, fungi, plants, and bacteria are capable of asexual reproduction as well. Yeast reproduce most commonly by budding. Fungi and plants reproduce asexually through spores. Plants can also reproduce by the asexual process of vegetative propagation. Bacterial asexual reproduction most commonly occurs by binary fission. Since the bacterial cells produced through this type of reproduction are identical, they are all susceptible to the same types of antibiotics.