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43.1: Reproduction Methods - Biology

43.1: Reproduction Methods - Biology


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Skills to Develop

  • Describe advantages and disadvantages of asexual and sexual reproduction
  • Discuss asexual reproduction methods
  • Discuss sexual reproduction methods

Animals produce offspring through asexual and/or sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions. On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce.

During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give species a better chance of surviving in an unpredictable or changing environment. Species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present.

Asexual Reproduction

Asexual reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms. There are a number of ways that animals reproduce asexually.

Fission

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 (PageIndex{1})) also reproduce through fission.

Budding

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 (PageIndex{2}), whereas in coral budding, the bud does not detach and multiplies as part of a new colony.

Link to Learning

Watch a video of a hydra budding.

Fragmentation

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 (PageIndex{3}) illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. 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

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

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

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 (PageIndex{4}), 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.

Sex Determination

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.

Review Questions

Which form of reproduction is thought to be best in a stable environment?

  1. asexual
  2. sexual
  3. budding
  4. parthenogenesis

A

Which form of reproduction can result from damage to the original animal?

  1. asexual
  2. fragmentation
  3. budding
  4. parthenogenesis

B

Which form of reproduction is useful to an animal with little mobility that reproduces sexually?

  1. fission
  2. budding
  3. parthenogenesis
  4. hermaphroditism

D

Genetically unique individuals are produced through ________.

  1. sexual reproduction
  2. parthenogenesis
  3. budding
  4. fragmentation

A

Free Response

Why is sexual reproduction useful if only half the animals can produce offspring and two separate cells must be combined to form a third?

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.

What determines which sex will result in offspring of birds and mammals?

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.

Glossary

asexual reproduction
form of reproduction that produces offspring that are genetically identical to the parent
budding
form of asexual reproduction that results from the outgrowth of a part of a cell leading to a separation from the original animal into two individuals
fission
(also, binary fission) method by which multicellular organisms increase in size or asexual reproduction in which a unicellular organism splits into two separate organisms by mitosis
fragmentation
cutting or fragmenting of the original animal into parts and the growth of a separate animal from each part
hermaphroditism
state of having both male and female reproductive parts within the same individual
parthenogenesis
form of asexual reproduction where an egg develops into a complete individual without being fertilized
sexual reproduction
mixing of genetic material from two individuals to produce genetically unique offspring

43.1: Reproduction Methods - Biology

Animals produce offspring through asexual and/or sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions. On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce.

During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give species a better chance of surviving in an unpredictable or changing environment. Species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present.


Concept in Action

Watch a video of a hydra budding.

Fragmentation

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 24.4 illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. 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.

Figure 24.4. Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is developing into a new individual.

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

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

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

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)

Sex Determination

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.


4 Methods of Plant Reproduction (With Experiment)

Some yeast are grown in sugar solution and observed under the microscope from time to time.

It is observed that one or more tiny outgrowths appear on one or more sides of the vegetative cells immersed in sugar solution. In some cases the outgrowths may be detached from the mother cell (which will grow into new individuals).

This method of outgrowth formation is known as budding. Often budding continues one after the other so that finally a chain of cells is formed. All the individual cells of the chain separate from one another and form new yeast plants.

Some gemmae cups are collected from Marchantia and placed in its natural habitat.

A new thallus of Marchantia is grown from a gemma cup.

This is a special method of vegetative reproduction. The gemmae develop in the gemmae cup and each gemma is a small, more or less circular flattened structure with a conspicuous depression on each side. The growing point lies in the depression.

The leaf tip of adiantum is made to touch the soil in which it grows and after a few days observation is made.

Observation and inference:

When the leaf bends down and touches the ground the tip strikes root and forms a bud. The bud grows into a new independent fern plant.

(i) By underground stems:

Some tubers of potato or bulbs of onion or rhizomes of ginger are planted in pots. Observation is made after 10 to 15 days.

Many flowering plants reproduce themselves by means of rhizome, tuber, bulb or the corm. New buds are produced on these modified stems which gradually grow up into new plants.

(ii) By sub-aerial stems:

Some Pistia, Hydrocotyle, Colocasia or Chrysanthemum plants are observed.

Observation and inference:

Here vegetative propagation takes place by means of sub-aerial stems, e.g., by runner in Hydrocotyle, by offset in Pistia, by stolon in Colocasia and by sucker in Chrysanthemum.

(iii) By Adventitous buds:

Some Bryophyllutn leaves are kept in moist soil.

Observation and inference:

A series of adventitious foliar buds arc produced on the leaf margin (notch), each at the end of vein. These buds grow up into new plants.

In Globba bulbifera or Allium sativum some of the lower flowers of the inflorescence become modified into small multicellular bodies, known as bulbils (Figure 44). Some bulbils are kept on moist ground and observed after a few days.

Observation and inference:

Bulbils grow up into new plants (sometimes they are seen to grow to some extent Fig. 44 on the plant itself).

B. Artificial Methods of Propagation:

When some stem cuttings of rose, china-rose, Moringa, Coleus, etc., are put (the physiological polarity should be maintained) into moist soil they strike roots at the base and develop adventitious buds which grow up.

N.B. The rooting of the stems may be hastened by treatment of the cuttings with growth hormones like IBA, NAA, NOA, etc. in lanolin paste or in solution.

When some root cuttings of lemon. Citrus, tamarind, etc. are put into moist soil they sprout forming roots and shoots.

In this case a lower branch of lemon, Ixora, rose, jasmine, etc., is bent down, a ring of bark to the length of 1 to 2 inches are removed and this portion is pushed into the soft ground keeping the upper part free. The bent portion is covered with soil and Removed bark a stone or a brick is placed on it (Figure 45).

When roots have developed (usually within two to four months), the branch is cut out from the mother plant and grown separately.

This method is usually employed for propagating lemon, orange, guava, and litchi. Magnolia, etc. During early rains a healthy, somewhat woody branch is selected and a ring of bark (1 to 2 in­ches in length) is sliced off from it.

A sufficiently thick plaster of grafting clay (clay two parts, cow dung one part and some finely-cut hay mixed with water) is applied all-round the ringed portion which is then wrapped up with straw (or polythene bag) and tied in a secured manner (Figure 46).

It should be wetted with water every morning and afternoon or may be kept moistened with the help of a wick from a water reservoir. Usually within one to three months the gootee is ready as is indicated by its striking roots. It is then cut out below the bandage and grown separately.

(а) Inarching or approach grafting:

By this method a branch (scion) of a plant is made to unite with a seedling (stock) by firmly tieing them together by means of a chord (Figure 47).

Before doing this, a small portion of the bark is sliced off from each to ensure a closed contact and quicker union between the two. When proper fusion has taken place (usually within two to three months), the stock is cut-out above the joining and the scion below, thus leaving the scion standing on the stock. Some of the fruit trees like mango, litchi, guava, plum, etc., readily res­pond to this method.

For this method a T-shaped (Figure 48) incision is made in the bark of the stock, and a bud, cut out clean from a selected plant, is inserted into the T-shaped slit and properly bandaged. By this method it has been found possible to grow several varieties of roses on one rose stock good varieties of orange, lemon, etc., on inferior stocks, several varieties of China rose on one, cacti on one and so on.

(c) Whip or tongue grafting:

The stock, usually half to three-fourth inch thick, is cut down a few inches above the ground, sloping cuts are then made in it 2 or 3 inches long, as shown in Figure 49.

Scion of the same thickness is also cut in such a ways to fit exactly into the stock. It is then inserted into the stock and tied firmly. The wound is of course covered with grafting wax (a mixture of tallow (animal fat) one part plus bees wax one part plus resin four parts, melted together and made into a soft dough under water). All buds are removed from the stock but not from the scion.

The stock is cut 8 to 10 inches above the ground and the wood of the Stem incised with clean cut in the form of a ‘V’. The scion, cut obliquely downward so as to closely fit into the stock, is inserted into the stock and tied firmly (Figure 50). Grafting wax is used for covering the wound. All buds are removed from the stock but not from the scion.

An old tree may be rejuvenated by this method. The stem is cut across 8 to 10 inches above the ground. The bark of the stock is cut through from the surface downward to a length of 5 to 6 inches.

The bark is partially opened on either side. Prior to this a small branch cut out from a tree is incised at the base with a sloping cut and this is now inserted into the slit in the bark and tied firmly. The wound is covered with grafting wax (Figure 51).

Method # 2. Asexual Reproduction:

A. By Fission:

In many unicellular algae, fungi and bacteria, the mother cell splits into two new cells. The new cells, thus formed, contain all materials of the mother cell and soon they grow to the size of the latter, becoming a new independent plant. This process of fission may be observed in case of yeast cells under the microscope.

B. By Spore Formation:

Zoospores of algae like Ulothrix, Vaucheria, etc., may be observed under the microscope. The zoospores after escape from the mother cell germinate giving rise to new filaments.

Non-motile spores of some algae, e.g., Nostoc, Penicillium, Phytophthora,

etc., are well adapted for dispersal by wind and at the same time to meet the ever-changing conditions of the environment. In favourable conditions they germinate and give rise to new plants.

True spores are always borne by sporophytes of bryophyta and pteridophyta. The germination of these spores may be observed in the labor­atory under suitable culture conditions.

Method # 3. Sexual Reproduction:

A. By Conjugation:

In lower algae and fungi the pairing gametes are not differentiated into male and female (isogamous). The union of such similar gametes (conjugation) forms the zygote called zygospore. Conjugation of Spirogyra or Mucor may be observed under the microscope. Both scalariform and lateral conjugation may be observed in Spirogyra and only scalar form in case of Mucor.

B. By Fertilisation:

In higher plants the uniting gametes are differentiated into male and female and the union of the two (fertilization) forms the zygote (oospore). When some pollen grains are dusted on the stigma of the flower, the pollen grains germinate forming a tube like outgrowth called pollen tube- The pollen tube contains sperms which unite with the egg of the nucleus.

The pollen tube grows through the style and ultimately reaches the ovule. The growth of the pollen tube may be studied by cutting and staining the serial longitudinal sections of the style at different time intervals.

Method # 4. Induced Sex Modification:

A. By Growth Hormones:

IAA and other synthetic growth substances like 2, 4-P or NAA seem to be effective the induction and modification of sex organs.

(i) Conversion of male flowers into female flowers by NAA:

In dioeciously species of Cannabis saliva genetically male plant could be induced to produce female flowers if during the period of differentiation of flower buds the third and fourth leaves of male plant are treated with 50 ppm NAA and also by 900 ppm ethrel.

This indicates that the sexua­lity in this plant is determined by the concentration of native auxin and ethylene during the period of primordium differentiation and that femaleness is associated with a relatively high auxin level.

(ii) Increased production of female flowers by NAA:

In some monoecious cucumber plant painting of the lower surface of’ the leaf with 50 ppm NAA causes an increase in the proportion of female flowers, sometimes altogether suppressing the production of male flowers. This also indicates that the female flowers tend to differentiate under higher concentration of auxins than do male flowers.

(iii) Promotion of maleness in sex expression by GA:

Gibberellin increases the staminate flowers in some cases, e.g., cucumber in contrast to promotion of femaleness by auxin.

B. By Growth Retardants:

Growth retardants also modify sex expression. The growth retardants are not the causative agents of sex expression in flowers. But the influence they exert may be due to differently regulated vegetative growth, and antagonistic reactions with growth promoters.

Inhibition of Staminate flowers:

The synthetic growth retardants such as CCC (2-Chlorocthyl trim ethyl ammonium chloride) and its derivatives MAB (Methyl ammonium bromide) and BCB (2-Biomoethyl trim ethyl ammonium bromide) and Ethrel (2-Chloroethyl phosphoric acid) all these inhibit staminate flower production and greatly increase pistillate flower formation in cucumber and spinach.

The greatest increase in pistillate flower formation is caused by MAB under high light intensities.

C. By Other Methods:

(i) By carbon monoxide (CO) gas:

In some Euphorbiaceae CO gas .in low concentration sometimes reduces the number of male flowers in genetically monoecious types. The effect is presumably due to the effect of CO on auxin.

Increase in salinity of the medium, particularly NaCL, may change the expression of sex in plants. In dioeciously species, salinity may sometimes significantly cause an increase in the production of staminate flowers.

(iii) By Photoperiod treatment:

The length of photoperiod also affects the differentiation of sexes. If seeds of some Euphorbiaceae are grown in short days instead of long days (in long days the plants are predominantly staminate) the percentage of pistillate plants significantly increases.

Similar results are also obtained with unisexual plants of Ambrosia of composite. Cannabis sativa growing under 16 hours photoperiod produces about half staminate and half pistillate plants. But if the day length is shortened to 8 hours the plants are about half bisexual, half females and none males. Long days seem to favour production of male sex organs.


Fragmentation

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. [link] illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. 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.


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 ( [link] ) also reproduce through fission.

Coral polyps reproduce asexually by fission. (credit: G. P. Schmahl, NOAA FGBNMS Manager)


226 Reproduction Methods

By the end of this section, you will be able to do the following:

  • Describe advantages and disadvantages of asexual and sexual reproduction
  • Discuss asexual reproduction methods
  • Discuss sexual reproduction methods

Animals produce offspring through asexual and/or sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions. On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce.

During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give species a better chance of surviving in an unpredictable or changing environment. Species that reproduce sexually must maintain two different types of individuals, males and females, which can limit the ability to colonize new habitats as both sexes must be present.

Asexual Reproduction

Asexual reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms. There are a number of ways that animals reproduce asexually.

Fission

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)) also reproduce through fission.


Budding

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), whereas in coral budding, the bud does not detach and multiplies as part of a new colony.


Watch a video of a hydra budding.

Fragmentation

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) illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. 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

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 fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones). If eggs are fertilized, diploid females develop, and if the fertilized eggs are fed a special diet (so called royal jelly), a queen is 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

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

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), 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.


Sex Determination

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.

Section 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.

Review Questions

Which form of reproduction is thought to be best in a stable environment?


Watch the video: What Is Asexual Reproduction. Genetics. Biology. FuseSchool (May 2022).