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7.1: Sexual Reproduction - Biology

7.1: Sexual Reproduction - Biology


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Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. That energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations only have female individuals, so every individual is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Because of this, an asexual population can grow twice as fast as a sexual population in theory. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis. Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.

EVOLUTION IN ACTION: The Red Queen Hypothesis

There is no question that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. The problematic question is why, even in the face of fairly stable conditions, sexual reproduction persists when it is more difficult and produces fewer offspring for individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973.1 The concept was named in reference to the Red Queen's race in Lewis Carroll's book, Through the Looking-Glass, in which the Red Queen says one must run at full speed just to stay where one is.

All species coevolve with other organisms. For example, predators coevolve with their prey, and parasites coevolve with their hosts. A remarkable example of coevolution between predators and their prey is the unique coadaptation of night flying bats and their moth prey. Bats find their prey by emitting high-pitched clicks, but moths have evolved simple ears to hear these clicks so they can avoid the bats. The moths have also adapted behaviors, such as flying away from the bat when they first hear it, or dropping suddenly to the ground when the bat is upon them. Bats have evolved “quiet” clicks in an attempt to evade the moth’s hearing. Some moths have evolved the ability to respond to the bats’ clicks with their own clicks as a strategy to confuse the bats echolocation abilities.

Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a coevolving species to keep its own share of the resources is also to continually improve its ability to survive and produce offspring. As one species gains an advantage, other species must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among progeny of sexual reproduction provides all species with a mechanism to produce adapted individuals. Species whose individuals cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species.

Life Cycles of Sexually Reproducing Organisms

Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the resulting gamete’s chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid stage is the most obvious life stage (and there is no multicellular haploid stage), as with most animals including humans; haploid-dominant, in which the multicellular haploid stage is the most obvious life stage (and there is no multicellular diploid stage), as with all fungi and some algae; and alternation of generations, in which the two stages, haploid and diploid, are apparent to one degree or another depending on the group, as with plants and some algae.

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. The gametes are produced from diploid germ cells, a special cell line that only produces gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 7.1.1a).

ART CONNECTION

If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

Most fungi and algae employ a life-cycle strategy in which the multicellular “body” of the organism is haploid. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores (Figure 7.1.1b).

The third life-cycle type, employed by some algae and all plants, is called alternation of generations. These species have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes. Meiosis is not involved in the production of gametes in this case, as the organism that produces gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will develop into the gametophytes (Figure 7.1.1c).

Section Summary

Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces genetically unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and alternation of generations, demonstrated by plants and some algae.

Art Connections

Figure 7.1.1 If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

Figure 7.1.1 Yes, it will be able to reproduce asexually.

Multiple Choice

What is a likely evolutionary advantage of sexual reproduction over asexual reproduction?

A. sexual reproduction involves fewer steps
B. less chance of using up the resources in a given environment
C. sexual reproduction results in greater variation in the offspring
D. sexual reproduction is more cost-effective

C

Which type of life cycle has both a haploid and diploid multicellular stage?

A. an asexual life cycle
B. diploid-dominant
C. haploid-dominant
D. alternation of generations

D

Which event leads to a diploid cell in a life cycle?

A. meiosis
B. fertilization
C. alternation of generations
D. mutation

B

Free Response

Explain the advantage that populations of sexually reproducing organisms have over asexually reproducing organisms?

The offspring of sexually reproducing organisms are all genetically unique. Because of this, sexually reproducing organisms may have more successful survival of offspring in environments that change than asexually reproducing organisms, whose offspring are all genetically identical. In addition, the rate of adaptation of sexually reproducing organisms is higher, because of their increased variation. This may allow sexually reproducing organisms to adapt more quickly to competitors and parasites, who are evolving new ways to exploit or outcompete them.

Describe the two events that are common to all sexually reproducing organisms and how they fit into the different life cycles of those organisms.

The two events common to all sexually reproducing organisms are meiosis and fertilization. Meiosis reduces a diploid cell to a haploid state. The haploid cell may divide mitotically to produce an organism, some of whose cells will combine during fertilization, or the haploid cells produced by meiosis may immediately combine in fertilization to produce a diploid cell that divides to produce an organism.

Footnotes

  1. 1 Leigh Van Valen, “A new evolutionary law,” Evolutionary Theory 1 (1973): 1–30.

Glossary

alternation of generations
a life-cycle type in which the diploid and haploid stages alternate
diploid-dominant
a life-cycle type in which the multicellular diploid stage is prevalent
haploid-dominant
a life-cycle type in which the multicellular haploid stage is prevalent
gametophyte
a multicellular haploid life-cycle stage that produces gametes
germ cell
a specialized cell that produces gametes, such as eggs or sperm
life cycle
the sequence of events in the development of an organism and the production of cells that produce offspring
meiosis
a nuclear division process that results in four haploid cells
sporophyte
a multicellular diploid life-cycle stage that produces spores

37 7.1 Sexual Reproduction

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis. Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.

The Red Queen Hypothesis

There is no question that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. The problematic question is why, even in the face of fairly stable conditions, sexual reproduction persists when it is more difficult and produces fewer offspring for individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973. 1 The concept was named in reference to the Red Queen’s race in Lewis Carroll’s book, Through the Looking-Glass, in which the Red Queen says one must run at full speed just to stay where one is.

All species coevolve with other organisms. For example, predators coevolve with their prey, and parasites coevolve with their hosts. A remarkable example of coevolution between predators and their prey is the unique coadaptation of night flying bats and their moth prey. Bats find their prey by emitting high-pitched clicks, but moths have evolved simple ears to hear these clicks so they can avoid the bats. The moths have also adapted behaviors, such as flying away from the bat when they first hear it, or dropping suddenly to the ground when the bat is upon them. Bats have evolved “quiet” clicks in an attempt to evade the moth’s hearing. Some moths have evolved the ability to respond to the bats’ clicks with their own clicks as a strategy to confuse the bats echolocation abilities.

Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a coevolving species to keep its own share of the resources is also to continually improve its ability to survive and produce offspring. As one species gains an advantage, other species must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among progeny of sexual reproduction provides all species with a mechanism to produce adapted individuals. Species whose individuals cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species.



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Chapter 7: Introduction to the Cellular Basis of Inheritance

Figure 7.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.

Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.


Explain that variation among offspring is a potential evolutionary advantage resulting from sexual reproduction

Figure 7.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

Chapter Outline 7.1: Sexual Reproduction

Introduction The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.

Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.

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

• Explain that variation among offspring is a potential evolutionary advantage resulting from sexual reproduction

Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction.

Chapter 7 | The Cellular Basis of Inheritance 153

And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. That energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations only have female individuals, so every individual is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Because of this, an asexual population can grow twice as fast as a sexual population in theory. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis. Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.

The Red Queen Hypothesis There is no question that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. The problematic question is why, even in the face of fairly stable conditions, sexual reproduction persists when it is more difficult and produces fewer offspring for individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973.


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7.1 Sexual Reproduction

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Meiosis and Sexual Life Cycles

Vocational-technical education. Students who complete approved vocational-technical education programs shall have their occupational competency assessed by completion of the appropriate assessment under the Pennsylvania Skills Certificate Program or by completion of another occupational competency assessment approved by the Department. When appropriate, vocational-technical education programs must adopt, in program areas for which they are available, industry recognized skills standards and may also include cooperative vocational-technical education and participation in vocational student organizations to develop leadership skills. Course announcements, guidance materials and other communications must convey the philosophy of equal access to students considering enrolling in AVTSs and include a description of admissions policies.

Chapter 15. Meiosis & Sexual Reproduction

The policies must assure that when admissions to AVTSs must be limited, the admissions shall be on a nondiscriminatory basis. Standards and reports.

Meiosis Study Guide Answers

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Chapter Meiosis And Sexual Life Cycles - - medicoguia.com

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Meiosis Reading Guide Packet Answers

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Quiz 13: Meiosis and Sexual Life Cycles

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Answer Key To Meiosis

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Chapter 13: Meiosis And Sexual Life Cycles -

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Sexual reproduction and recombination in the aflatoxin-producing fungus Aspergillus parasiticus

The fungal phylum Ascomycota comprises a large proportion of species with no known sexual stage, despite high genetic variability in field populations. One such asexual species, Aspergillus parasiticus, is a potent producer of carcinogenic and hepatotoxic aflatoxins, polyketide-derived secondary metabolites that contaminate a wide variety of agricultural crops. In this study, individuals of A. parasiticus from a population showing an evolutionary history of recombination were examined for sexual reproduction. Crosses between strains with opposite mating-type genes MAT1-1 and MAT1-2 resulted in the development of ascospore-bearing ascocarps embedded within stromata. Sexually compatible strains belonged to different vegetative compatibility groups. Recombination through the independent assortment of chromosomes 3 and 6 was detected using loci for mating type, aflatoxin gene cluster, and a protein-encoding gene. Our discovery of the sexual stage in A. parasiticus has important implications for current biological control strategies using nontoxigenic strains to reduce aflatoxin contamination in crops.


Puberty in the Rat

The Male

Male sexual maturation initiates during embryonic development at the onset of testis determination and as gonadal androgens begin to be produced for reproductive tract development. Therefore, the testis becomes an endocrine organ earlier than the ovary, which initiates steroidogenesis postnatally. Although the testis has the capacity to respond to gonadotropins and steroids, testis formation and embryonic development are hormone independent. This has been shown using LHRH, FSH, FSH receptor, and LH receptor knockout models ( 66 , 67 , 249 ) that demonstrate normal testis development, cell growth, and cellular differentiation occurring in the absence of these endocrine agents.

Contrary to the female, the capacity for interrelations between the hypothalamic–pituitary unit and the male gonads are already functional before birth, but similar to the ovary a functional unit develops prepubertally to allow a gonadal axis to form. Also similar to the female, the primary events that set into motion the onset of male puberty originate within the CNS. Studies using male hamsters have shown that the major endocrine events associated with puberty are preceded by an increase in the size of LHRH neuronal perikarya ( 795 ). These changes may reflect changes in the synthesis and/or alteration in the secretory pattern of LHRH, leading to the initiation or enhancement of the pubertal rise of gonadotropin secretion. Such endocrine developments in turn influence the growth and maturation of the testes. One of the most significant testicular developments occurring at this time involves an increased steroidogenic capacity associated with an increased production of testosterone. In addition, as the animal matures, the testes become more sensitive to the stimulatory actions of the gonadotropins, primarily because of elevated FSH secretion, and so the level of T production becomes enhanced. Meanwhile, both T and the gonadotropins provide the basic stimulus for initiating spermatogenesis. Endocrine procedures and knockout mouse models have shown that T is essential to initiate and obtain adult testis function. FSH facilitates and allows a quantitative optimal testis function to be obtained. In the absence of FSH, the onset of puberty is delayed and progression is prolonged, and testis development can occur qualitatively but not quantitatively. Therefore, FSH actions on Sertoli cells help coordinate the onset and progression of puberty, but androgen is the primary driver of the process. The increased output of T also produces the physical alterations that are typically associated with puberty, namely the development and maintenance of the accessory sex organs. In addition, T and inhibin act as negative feedback agents to attenuate gonadotropin secretion. It has previously been hypothesized that the onset of male puberty results from a decrease in sensitivity of the hypothalamic–pituitary unit to negative feedback. This change in sensitivity would permit gonadotropin secretion to increase in spite of an enhanced production of androgens by the developing testes. The hypothesis, however, does not explain the divergent patterns of FSH and LH secretion observed in developing males.

In summary, the central neuroendocrine regulation of LHRH production and gonadotropin secretion initiates the onset of puberty through LH actions on the Leydig cells to increase androgen production and FSH actions on the Sertoli cell to enhance Leydig cell function and initiate Sertoli cell differentiation needed for the induction of spermatogenesis. The negative feedback of androgens and inhibin regulate gonadotropin production during the progression of puberty to the adult stage. The coordinated control of this hypothalamic–pituitary axis with the testis involves molecular events at the LHRH neuron and pituitary levels and at the testis somatic cell level ( 796 , 797 ). The resulting endocrine events control the onset and progression of puberty in the rat.


Watch the video: The Dividing Starfish (May 2022).