Chapter 48 - Chapter 48: Animal Reproduction Key Concepts...

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Chapter 48: Animal Reproduction Key Concepts The reproductive systems of animals are highly variable. Many animals can switch between asexual and sexual reproduction. When sexual reproduction occurs, fertilization may be external or internal and egg development may take place inside or outside the mother's body, depending on the species. In humans, the male reproductive system is composed of structures specialized for the production and storage of sperm, the synthesis of important molecules found in semen, or the transport and delivery of semen. The female reproductive system is composed of structures specialized for producing eggs, receiving sperm, and nourishing offspring during early development. In humans, the menstrual cycle is regulated by hormones from the pituitary gland and female reproductive organs. These hormones may interact with each other via positive or negative feedback. Pregnancy is maintained by hormonal signals from the embryo and from the mother's reproductive organs. Section 48.1 Outline: Asexual and Sexual Reproduction How Does Asexual Reproduction Occur? Switching Reproductive Modes: A Case History Mechanisms of Sexual Reproduction: Gametogenesis Asexual reproduction is based on mitosis and results in offspring that are genetically identical to one another and to their parent. Sexual reproduction is based on meiosis and fusion of gametes. Genetic recombination during meiosis and the fusion of haploid gametes from different parents during fertilization result in offspring that are genetically different from one another and from their parents. How Does Asexual Reproduction Occur? o Asexual reproduction occurs in a variety of ways and in a wide variety of animal groups. o In budding , an offspring begins to form within or on a parent and is completed when the offspring breaks free and begins to grow on its own. The offspring is a miniature version of the parent o In fission , an individual simply splits into two or more descendants. o In parthenogenesis , female offspring develop from unfertilized eggs. o These offspring are genetically identical to the mother. o If a species reproduces exclusively by parthogenesis, no males exist. Switching Reproductive Modes: A Case History o Daphnia reproduce asexually by parthenogenesis.
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o Only diploid female offspring are produced throughout the spring and summer by parthogenesis. o In late summer/early fall, many females begin producing male offspring. o Haploid sperm from these males fertilize haploid eggs that females produce by meiosis. o In spring, the sexually produced offspring hatch and begin reproducing asexually. o Daphnia were shown to require three different cues from the environment to switch to sexual reproduction (Figure 48.2c): crowding, low food availability, and short day lengths.
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Chapter 48 - Chapter 48: Animal Reproduction Key Concepts...

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