CHAPTER SUMMARY
42.1 REPRODUCTION IS A FUNDAMENTAL FEATURE OF LIVING ORGANISMS AND OCCURS ASEXUALLY AND SEXUALLY.
- Asexual reproduction involves a single parent. Offspring are produced by binary fission or mitosis and are clones of the parent.
- Examples of asexual reproduction include budding, fragmentation, and parthenogenesis.
- Sexual reproduction involves the production of haploid gametes by meiosis and the fusion of gametes to make a diploid zygote.
- Many organisms reproduce both sexually and asexually. For some organisms, environmental conditions favor one mode or the other.
- Asexual reproduction allows for rapid population growth but produces clones and results in genetic uniformity.
- Sexual reproduction produces genetically unique offspring, a feature that is thought to explain why most eukaryotes reproduce sexually at least some of the time.
42.2 THE MOVEMENT OF VERTEBRATES FROM WATER TO LAND INVOLVED CHANGES IN REPRODUCTIVE STRATEGIES, INCLUDING INTERNAL FERTILIZATION AND THE AMNIOTIC EGG.
- Fertilization can occur outside or inside the body of the female. External fertilization can occur only in an aquatic environment. Internal fertilization is an adaptation to terrestrial living.
- External fertilization is usually associated with the production of many offspring, little or no parental care, and high mortality. These are characteristics of r-strategists.
- Internal fertilization is often associated with the production of fewer offspring, increased parental care, and low mortality. These are characteristics of K-strategists.
- A watertight sac, called the amnion, is an adaptation for reproduction on land.
- Oviparous animals lay eggs and development occurs outside the female; viviparous animals give birth to live young and development occurs inside the female.
42.3 THE MALE REPRODUCTIVE SYSTEM IS ADAPTED FOR THE PRODUCTION AND DELIVERY OF SPERM, AND THE FEMALE REPRODUCTIVE SYSTEM IS ADAPTED FOR THE PRODUCTION OF EGGS AND, IN MANY CASES, SUPPORT OF THE DEVELOPING FETUS.
- Male gametes, called sperm, develop in the seminiferous tubules of the testes.
- Sperm travel from the seminiferous tubules to the epididymis to the vas deferens to the ejaculatory duct and finally to the urethra before being released during ejaculation.
- Semen is a mixture of sperm and fluid from the prostate gland, seminal vesicles, and bulbourethral glands.
- Developing female gametes, called oocytes, are produced in the ovaries. Mature oocytes are called eggs, or ova.
- An oocyte is released from the ovary, travels through the fallopian tube, and implants in the uterus if it is fertilized.
- The reproductive system responds to and produces hormones.
- The hypothalamus releases GnRH, which stimulates the anterior pituitary gland to release FSH and LH, which in turn stimulate the ovaries to release estrogen and progesterone or the testes to secrete testosterone.
- In males, LH stimulates Leydig cells to secrete testosterone, and FSH and testosterone act on Sertoli cells to support sperm production.
- In females, monthly menstrual cycles are driven by the interplay of hypothalamic, anterior pituitary, and ovarian hormones, leading to ovulation, which is the maturation and release of an oocyte from the ovary, and to changes to the uterine lining.
42.4 HUMAN REPRODUCTION INVOLVES THE FORMATION OF GAMETES, FERTILIZATION, AND GROWTH AND DEVELOPMENT.
- Gametogenesis is called spermatogenesis in males and oogenesis in females.
- In males, spermatogonia differentiate into primary spermatocytes that undergo meiosis to make immature haploid spermatids and mature spermatozoa or sperm.
- Spermatogenesis begins at puberty and continues throughout life.
- In females, oogonia undergo differentiation into primary oocytes during fetal development, which begin meiosis but arrest in prophase I. During a menstrual cycle, primary oocytes are released from arrest, finish meiosis I, but then arrest in metaphase II. They remain arrested in metaphase II until they are fertilized.
- Fertilization is the fusion of the oocyte and sperm plasma membranes. It usually occurs in the fallopian tube and results in the formation of a diploid zygote.
- Cleavage is the division of the zygote by mitosis into smaller cells. At the blastocyst stage, it implants in the uterus.
- Gastrulation results in the formation of the three germ layers—ectoderm, mesoderm, and endoderm.
- Organogenesis is the formation of organs.
- Pregnancy lasts about 38 weeks from fertilization until birth and is divided into three trimesters.
- Childbirth, mediated by the posterior pituitary hormone oxytocin, involves changes in the cervix, delivery of the baby, and delivery of the placenta.
Self-Assessment Question 1
List similarities and differences between asexual and sexual reproduction.
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Model Answer:
Asexual and sexual reproduction are two ways that organisms can reproduce. They both provide a way for a parent to transmit genetic information into a new generation. In asexual reproduction, a single individual will divide in two, bud, or fragment, forming genetically identical clones. In contrast, in sexual reproduction, two genetically different organisms will come together to form a genetically unique progeny. The process of mitosis happens in both forms of reproduction, whereas meiosis only happens with sexual reproduction.
Self-Assessment Question 2
Explain the roles of meiosis and fertilization in sexual reproduction.
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Model Answer:
The role of meiosis in sexual reproduction is to create gametes or spores containing half the amount of chromosomes as the parent organism. Fertilization is the fusion of two gametes or spores, and restores the original chromosome content of the parent organisms. Both processes contribute to genetic variation in the offspring.
Self-Assessment Question 3
Describe the costs and benefits of asexual and sexual reproduction.
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Model Answer:
Asexual reproduction eliminates the need for the organism to find and attract a mate, which takes time and energy. It is also rapid, allowing organisms to increase their numbers quickly, often exponentially. However, asexual reproduction does not introduce genetic variability—the only source of genetic variation in asexual reproduction is chance mutations—and therefore may have dire evolutionary consequences. Sexual reproduction exacts costs in terms of energy expenditure and the rate of population growth, but the benefit is that it produces offspring that are genetically different from one another and from their parents.
Self-Assessment Question 4
Provide an explanation for the observation that there are very few, if any, ancient asexual organisms.
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Model Answer:
There are few, if any, ancient asexual organisms because a certain amount of genetic variability is favored. For a population to adapt, there must be a higher rate of genetic variability than just random mutation alone. Sexual reproduction gives these populations the level of genetic diversity they need in order to be evolutionarily fit.
Self-Assessment Question 5
Name three adaptations that allow reproduction to take place on land.
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Model Answer:
Internal fertilization, oviparity (in particular the development of the amnion), and viviparity are all adaptations that allow reproduction to take place on land.
Self-Assessment Question 6
Describe the pathway of sperm from its site of production to the urethra.
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Model Answer:
Sperm are produced within the testes in the seminiferous tubules, then travel through the epididymis, vas deferens, ejaculatory duct, and finally the urethra.
Self-Assessment Question 7
Describe the pathway of an oocyte (and the egg it develops into) from its site of production to the uterus.
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Model Answer:
Oocytes are produced in the ovaries and then travel through the fallopian tubes to the uterus.
Self-Assessment Question 8
Explain the relationships among changes in levels of anterior pituitary hormones, ovarian hormones, oocyte development, and the uterine lining during a menstrual cycle.
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Model Answer:
In females, the anterior pituitary gland secretes luteinizing hormone (LH) and follicle-stimulating hormone (FSH), in a cyclical pattern. FSH acts on follicle cells in the ovary, causing oocyte maturation. A surge of LH from the anterior pituitary triggers release of the mature oocyte from the ovary, or ovulation. Maturing follicle cells secrete estradiol (a form of estrogen), which causes the uterine lining to thicken, and the corpus luteum (a structure formed by the follicle cells after oocyte release) secretes progesterone, which helps maintain the thickened uterine lining. If the oocyte is fertilized, it implants into the uterine lining and secretes human chorionic gonadotropin (hCG), which maintains the corpus luteum and hence the thickened uterine lining. Eventually, the placenta takes over the estrogen and progesterone production to maintain the uterine lining and stimulate the growth of the uterus. If the oocyte is not fertilized, the corpus luteum degenerates, estrogen and progesterone levels drop, and the uterine lining is shed.
Self-Assessment Question 9
Describe similarities and differences between male and female gametogenesis.
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Model Answer:
Male and female gametogenesis are similar in that a primordial germ cell undergoes mitotic and meiotic cell divisions to form a primary spermatocyte or oocyte, but there are striking differences in timing, and the number of gametes produced from each parental diploid cell. In spermatogenesis, the primary spermatocyte undergoes meiosis I and II to form four haploid sperm cells. Spermatogenesis begins at puberty and continues throughout life, and the entire process takes 2-3 months. In contrast, oogenesis begins in the developing female embryo, with formation of primary oocytes. The primary oocytes enter the first meiotic division, but arrest immediately in prophase I. By the time the female is born, she has 1-2 million primary oocytes arrested in prophase I. These cells remain arrested for at least 12 years (until the age of menarche) and up to 50 years (until the age of menopause). A primary oocyte completes its first round of meiotic division during each menstrual cycle; the asymmetric cell division produces one secondary oocyte and one polar body. The secondary oocyte enters the second meiotic division, but arrests in metaphase II until fertilization takes place. Again, the division is asymmetric, producing one ovum and one polar body. Thus, in contrast to spermatogenesis, in oogenesis the primary oocyte undergoes two rounds of meiosis but only produces one viable ovum. The other three cells are polar bodies and remain arrested in the metaphase II phase. See Figure 42.19.
Self-Assessment Question 10
Name and describe three key developmental steps in the development of a single-celled zygote to a multicellular individual.
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Model Answer:
Three key developmental steps in the development of a single-celled zygote to a multicellular individual are cleavage, gastrulation, and organogenesis. Cleavage is where the single-celled zygote divides by mitosis. This divides the single large egg into many smaller cells. Gastrulation is a highly coordinated set of cell movements that leads to a fundamental reorganization of the embryo into three germ layers. Organogenesis is the transformation of the three germ layers into all the organ systems of the body.