Unit Six. Animal Life
Few subjects pervade our everyday thinking more than sex; few urges are more insistent. They are no accident, these strong feelings. They are a natural part of being human. All animals share them. The cry of a cat in heat, insects chirping outside the windows, frogs croaking in swamps, wolves howling in a frozen northern scene—all these are the sounds of the living world’s essential act, an urgent desire to reproduce that has been patterned by a long history of evolution. It is a pattern that all of us share. The reproduction of our families spontaneously elicits in us a sense of rightness and fulfillment. It is difficult not to return the smile of a new infant, not to feel warmed by it and by the look of wonder and delight to be seen on the faces of parents like this nursing mother. This chapter deals with sex and reproduction among the vertebrates, of which we human beings are one kind. Few subjects are of more direct concern to students than sex. Because many students must make important decisions about sex, the subject is of far more than academic interest, and is one about which all students need to be well informed.
31.1. Asexual and Sexual Reproduction
Not all reproduction involves two parents. Asexual reproduction, in which the offspring are genetically identical to one parent, is the primary means of reproduction among protists, cnidari- ans, and tunicates, and also occurs in some more complex animals.
Through mitosis, genetically identical cells are produced from a single parent cell. This permits asexual reproduction to occur in the Euglena in figure 31.1 by division of the organism, or fission. The DNA replicates and cell structures, such as the flagellum, duplicate. The nucleus divides with identical nuclei going to each daughter cell. Cnidaria commonly reproduce by budding, where a part of the parent’s body becomes separated from the rest and differentiates into a new individual. The new individual may become an independent animal or may remain attached to the parent, forming a colony.
Unlike asexual reproduction, sexual reproduction occurs when a new individual is formed by the union of two cells. These cells are called gametes, and the two kinds that combine are generally called sperm and eggs (or ova). The union of a sperm and an egg produces a fertilized egg, or zygote, that develops by mitotic division into a new multicellular organism. The zygote and the cells it forms by mitosis are diploid; they contain both members of each homologous pair of chromosomes. The gametes, formed by meiosis in the sex organs, or gonads—the testes and ovaries—are haploid (see chapter 9). The processes of spermatogenesis (sperm formation) and oogenesis (egg formation) are described in later sections.
Figure 31.1. Asexual reproduction in protists.
The protist Euglena reproduces asexually: A mature individual divides by fission, and two complete individuals result.
Parthenogenesis, a type of reproduction in which offspring are produced from unfertilized eggs, is common in many species of arthropods. Some species are exclusively parthenogenic, whereas others switch between sexual reproduction and parthenogenesis in different generations. In honeybees, for example, a queen bee mates only once and stores the sperm. She then can control the release of sperm. If no sperm are released, the eggs develop parthenogenetically into drones, which are males; if sperm are allowed to fertilize the eggs, the fertilized eggs develop into other queens or worker bees, which are female.
The Russian biologist Ilya Darevsky reported in 1958 one of the first cases of unusual modes of reproduction among vertebrates. He observed that some populations of small lizards of the genus Lacerta were exclusively female, and he suggested that these lizards could lay eggs that were viable even if they were not fertilized. In other words, they were capable of asexual reproduction in the absence of sperm, a type of parthenogenesis. Further work has shown that parthenogenesis occurs among populations of other lizard genera.
Hermaphroditism, another variation in reproductive strategy, is when one individual has both testes and ovaries and so can produce both sperm and eggs. The hamlet bass in figure 31.2a are hermaphroditic, producing both eggs and sperm. During mating each fish switches from producing eggs that are fertilized by its partner, to producing sperm that fertilizes its partner’s eggs. A tapeworm is hermaphroditic and can fertilize itself as well as cross fertilize, a useful strategy because it is unlikely to encounter another tapeworm living inside its host. Most hermaphroditic animals, however, require another individual to reproduce. Two earthworms, for example, are required for reproduction—like the hamlet bass, each functions as both male and female. Each leaves the encounter with fertilized eggs.
Sequential hermaphroditism, in which individuals can change their sex, occurs in numerous fish genera. Among coral reef fish, for example, both protogyny (“first female,” a change from female to male) and protandry (“first male,” a change from male to female) occur. In the protogynous blue-head wrasse in figure 31.2b, the sex change appears to be under social control. These fish commonly live in large groups, or schools, where successful reproduction is typically limited to one or a few large, dominant males. If those males are removed, the largest female rapidly changes sex and becomes a dominant male (the blue-headed fish in the photo).
Figure 31.2. Hermaphroditism and protogyny.
(a) The hamlet bass (genus Hypoplectrus) is a deep-sea fish that is a hermaphrodite. In the course of a single pair-mating, one fish may switch sexual roles as many as four times. Here, the fish acting as a male curves around its motionless partner, fertilizing the upward- floating eggs. (b) The bluehead wrasse Thalassoma bifasciatium is protogynous. Here a large male, or sex-changed female, is seen among females, which are typically much smaller.
Among the fish just described, and in some species of reptiles, environmental changes can cause changes in the sex of the animal. In mammals, the sex is determined early in embryonic development. The reproductive systems of human males and females appear similar for the first 40 days after conception. During this time, the cells that will give rise to ova or sperm migrate to the embryonic gonads, which have the potential to become either ovaries in females or testes in males. If the embryo is XY, it is a male and will carry a gene on the Y chromosome whose product converts the gonads into testes (as on the left in figure 31.3). In females, who are XX, this Y chromosome gene and the protein it encodes are absent, and the gonads become ovaries (as on the right). Recent evidence suggests that the sex-determining gene may be one known as SRY (for “sex-determining region of the Y chromosome”). The SRY gene appears to have been highly conserved during the evolution of different vertebrate groups.
Figure 31.3. Sex determination.
Sex determination in mammals is made by a gene on the Y chromosome designated SRY. Testes are formed when the Y chromosome and SRY are present; ovaries are formed when they are absent.
Once testes form in the embryo, they secrete testosterone and other hormones that promote the development of the male external genitalia and accessory reproductive organs (indicated in the blue box). If testes do not form, the embryo develops female external genitalia and accessory reproductive organs. The ovaries do not promote this development of female organs because the ovaries are nonfunctional at this stage. In other words, all mammalian embryos will develop female sex accessory organs and external genitalia by default unless they are masculinized by the secretions of the testes.
Key Learning Outcome 31.1. Sexual reproduction is most common among animals, but many reproduce asexually by fission, budding, or parthenogenesis. Sexual reproduction generally involves the fusion of gametes derived from different individuals of a species, but some species are hermaphroditic.