Answers to review questions – chapter 19
1.List the advantages and disadvantages of asexual reproduction and sexual reproduction. (pp. 423–425)
An advantage of asexual reproduction is that it allows offspring to form without the need to get two individuals of different sex into reproductive condition at the same place at the same time. A disadvantage is that, because the offspring are genetically identical to the parent and to each other, genetic diversity is limited, which limits the ability of the organism to adapt to new or changing conditions.
In contrast, while sexual reproduction imposes the necessity of getting individuals of different sex together at the same time and place in reproductive condition, by allowing DNA from one organism to be combined with that from another, sexual reproduction results in unique individuals and generates genetic variation within populations.
2.Why can parthenogenesis in the whiptail skink be considered a degenerate form of reproduction? (p. 424)
While only females are present in the population and eggs develop by parthenogenesis, ovulation can take place only after another female has mimicked the male role in courtship behaviour. This suggests that males were originally present in the population and were involved in reproduction, the retention of the need for courtship behaviour being a remnant of this stage in the species’ evolution.
3.Explain why mutations that take place in muscle cells during the life of an organism are not passed down to the next generation. (pp. 435–440)
For a mutation to be inherited it must be in a gamete or in a cell that gives rise to gametes. This way it is transmitted to a new individual at fertilisation. Mutations in muscle cells have no way of influencing the genetic material of gametes.
4.What are the roles of follicle cells and nurse cells in oogenesis? (pp. 439–440)
In many invertebrates, including some insects, annelids and molluscs, the oocyte is connected to a number of nurse cells; these cells are in fact sister cells. RNA produced by the nurse cells is transported to the oocyte via cytoplasmic bridges. In mammals, the oocyte is surrounded by layers of cells, called follicle cells, one of whose functions is to regulate the movement of substances such as yolk from the blood stream to the oocyte. As the egg cell matures, it becomes surrounded by extracellular membranes (envelopes), which are secreted by the oocyte and/or the follicle cells, and which serve as protection after the egg is released.
5.The mechanisms involved in bringing gametes together for fertilisation vary between species. Use examples to explain how these mechanisms are related to an organism’s lifestyle. (pp. 441–442)
Species that live in water can use the external environment as the medium for fertilisation. Thus sedentary species in water usually release enormous numbers of gametes. Ensuring that gamete release is synchronised through environmental cues, such as water temperature, tide and day length, increases the chances of fertilisation in any one area. In addition, when one animal starts to spawn, pheromones released along with the gametes stimulate nearby individuals to spawn, which in turn releases more pheromones, resulting in coordinated spawning over a wide area.
Terrestrial animals usually use internal fertilisation, since unprotected eggs and sperm are prone to desiccation and consequent death. Many variations are seen, but usually sperm are placed directly in the female’s reproductive tract, usually using a special organ, the penis. A number of animals use indirect sperm transfers via packets of sperm called spermatophores. These are deposited by the male and subsequently picked up by the female.
6.Most hermaphrodite species prevent self-fertilisation. Describe the various ways by which they do this. (pp. 425–426)
Hermaphrodite species that allow self-fertilisation lose the advantage of genetic variation in their offspring, and thus hermaphrodite species usually have several mechanisms in place to prevent this. These can be anatomical, such as in the earthworm where there is physical separation of the gametes and a complex mating procedure. Alternatively, in many animals that release eggs and sperm to the environment simultaneously, the eggs become fertile only some time after release, by which time the sperm are no longer capable of fertilisation.
7.Explain why changing sex is an advantage for sexual reproduction in both the salmon and the blue wrasse. (p. 426)
In the salmon, the small fish as males can produce enough sperm to fertilise a large number of eggs, as the cost of sperm production is low compared with that of producing eggs. In contrast, the females have to be substantial to sustain the cost of producing a large number of large yolky eggs.
The wrasse runs a harem system for reproduction and thus it is an advantage to be large, because large males can control a bigger harem. Again, sperm production is not a significant cost and a single male can fertilise the eggs of many females.
8.How is mass spawning of corals triggered? What advantages and disadvantages does it have for the organisms? (Box 19.4)
The production of many marine organisms of coral reefs is triggered by environmental cues. One of the biggest and most spectacular of these events occurs on the Great Barrier Reef. Spawning occurs during spring and early summer one night or so after a full moon. When one animal starts to spawn, pheromones released along with the gametes stimulate nearby individuals to spawn, which in turn releases more pheromones, resulting in coordinated spawning over a wide area. Spawning reaches its peak on the fourth, fifth and sixth nights during a period of neap tides and rising sea temperatures. The advantages of this timing are that the sea is usually calm with little tidal movement to remove the gametes from the vicinity of the reef and away from suitable habitats, and that the gametes from many individuals are released simultaneously so that the chances of fertilisation are greatly increased. One major disadvantage is that the mass spawning often attracts a mass of predators, though the sudden glut of planktonic food is often so great that predators rapidly become sated and a high proportion of gametes and larvae escape.
9.Explain why animals that produce large numbers of eggs often undergo indirect development, while those that produce fewer eggs often undergo direct development. (pp. 426–428)
When vast numbers of gametes are released the females can invest only a relatively small amount of energy in each individual egg. As a consequence the egg can sustain only a very brief period of embryonic development and thus hatches into a larval form that feeds and grows before embarking on the final metamorphosis that will transform it into its final adult form. In contrast, animals showing direct development typically are those where the females produce large yolky eggs with sufficient food reserves for the offspring to reach their final adult form.
10. Outline some of the potential costs of sexual reproduction for animals. (pp. 432–433)
The basic costs of developing the gametes within the body can be very high and often place the animal, particularly the females, at a competitive disadvantage. This disadvantage is increased if a large amount of effort is invested in developing or nurturing the young in utero or via sucking. The cost of finding a mate, especially in normally solitary species, can be considerable. For example, many species change their plumage to make themselves more attractive, but at the same time this increases the risk of becoming the prey of their predators. Equally, other species advertise their presence by calling, again risking predation. Aggressive species need to sublimate their aggressive instincts when mating, though in many species that compete for the attention of the opposite sex, the cost of such competition can be very high.