recap

901

42.2 recap

Sexual reproduction involves gametogenesis, mating, and fertilization. Fertilization can be external or internal and involves mechanisms for ensuring that only one sperm from the right species enters the egg. Oviparous female animals lay eggs in the environment, and their embryos develop outside the mother’s body. Viviparous female animals retain the developing embryo in the uterus until it is born.

learning outcomes

You should be able to:

  • Explain the connection between sex chromosome asymmetry and spermatocytes remaining in cytoplasmic contact.

  • Compare oogenesis and spermatogenesis.

  • Summarize the interactions between sperm and eggs in sea urchins.

  • Summarize ways aquatic animals enhance the likelihood that sperm and eggs of the same species will meet.

  • Identify and describe conditions that favor the evolution of hermaphroditism.

Question 1

Crossing over is the reciprocal exchange of segments in homologous chromatids. Why does it occur more frequently in human oocytes than in human spermatocytes?

Human oocytes remain in prophase of the first meiotic division much longer than spermatocytes, and therefore the opportunity for crossing over is much greater.

Question 2

If a mutation in a mammal eliminated the maintenance of cytoplasmic bridges between spermatids, would you expect all offspring sired by that mammal to be male or female? Explain your answer.

Without cytoplasmic bridges between the spermatids of mammals, all of the spermatids would not receive gene products of the X chromosome that seem to be necessary for sperm survival. Thus all surviving sperm would lack a Y chromosome, and the offspring of that individual would all be female.

Question 3

In animals that spawn, what mechanisms ensure that an egg is fertilized by sperm of the same species, and by only one sperm?

The egg is covered by protective layers that prevent sperm from contacting the egg cell membrane. The egg emits species-specific chemical attractants that stimulate sperm of its species to swim toward the egg. The protective coat of the egg also contains species-specific signals that stimulate the acrosomal reaction of the sperm, releasing enzymes that digest a path through the protective coat. The sperm head is coated with molecules (bindin) that have receptors on the inner protective coat of the egg, and when those molecules bind their receptors, the sperm contacts and fuses with the egg cell membrane. That event stimulates the blocks to polyspermy (fast and slow) that prevent additional sperm from fusing with the egg cell membrane.

Question 4

What are three situations in which hermaphroditism would be advantageous?

  1. The possibility of finding a mate is low.
  2. Sequential hermaphroditism would be advantageous to avoid inbreeding in a species that produces large numbers of offspring that remain in the same area.
  3. Sequential hermaphroditism would also be advantageous in a population where single dominant females control essential resources such as territory. Male offspring in that population would have the chance to reproduce, but females would not until the dominant female was removed. At that time a male could transition to female and take the place of the single dominant female in the group.

Now that we have covered some of the general aspects of gametogenesis and fertilization and have briefly discussed the great diversity of mating systems, we will next consider the human reproductive systems in detail.