Chapter 42

1. Budding and regeneration begin with somatic (body) cells of the parent, whereas parthenogenesis begins with a germ (sex) cell, an egg that is not fertilized.

2. Global climate change is more likely to have a negative impact on asexually reproducing species, because they have less genetic variability to facilitate the evolution of adaptive changes that meet the new challenges imposed by climate change.

3. Flora must have reproduced through parthenogenesis, an ability that would be adaptive for an island-dwelling species should a weather event eliminate all potential mates or cause a female to be relocated to another island with no potential mate. If the offspring of a parthenogenic reproductive event are male, they would be able to mate with their mother to initiate a new population of sexually reproducing individuals.

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

2. 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.

3. 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.

4. 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.

1. Semen contains fibrinogen secreted by the seminal vesicles. The prostate secretes a clotting enzyme that causes the fibrinogen to polymerize and form a clot. The prostate also secretes the enzyme fibrinolysin, which eventually breaks down the clot and liberates the sperm.

2. Sertoli cells nurture and promote the maturation of sperm, beginning with the primary spermatocyte stage. They provide a suitable environment and essential nutrients. Tight junctions between Sertoli cells protect the developing sperm from water-soluble toxic substances that might be circulating in the blood.

3. Sexual stimulation of the male causes nerve endings in the penis to release a neurotransmitter that stimulates endothelial cells in the blood vessels to release NO. NO diffuses into the muscle cells controlling the diameter of the penile arteries, causing them to relax through increased production of cGMP. The resulting increase in blood flow in those arteries compresses the veins carrying blood out of the penis, and it becomes engorged with blood (an erection). A phosphodiesterase breaks down cGMP, so a phosphodiesterase inhibitor prolongs the effects of cGMP production.

4. Since prepubertal testes are responsive to LH and FSH, the cause of their inactivity must either be low production and release of LH and FSH by pituitary gonadotropes or low production of GnRH by the hypothalamus.

1. The events in the ovary that are responsible for the onset of menses are the degradation of the corpus luteum and the consequent decrease in production and release of estrogen and progesterone. However, if a blastocyst implants in the uterus, it produces human chorionic gonadotropin (hCG), which maintains the corpus luteum until the extraembryonic membranes take over the production of estrogen and progesterone.

2. The granulosa cells nurture the developing egg, providing it with nutrients, growth factors, and mRNAs. The granulosa cells also produce estrogen and progesterone in response to FSH. Estrogen is produced by the granulosa cells from testosterone that is produced in the outer cells of the follicle, the thecal cells. Estrogen stimulates the granulosa cells to produce FSH receptors, but at the same time estrogen exerts negative feedback on the production and release of FSH from the anterior pituitary. Since multiple follicles begin to mature in each menstrual cycle, only the one with the most FSH receptors fully matures as the estrogen levels rise and FSH levels fall.

3. Prior to the onset of labor, the contractility of the uterine muscle increases, and at the same time the growing fetus is causing more stretch of the uterine muscle. Stretch of the uterine muscle stimulates contractions, pressing the head of the fetus onto the cervix. That stimulates neural signals to the hypothalamus, causing it to release oxytocin from the posterior pituitary. Oxytocin increases the uterine contractions, which in turn puts more pressure on the cervix, resulting in even more oxytocin being released from the posterior pituitary—a positive feedback loop.

1. The condom is the only contraceptive technology that can protect against STDs because if used properly it protects each partner from contact with the body fluids of the other.

2. The pill delivers a high dose of progesterone that exerts negative feedback on the hypothalamic–pituitary axis to inhibit the production and release of LH and FSH, which are essential for the maturation and release of an egg from the ovary. RU-486 is an antiprogestive drug. During the luteal stage of the menstrual cycle, high levels of progesterone from the corpus luteum maintain the endometrium of the uterus in a receptive condition to receive a blastocyst. Blocking that action of progesterone can prevent implantation of the blastocyst or promote sloughing of the endometrium.

3. IVF can be used to prevent transmission of a genetic disease because cells can be taken from the early embryo in culture and analyzed for the suspected defective gene. Only embryos that do not carry the defective gene can then be selected for implantation into the uterus of the mother.

1. 

   The data for this group were mixed, with minor peaks in mean PRL for the group occurring on days 7 and 10, indicating that some of the wallabies had broken diapause over that period of time. These rises occurred later than measured in the saline-treated (control) wallabies, but sooner than in the daily-treated wallabies. Thus the 48-hr PRL treatments were marginally effective in preventing the breaking of diapause.

1. Following the mating behavior between the male and group A female koalas, the female koala’s pituitary gland released a surge of LH into the bloodstream. This surge in LH stimulated ovulation. An egg was fertilized by the male’s sperm, beginning a period of pregnancy that lasted about 35 days. No LH increases or pregnancies occurred in the Group B females. This experiment eliminates the possibility that the signal for ovulation could be a stimulus, such as a pheromone, from close proximity to a male koala.

2. The results from groups A and C show that neither artificial insemination procedures without semen nor tactile stimulation of the urogenital sinus stimulate LH surges or ovulation. However, AI with semen or artificial stimulation followed by semen injection both result in LH surges and ovulation. These results lead to the hypothesis that some component of the male semen is both the necessary and sufficient stimulus for the LH surge and subsequent ovulation. Clearly tactile stimulation alone or injection of a liquid volume into the urogenital sinus did not induce ovulation.

3. A possible selective advantage of reflex ovulation is that the female can choose the optimal time and the optimal mate for reproduction. If she has mature eggs available to ovulate, but she is not sexually receptive unless environmental conditions are good and a potential mate is at hand, reflex ovulation can optimize her chances of reproductive success.