Chapter
7 The Reproductive ProcessChapter Outline
7.1. Nature of the Reproductive Process
A. Mechanisms (Figure 7.1)
1. Asexual reproduction involves only one parent.
a. There are no special reproductive organs or cells involved.
b. Genetically identical offspring are produced.
c. Production of offspring is simple, direct and rapid.
2. Sexual reproduction generally involves two parents.
a. Special germ cells (gametes) unite to form a zygote.
b. By receiving genetic material from both parents, the offspring are unique.
c. Sexual reproduction recombines parental characters and makes possible a richer and more diversified population.
3. In haploid asexual organisms, mutations are expressed and selected quickly.
4. In sexual reproduction, a normal gene on the homologous chromosome may mask a gene mutation.
B. Asexual Reproduction: Reproduction Without Gametes
1. Neither eggs nor sperm are involved.
2. Unless mutations occur, all offspring have the same genotype and are clonal.
3. Asexual reproduction is widespread in bacteria, unicellular eukaryotes and many invertebrate phyla.
4. Asexual reproduction ensures rapid increase in numbers.
5. Binary fission is common among bacteria and protozoa.
a. The parent divides by mitosis into two parts; each grows into an individual similar to the parent.
b. Binary fission can be lengthwise or transverse.
c. In multiple fission, the nucleus divides repeatedly; cytoplasmic division produces many daughter cells.
d. Sporogony is spore formation, a form of multiple fission in parasitic protozoa.
6. Budding is unequal division of an organism.
a. The bud is an outgrowth of the parent; it develops organs and then detaches.
b. Budding occurs in cnidarians and some other animal phyla.
7. Gemmulation is formation of a new individual from an aggregation of cells from the parent individual surrounded by a resistant capsule (gemmule).
a. Freshwater sponges survive winter in the dried or frozen body of the parent.
b. In good conditions, the enclosed cells become active, emerge and grow a new sponge.
8. Fragmentation involves a multicellular animal breaking into many fragments that become a new animal.
C. Sexual Reproduction: Reproduction With Gametes
1. Bisexual Reproduction
a. Also called biparental, bisexual reproduction produces offspring from union of gametes from two genetically different parents.
b. Offspring therefore have a genotype different from either parent. (Figure 7.2)
c. Generally, individuals are male or female and produce spermatozoa or ova, respectively.
1) The female produces the ovum; it is large with stored yolk and is nonmotile.
2) The spermatozoon is produced by the male; it is small, motile and much more numerous.
d. Most vertebrates and many invertebrates have separate sexes; they are dioecious.
e. Some animals have both male and female organs; they are monoecious or hermaphrodites.
f. Meiosis (duplication and two divisions) produces four haploid cells.
g. In fertilization, two haploid cells combine to restore the diploid chromosome number in the zygote.
h. A zygote divides by mitosis for all somatic (body) cells.
i. Many unicellular organisms can reproduce both sexually and asexually.
j. When sexual parents merely join together to exchange nuclear material (conjugation), distinct sexes do not occur.
k. Organs that produce germ cells are gonads; testes produce sperm and ovaries produce eggs.
l. Gonads are primary sex organs; some animals lack any other "accessory" sex organs.
m. Additional accessory sex organs include penis, vagina, uterine tubes and uterus.
2. Hermaphroditism (Figure 7.3)
a. Hermaphrodites have both male and female organs in the same individual.
b. Many sessile, burrowing and/or endoparasitic invertebrate animals and a few fish are hermaphroditic.
c. Most avoid self-fertilization and exchange germ cells with another member of the same species.
d. Each individual is reproductive, in contrast to dioecious species where about half is male.
e. In sequential hermaphroditism, a fish starts life as one sex and is genetically programmed to change to the other sex later.
3. Parthenogenesis
a. Parthenogenesis is the development of an embryo from an unfertilized egg.
b. The male and female nuclei may fail to unite after fertilization.
c. In ameiotic parthenogenesis, no meiosis occurs and the egg forms by mitotic division.
d. In meiotic parthenogenesis, the haploid ovum is formed by meiosis and develops without fusion with the male nuclei.
1) Sperm may be absent or they may only activate development.
2) In some species, the haploid egg returns to a diploid condition by chromosomal duplication.
e. Haplodiploidy occurs in bees, wasps and ants.
1) The queen controls whether the eggs are laid fertilized or unfertilized.
2) Fertilized eggs become female workers or queens; the unfertilized eggs become drones.
f. Some desert lizards have modified meiosis so offspring are clones of the female parent.
g. Parthenogenesis avoids the energy and dangers of bringing two sexes together; but it narrows the diversity available for adaptation to new conditions.
4. Why do so many animals reproduce sexually rather than asexually? (Figure 7.4)
a. Sexual reproduction is more common among animals.
b. The costs of sexual reproduction are greater.
1) It is more complicated, requires more time and uses more energy than asexual.
2) The cost of meiosis to the female is passage of only half of her genes to offspring.
3) Production of males reduces resources for females that could produce eggs.
c. Sexual organisms produce more novel genotypes to survive in times of environmental change.
d. Asexual organisms can have more offspring in a short time to colonize new environments.
e. In crowded habitats, selection is intense and diversity prevents extinction.
f. On a geological time scale, asexual lineages with less variation are prone to extinction.
g. Many invertebrates with both sexual and asexual modes enjoy the advantages of both.
7.3. Reproductive Patterns
A. Live-birth Versus Egg-bearing
1. Oviparous animals lay eggs in the environment for development.
a. Fertilization may be internal (before eggs are laid) or external (after laid).
b. Some animals abandon eggs; others provide extensive care.
2. Ovoviviparous animals retain eggs in their body.
a. Essentially all nourishment is derived from the yolk.
b. This is common in some invertebrate groups and certain fishes and reptiles.
c. Fertilization is internal.
3. Viviparous animals give live birth.
a. Eggs develop in an oviduct or uterus.
b. Embryos continuously derive nourishment from the mother.
c. Fertilization is internal.
d. This occurs in mammals and some fishes; it provides more protection to offspring.
7.4. Plan of Reproductive Systems
A. Components
1. Primary organs are the gonads that produce sperm, eggs and sex hormones.
2. Accessory organs assist gonads in formation and delivery of gametes and may support embryos.
B. Invertebrate Reproductive Systems (Figure 7.11)
1. Invertebrates that transfer sperm for internal fertilization require complex organs.
2. Invertebrates that release sperm into water for external fertilization may be simple.
a. Polychaete annelids have no permanent reproductive organs; gametes are cells from the body cavity.
b. Mature gametes may be released through ducts or spill out through ruptures.
3. Insects have separate sexes and accomplish internal fertilization using complex systems.
a. Sperm from testes are stored in seminal vesicles before ejaculated.
b. Female insects have ovaries in a series of egg tubes.
c. Mature ova pass to a common genital chamber and short vagina.
d. Sperm inserted by male are stored in a seminal receptacle in female.
e. One mating may provide enough sperm to last the reproductive life of a female insect.
C. Vertebrate Reproductive Systems
1. Urogenital system of vertebrates shows close connections of reproductive and excretory systems.
2. The mesonephric duct drains the kidney and carries sperm in male fishes and amphibians.
3. The mesonephric duct is composed of the vas deferens and a separate ureter develops in male reptiles, birds and mammals.
4. The cloaca is the common chamber for intestinal, reproductive and excretory canals, except in mammals.
5. The uterine duct of the oviduct has an independent duct opening into cloaca when present.
D. Male Reproductive System
1. Paired testes are sites of sperm production.
2. Testes contain numerous seminiferous tubules where sperm develop. (Figure 7.12)
3. Sperm are surrounded by sustentacular cells that nourish developing sperm.
4. Between tubules are interstitial cells that produce testosterone.
5. A sac-like scrotum suspends testes outside the warm body cavity; the lower temperature of scrotum is vital to normal sperm production.
6. Sperm pass from the testes to vasa efferentia and to coiled epididymis for maturation.
7. The vas deferens carries sperm from the epididymis to the urethra, where it exits the penis.
8. The penis is a copulatory organ used to introduce spermatozoa into the female vagina.
9. Seminal vesicles, prostate gland and bulbourethral glands form seminal fluid.
a. Seminal vesicles secrete a thick fluid containing nutrients for use by sperm.
b. The prostate gland secretes a milky, slightly alkaline solution that counters acidity.
c. Bulbourethral glands release mucus secretions that provide lubrication.
E. Female Reproductive System
1. Ovaries in female vertebrates produce ova and the female sex hormones, estrogen and progesterone.
2. In jawed vertebrates, mature ova from ovaries enter funnel-like uterine tubes or oviducts.
3. The terminal end of uterine tube is specialized in cartilaginous fishes, reptiles and birds to produce shelled eggs; special regions produce albumin and shell.
4. The terminal portion of amniote uterine tube expands into a muscular uterus.
a. Shelled eggs may be retained here before laying.
b. Embryos may complete their development here.
c. Placental mammals use the walls of the uterus to intermingle vascular tissue as a placenta.
5. Ovaries are paired and slightly smaller than male testes. (Figure 7.13)
a. Oocytes develop within a follicle that enlarges to release a secondary oocyte. (Figure 7.10)
b. Unless fertilization occurs, women release about 13 oocytes per year, 300-400 per a 30-year reproductive lifetime.
c. 300-400 primary oocytes, of ca. 400,000 in ovaries at birth, reach maturity while the rest degenerate and are absorbed.
6. Uterine tubes or oviducts are lined with cilia that propel the egg.
7. The oviducts enter the upper corners of the uterus.
8. Uterus
a. The uterus is specialized to house the embryo for nine months.
b. The uterus has thick muscular walls and is stretchable.
c. The endometrium is the specialized lining rich in blood vessels.
d. Ancestrally, the uterus was paired but is fused in eutherian mammals.
9. The vagina is muscular tube that receives the male’s penis and serves as birth canal.
10. The cervix is the end of the uterus that extends into the vagina.
11. The vulva is external genitalia in human females.
a. Labia majora and labia minora enclose urethral and vaginal openings.
b. The clitoris is a small erectile organ equivalent to the glans penis of male.