Ch 46-Animal Reproduction Learning Objectives

what are the pros of sexual reproduction

increase genetic variation

what are the minuses of sexual reproduction

• requires twice fitness (both parents must be fit)

• loss of “ideal” individual (offspring are not clones)

• less efficient

• slower population growth (asexual reproduction produces more after generations)

Vicar of Bray Hypothesis

• sexual reproduction increase variation which means greater chance of getting more fit traits

• more variation is beneficial in changing environments

Red Queen Hypothesis

• evolutionary arms race between hosts and pathogens

• pathogens adapt to common host genotypes

• sexual reproduction produces rare hosts genotypes

• increased host variation (sexual reproduction) makes it difficult for pathogen to specialize

what are the different ways animals reproduce

• asexual reproduction — somewhat common in invertebrates

  • budding

  • parthenogenesis

• sexual reproduction

  • hermaphroditism

  • gonochorism

    • external and internal fertilization

budding

makes clone of parent

parthenogenesis

unfertilized egg makes offspring (in some vertebrates)

hermaphroditism

• animals produce both female and male gametes

  • simultaneous: they have both parts at the same time

  • sequential: they can start one gender and end the other

gonochorism

separate sexes (as the kinds of gametes produced, not gender or sexually)

external fertilization

• often associated with aquatic animals

• releases eggs and sperm into the water where sperm swim to eggs

• many (not all) have a planktonic larval stage

internal fertilization

sperm deposited in or near the female reproductive tract fertilize eggs within the tract

how does sexual reproduction solve the problem of producing gametes

male and female reproductive structures

describe male reproductive structures

• male gametes (sperm) is produced in the testes

• composed of highly coiled tubes

  • seminiferous tubules

  • leydig cells (produce testosterone)

  • sertoli cells (provide nutrients for developing sperm)

• from the seminiferous tubules the sperm pass into the epididymis

• spermatogenesis: make sperm

  • during this process the cells move from the outer edge of the tubule toward the center

  • the sperm cells mature in the epididymis

  • spermatogonia (diploid) through mitosis

    • cells undergo meiosis (spermatocytes initially diploid)

    • produce haploid spermatids

    • mature into final sperm cells haploid

describe female reproductive structures

• the ovaries are composed of an outer covering surrounding up to 400,000 follicles (present at birth)

• from puberty to menopause follicles mature and egg cells are released (ovulation) - monthly

• oogenesis: production of eggs

  • oogenesis (diploid) enter meiosis and become primary oocytes

  • but stop in prophase I of meiosis and become primary oocytes

  • after puberty FSH stimulates one or more primary oocytes to complete the first meiotic division and pause in metaphase II

  • the mature follicle ruptures releasing the secondary oocyte from the ovary

  • at the time of ovulation the egg is in metaphase II of meiosis

  • after a sperm penetrates the egg it is triggered to complete meiosis

how does sexual reproduction solve the problem of bringing gametes together

• sperm passage

• ovulation

• fertilization

describe sperm passage

• during ejaculation the sperm are propelled from the epididymis through the vas defernes and out the urethra

• 3 sets of glands add secretions to the sperm to form the semen

  • seminal vesicle: fructose (60% total volume)

  • prostate gland: anticoagulant enzymes and citrate

  • bulbourethral gland: neutralizes the acidic environment of the urethra

describe ovulation

• the egg travels into the oviduct. the remaining cells of the follicle form the corpus lutem

• helped by the cilia that line the oviduct and travels along to the uterus

  • after a sperm cell penetrates, the egg is triggered to complete the meiosis

describe fertilization of egg and sperm

• sperm travel through the outer later of egg cells to reach the zona pellucida —the extracellular matrix of the egg

• when the sperm binds to a receptor in the zona pellucida, it triggers a slow block to polyspermy (solves the limit of fertilization of 2 gametes)

• if fertilized the developing embryo will implant in the thickened lining of the uterus (endometrium)

describe hormonal regulation in males

• hypothalamus

  • GnRH (gonadotropin releasing hormone)

• Anterior pituitary gland

  • FSH and LH (follicle stimulating hormone and luteinizing hormone)

• Testes

  • FSH acts on sertoli cells supporting spermatogenesis

  • LH stimulates leydig cell production of androgens (testosterone—also needed for spermatogenesis)

• inhibin provides negative feedback that inhibits the production of FSH and LH

• too much testosterone provides negative feedback

describe hormonal regulation in females

• hypothalamus

  • GnRH (gonadotropin releasing hormone)

• Anterior pituitary gland

  • FSH and LH (follicle stimulating hormone and luteinizing hormone)

• Ovaries

  • FSH stimulates maturation of follicle (estrogen)

  • LH stimulates ovulation and development of corpus luteum

• day 0-13 follicle is growing

• day 14 estradiol gives positive feedback

• the big peak in LH stimulates ovulation

  • the estradiol peak guides peak in LH

• estradiol and progesterone inhibit production of GnRH

  • releases the egg

• ruptured follicle = corpus luteum

• corpus luteus produced progesterone as it degenerates

• endometrium thickens as a result of progesterone

  • preparation for fertilized egg implantation

• day 15-28: negative feed pack

  • no more eggs released