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Chapter 46: Animal Reproduction

Chapter 46: Animal Reproduction. Asexual reproduction Offspring from one parent; growth by mitosis Types Fission- equal division of cytoplasm (ex. Bacteria, paramecia) Budding- unequal division of cytoplasm (ex. Cnidarians, yeast colonies, gemmule production in Porifera)

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Chapter 46: Animal Reproduction

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  1. Chapter 46: Animal Reproduction Asexual reproduction Offspring from one parent; growth by mitosis Types Fission- equal division of cytoplasm (ex. Bacteria, paramecia) Budding- unequal division of cytoplasm (ex. Cnidarians, yeast colonies, gemmule production in Porifera) Fragmentation- body broken into pieces Regeneration of lost parts Found in sponges, cnidarians, annelids, tunicates, & enchinoderms Advantages Isolated individuals can reproduce without a mate Less time between generations Successful genotypes perpetuate

  2. Sexual reproduction • 2n offspring (zygote) produced by the fusion of egg & sperm • Gametes formed by meiosis • Female gamete=ovum • Male gamete=sperm • Offspring have characteristics of both parents resulting in species variation • Reproductive cycles & patterns vary among animals • Often seasonally based & controlled by hormones • Parthenogenesis • Development of an unfertilized egg into new individuals • Ex. Aphids, bees, daphnia • Hermaphrodites • Individuals have functioning ovaries & testes • Self fertilization is rare • Found in slow moving, energy conserving organisms • Sequential hermaphrodites • Individual reverses its sex • Ex. Some fish & oysters

  3. Mechanisms of sexual reproduction • External fertilization • Occurs in moist habitats to prevent desiccation & heat stress • Environmental cues & pheromones trigger release of gametes • Vertebrates (fish, amphibians) exhibit courtship behavior to ensure fertilization • Large numbers of zygotes result • few reach sexual maturity • Gelatinous coat covers the egg to permit gas & H2O exchange

  4. Internal fertilization • Fewer zygotes produced but more parental protection • Requires • Cooperative behavior • Copulatory organ for sperm delivery & a sperm receptacle for storage/transport • Mating behaviors to signal copulatory receptivity • Adaptations include • Shelled egg (“pond away from the pond”) in egg laying animals • Reptiles & monotremes- amniotic egg with protein shell • Birds- amniotic egg with calcium shell • Mammals have internal embryonic development • Marsupials- embryo in uterus for short time; completes development in maternal pouch • Eutherians (placental)- embryo in uterus until development complete

  5. Reproductive system complexity is not necessarily an indicator of phylogenetic relationship • Parasitic flatworms reproductive system is one of the most complex • Polycheates have separate sexes without distinct gonads • Insects have separate sexes & complex reproductive tracts; females can store sperm for years after a single mating in the spermatheca • Vertebrates have similar reproductive systems with some differences • Mammals have separate openings for digestion, excretion, & reproduction • Non-mammals have single opening=cloaca • Some mammals, birds, & snakes have a single branched uterus; most mammals have a divided uterus • Non-mammalian vertebrates do not have well developed penises & use other means to transfer spermatozoa

  6. Mammalian Reproduction • Human male • Testes develop & descend from the abdomen into the scrotum • Sperm production requires a lower temperature • Penis is copulatory organ; ejaculatory duct joins urethra • Human female • More complicated than the male • Produces female gamete • Houses embryo & developing fetus for gestation period

  7. Human Sexual Response Cycle • Coitus= sexual intercourse • Excitement • Vasocongestion of penis & clitoris (increased arterial blood flow) • Myotonia- muscle tension • Plateau • Stimulation of autonomic nervous system • Orgasm • Rhythmic involuntary contractions of reproductive structures in both sexes • Resolution • Reverses early responses

  8. Meiosis • Spermatogenesis • All 4 cell products become mature spermatozoa • Continuous process; 100 million +sperm/day • Oogenesis • Unequal cytokinesis results in one ovum & 3 polar bodies • All potential ova present at birth as primary oocytes in the ovaries

  9. Males • Androgens (testosterone) produce primary & secondary sex characteristics • GnRH from hypothalamus stimulates anterior pituitary to release LH & FSH • LH- stimulates testosterone production • FSH- stimulates sperm production in seminiferous tubules • Females • Estrogen produces primary & secondary sex characteristics • Estrous cycle occurs in non-primate mammals • “heat”; period of sexual activity surrounding ovulation • Ovulation occurs after thickening of endometrium • Menstrual cycle in primates & humans • Menstruation- shedding of endometrium • Proliferative phase- regeneration of endometrium • Secretive phase- endometrium thickens & vascularizes • Follicle phase- egg matures in follicle • Ovulation- egg is released • Corpus luteum phase- follicle fills with yellow tissue mass; secretes progesterone • Menopause- female stops menstruation & ovulation; ovaries no longer respond to LH & FSH • Gestation period (conception to birth) • Humans 266 days (38 weeks) • Rodents 21 days • Cows 270 days • Elephants 600 days • Dogs 60 days

  10. Development • 1st trimester • Conception in oviduct • Cleavage- 24 hours after conception • Implantation of blastocyst (sphere of cells) • HcG secreted by corpus luteum to maintain progesterone • Placenta formation • Organogenesis • 8 weeks= fetus • 2nd trimester • Fetus grows longer- up to 30 cm • Movement detected • Placenta takes over progesterone production so HcG declines as corpus luteum disintegrates • 3rd trimester • Fetus grows to 50 cm • Mass increases rapidly • Maternal organs compressed • Labor • Induced by interplay of hormones estrogen, oxytocin, & prostaglandins • Parturition (birth) • Dilation of cervix • Strong uterine contractions through positive feedback • expulsion

  11. Contraception • Abstinence • Rhythm method • Physical barriers… condoms, diaphragms, cervical caps, sponges • Implants… IUD- spiral shaped device implanted into uterine entrance to prevent sperm entry • Withdrawal • Chemical contraception…prevents release of gametes (pill) • Tubal ligation… closing oviducts • Vasectomy… cutting of vas deferens • Miscarriage…. Spontaneous abortion (1/3 of all pregnancies) • Abortions… 1.5 million/year in US • RU-484… blocks progesterone receptors on uterus so pregnancy is not maintained

  12. Modern technology • Ultrasound • Non-invasive • High frequency sound waves to “picture” fetus • Sampling maternal blood • Used to detect fetal metabolic waste • Amniocentesis • Invasive extraction of amniotic fluid to examine fetal cells • Chorionic villus sampling • Invasive biopsy of placental tissue to detect genetic disorders • In vitro fertilization • “test tube baby” • Mother egg & father sperm fuse in vitro (in glass) after several cleavages embryo is implanted into mother

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