Animal Reproduction

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Animal Reproduction

• AP Biology Chapter 46
• Campbell 6e

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Reproduction

• There are 2 modes of reproduction in the animal
  kingdom.
• Asexual creation of individuals whose genes
  come from one parent by mostly mitotic cell
  division.
• Sexual creation of individuals from the fusion
  of gametes to form a zygote.

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Asexual Reproduction

• Fission, budding, and fragmentation are some of
  the mechanisms that allow organisms to reproduce
  asexually.
• Fission the division of bacterial cells.
• Budding new individuals splitting off of the
  parent.
• Fragmentation breakage of the organism into
  several parts to form new organisms. (accompanied
  by regeneration of new body parts.)

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• It is advantageous in a constant environment
  because a large number of offspring can be
  produced in a short time.
• Flatworms can divide into 2 halves each half
  grows into a separate organism.
• Cnidarians undergo budding where a new individual
  grows from and then breaks off of the parent
  individual.
• Echinoderms can be cut to form new individuals.
• Insects- parthenogenesis (unfertilized egg
  develops.)

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Sexual Reproduction

• In sexual reproduction new individuals are
  produced by the fusion of haploid gametes to form
  a diploid zygote.
• Sperm are male gametes, ova (ovum singular) are
  female gametes.
• Meiosis produces cells that are genetically
  distinct from each other fertilization is the
  fusion of two such distinctive cells that
  produces a unique new combination of alleles,
  thus increasing variation on which natural
  selection can operate.

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• Sperm and eggs are sometimes produced by the same
  individual and sometimes produced by different
  individuals.
• The sexes of vertebrates are separate but some
  vertebrates can change sex.

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Variation

• Sexual reproduction promotes variation because
  offspring inherit genes from two different
  parents.
• In addition, when an individual produces gametes,
  crossing-over and independent assortment mix
  genes from that individuals parents. Gametes are
  haploid they contain some genes from the
  individuals mother and some from the father.
• Variation is advantageous to species in
  fluctuating environments.

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Reproductive Cycles

• Reproductive cycles in many vertebrates and
  invertebrates are related to changes in day
  length.
• Day length is a reliable indicator of season for
  timing circannual events such as reproductive
  behavior and migration.
• In many species, the pineal gland secretes
  melatonin in the dark. Increasing daylength
  (decreased melatonin levels) triggers
  reproductive behavior.

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Morphisms

• Sexually reproducing animals are either
• Monoecious male and female in same organism
• Dioecious male and female in different
  organisms.
• Dioecious species are either sexually
• Monomorphic (look the same)
• dimorphic. (look different)

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Monoecious

• Even though monoecious organisms have both male
  and female reproductive systems they rarely
  fertilize their own eggs.
• Either the two systems mature at different times
  or the events of sperm transfer and fertilization
  are separated.

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Monoecious
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• They do have one advantage that dioecious
  organisms do not have. When dioecious organisms
  mate, only one parent produces new offspring.
  That's not the case with monoecious animals. When
  monoecious meet, they both can receive sperm from
  their partners and will later fertilize their own
  eggs with the sperm they received when they
  mated.
• The result is that when monoecious animals mate,
  both parents produce young. Monoecious organisms
  are often sessile or slow moving, and there is an
  advantage to having both animals produce young
  when there is a chance encounter.

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Dimorphism

• The sexes of most sexually reproducing species
  differ in both primary sexual characteristics
  (sex organs) and secondary sexual characteristics
  associated with mating and bearing young.
• Males are characterized by brighter colors and
  more elaborate structures such as horns. Bright
  colors may increase the attractiveness of males
  to females, while horns may serve as weapons for
  fighting with other males for control of a female.

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Mono- and Dimorphic
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Dimorphism

• Dimorphism is most pronounced in insects where
  the morphologies of the penis and vagina are
  precisely matched for each species, apparently
  preventing incorrect matings between closely
  related species.

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Mechanisms of Sexual Reproduction

• Fertilization plays an important role in
  reproduction--- can occur in two ways.
• Internal
• External

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External Fertilization

• During external fertilization, many gametes are
  released into the water by each sex at the same
  time and place. 
• This type of fertilization requires water because
  animal sperm must swim to the eggs. Water also
  protects the gametes from drying out.
• Species which have external fertilization are
  either aquatic or return to water for
  reproduction.

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Amphibian Eggs
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Internal Fertilization

• Internal fertilization is practiced by species
  that lay shelled eggs or have a period of
  internal embryonic development.
• This type of fertilization enables animals to
  reproduce in a terrestrial environment because it
  enables sperm to swim to the egg and it prevents
  gametes from drying out.
• The male often has a copulatory organ (a penis)
  for transferring sperm.
• Males of some aquatic animals (ex sharks,
  skates, rays) have specialized pelvic fins which
  allow the sperm to be passed to the female.

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Eggs and Birth

• Animals can be grouped into those which
• Give birth to living offspring and
• Those which lay eggs that eventually hatch into
  offspring.
• Those animals which give birth to live offspring
  are called live-bearing or viviparous.
• Those animals which lay eggs are called
  egg-laying or oviparous. The difference is in the
  place where the offspring develops before it is
  born.

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Human Reproduction

• Human reproduction employs internal
  fertilization, and depends on the integrated
  action of hormones, the nervous system, and the
  reproductive system.
• Gonads are sex organs that produce gametes.
• Male gonads are the testes, which produce sperm
  and male sex hormones.
• Female gonads are the ovaries, which produce eggs
  (ova) and female sex hormones.

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Male Reproductive System

• Testes are suspended outside the abdominal cavity
  by the scrotum, a pouch of skin that keeps the
  testes close or far from the body at an optimal
  temperature for sperm development.
• Seminiferous tubules are inside each testis, and
  are where sperm are produced by meiosis. About
  250 meters (850 feet) of tubules are packed into
  each testis.
• Spermatocytes inside the tubules divide by
  meiosis to produce spermatids that in turn
  develop into mature sperm.

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Spermatogenesis

• Sperm production begins at puberty at continues
  throughout life, with several hundred million
  sperm being produced each day.
• Once sperm form they move into the epididymis,
  where they mature and are stored.

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Male Hormones

• The anterior pituitary produces
  follicle-stimulating hormone (FSH) and
  luteinizing hormone (LH).
• Action of LH is controlled by the
  gonadotropin-releasing hormone (GnRH).
• LH stimulates cells in the seminiferous tubules
  to secrete testosterone, which has a role in
  sperm production and developing male secondary
  sex characteristics.
• FSH acts on cells to help in sperm maturation.
  Negative feedback by testosterone controls the
  actions of GnRH.

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Semen

• Sperm pass through the vas deferens and connect
  to a short ejaculatory duct that connects to the
  urethra.
• The urethra passes through the penis and opens to
  the outside.
• Secretions from the seminal vesicles add fructose
  and prostaglandins to sperm as they pass.
• The prostate gland secretes a milky alkaline
  fluid.
• The bulbourethral (Cowpers) gland secretes a
  mucus-like fluid that provides lubrication for
  intercourse. Sperm and secretions make up semen.

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The Female Reproductive System

• The female gonads, ovaries, are located within
  the lower abdominal cavity.
• The ovary contains many follicles composed of a
  developing egg surrounded by an outer layer of
  follicle cells.
• Each egg begins oogenesis as a primary oocyte.
• At birth each female carries a lifetime supply of
  developing oocytes, each of which is in Prophase
  I. A developing egg (secondary oocyte) is
  released each month from puberty until menopause,
  a total of 400-500 eggs.

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The Ovarian Cycle

• After puberty the ovary cycles between a
  follicular phase (maturing follicles) and a
  luteal phase (presence of the corpus luteum).
  These cyclic phases are interrupted only by
  pregnancy and continue until menopause, when
  reproductive capability ends. The ovarian cycle
  lasts usually 28 days.

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• During the first phase, the oocyte matures within
  a follicle. At midpoint of the cycle, the oocyte
  is released from the ovary in a process known as
  ovulation.
• Following ovulation the follicle forms a corpus
  luteum which synthesizes and prepares hormones to
  prepare the uterus for pregnancy.
• The secondary oocyte passes into the oviduct
  (fallopian tube or uterine tube). The oviduct is
  connected to the uterus.

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Structures

• The uterus has an inner layer, the endometrium,
  in which a fertilized egg implants.
• At the lower end of the uterus the cervix
  connects the uterus to the vagina.
• The vagina receives the penis during intercourse
  and serves as the birth canal.

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External Genitals

• The female external genitals are collectively
  known as the vulva.
• The labia minora is a thin membrane of folded
  skin just outside the vaginal opening.
• The labia majora cover and protect the genital
  area.

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Hormones and Cycles

• The ovarian cycle is hormonally regulated in two
  phases. The follicle secretes estrogen before
  ovulation the corpus luteum secretes both
  estrogen and progesterone after ovulation.
• Hormones from the hypothalamus and anterior
  pituitary control the ovarian cycle.
• The ovarian cycle covers events in the ovary the
  menstrual cycle occurs in the uterus.

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Menstrual Cycle

• Menstrual cycles vary from between 15 and 31
  days.
• The first day of the cycle is the first day of
  blood flow (day 0) known as menstruation.
• During menstruation the uterine lining is broken
  down and shed as menstrual flow.
• FSH and LH are secreted on day 0, beginning both
  the menstrual cycle and the ovarian cycle. Both
  FSH and LH stimulate the maturation of a single
  follicle in one of the ovaries and the secretion
  of estrogen.

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• Rising levels of estrogen in the blood trigger
  secretion of LH, which stimulates follicle
  maturation and ovulation (day 14, or midcycle).
• LH stimulates the remaining follicle cells to
  form the corpus luteum, which produces both
  estrogen and progesterone.
• Estrogen and progesterone stimulate the
  development of the endometrium and preparation of
  the uterine inner lining for implantation of a
  zygote.
• If pregnancy does not occur, the drop in FSH and
  LH cause the corpus luteum to disintegrate. The
  drop in hormones also causes the sloughing off of
  the inner lining of the uterus by a series of
  muscle contractions of the uterus.

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Credits

• Most information from this site
• http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
  BioBookREPROD.html
• Other sites
• http//faculty.clintoncc.suny.edu/faculty/Michael.
  Gregory/files/Bio20102/Bio2010220lectures/Anima
  l20Reproduction/animal.htm
• http//www.saburchill.com/chapters/chap0031.html
• http//tidepool.st.usm.edu/Crswr/103animalreproduc
  tion.html