Chapter 12b Sexual reproduction in animals

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Chapter 12b- Sexual reproduction in animals

• Most animals reproduce sexually
• However, many have the capability to reproduce
  asexually by 2 main methods
• - budding - Hydra and all polyps (such as
  anemones) sponges.
• - fragmentation - Example planaria many polyps
  (such as corals).
• Asexual reproduction result in offspring with
  the same genetic makeup as the parents ? clones.
• Most animals that reproduce asexually live
  attached permanently to their substratum (i.e.,
  they dont have the capability of locomotion as
  adults).
• Animals that reproduce asexually do so under
  certain specific conditions, particularly
• - When the purpose is to rapidly cover a local
  habitat that has lots of space available.
• - When conditions are favorable at the place
  where they are.
• - Fragmentation is not reproduction per se, but
  it does result in clonal individuals who can
  colonize habitats.

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Sexual reproduction in animals

• Since most animals that reproduce both sexually
  and asexually are not capable of moving, sexual
  reproduction is favored in certain circumstances
• - Local habitat has become crowded and clonal
  individuals have no space to live.
• - Environmental conditions are not good, making
  long-term survival unlikely.
• Most animals who reproduce sexually have gonads
  that produce gametes
• - ovaries produce ova
• - testes produce sperm
• Many simple animals dont have separate sexes A
  single individual produces both eggs and sperm.
  Therefore, the individual has both types of
  gonads. In most cases, self-fertilization is
  avoided by various possible means
• - They dont mature as ? and ? at the same
  time of the year.
• - Even if they mature at the same time, they
  will release one time of gametes first, then he
  next.
• - Certain age groups within the population of
  the species are of each gender (they are born as
  one gender and as they grow they change)
• - The two types of gametes (ova, sperm) are
  produced in parts of the body that make it
  difficult for them to mix.
• Overall, the goal of sexual reproduction is to
  exchange genetic material among individuals, and
  therefore the process of outcrossing (opposite of
  self-fertilization) is favored by all means.

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External fertilization

• Fertilization is the process by which gametes
  from different sexes join, and ova is fertilized.
• The simplest sexually reproducing animals are
  aquatic. In aquatic habitats the most common
  method is external fertilization Individuals of
  the 2 sexes release gametes into the water ?
  Spawning.
• During spawning, the likelihood that any single
  egg or sperm will result in fertilization is
  very, very small, because
• - Gametes go into the water and are diluted and
  carried away. Eggs and sperm may never meet.
• - Many gametes will be eaten by other
  organisms, since after all, they are food.
• - Even if fertilization occurs, chances of
  survival are small.
• ? To maximize chances of fertilization,
  spawning by males and females must be
  synchronized.
• For these reasons, animals that use external
  fertilization tend to produce many gametes. This
  ensures the fertilization and survival of at
  least some of them. The gametes themselves are
  very small, so they are comparatively inexpensive
  to produce, which makes it possible to make many.
  However, frequently they are produced only during
  a certain period of time (a reproductive season).
  

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Internal fertilization

• Most terrestrial (land) animals, and a few
  aquatic animals use internal fertilization Males
  release sperm into the females reproductive
  organs. Several advantages and some consequences
• - Gametes are protected from dilution, damage,
  and from being eaten by others.
• - Fewer gametes are needed because survival is
  much higher.
• - Gametes are larger, and therefore more
  expensive to produce.
• Most amphibians (frogs, toads, salamanders) and
  some fish exhibit an intermediate situation
  between external and internal fertilization
• - Males deposit sperm over eggs deposited by
  the female moments before.
• - In amphibians, the eggs are contained in a
  gelatinous mass that hold tem together. In fish
  such as salmon, the unfertilized eggs are
  deposited into a nest, which also holds tem
  together temporarily. In both cases (the mass,
  or the nest), rate of fertilization is increased.
• - Both amphibians and fish such as salmon
  produce intermediate numbers of relatively large
  gametes, since some protection is given.
• Internal fertilization and parental care of
  fertilized gametes and offspring are all part of
  an evolutionary trend of larger, more complex
  organisms towards fewer gametes and offspring,
  but a higher investment into assuring the
  survival of each. Simpler organisms just produce
  many, and some will survive.
• Internal fertilization can be accomplished in
  different means by various types of organisms

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Amphibians
Amplexus (an intermediate method
without copulation, but without broadcast
spawning. fertilization is external)
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Eggs and Sperm

• Sperm is much smaller than ova (eggs), and thus
  cheaper. Partly for this reason, much more sperm
  is produced by a single individual. Sperm has a
  very short life once released into the
  environment (if it does not meet an egg very
  quickly, it dies).
• Eggs are larger because they store nutrients and
  organelles that will support the developing
  embryo. In contrast, the only contribution of the
  sperm is genetic information. Eggs have a longer
  life but also die quickly unless fertilization
  has occurred.
• Animals with external fertilization
• - Synchronicity of spawning by males and females
  is crucial to ensure successful fertilization.
• - Synchronicity occurs at the population level,
  so most individuals of a population of a species
  will spawn at the same time. During these events,
  the water will be literally full of eggs and
  sperm, maximizing the chances of fertilization by
  reducing the distance sperm and eggs travel to
  meet.
• Animals with internal fertilization
• - Synchronicity is also important for
  fertilization
• - Most species dont reproduce at any time of
  the year, but within certain seasons when
  individuals congregate (i.e., migrations), food
  is abundant (assuring survival or offspring), or
  obey to cycles of fertilility/infertility
  (menstrual cycle or equivalent)

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More on eggs and sperm

• In humans and many mammals, sperm and eggs also
  are short-lived (from hours to a few days), so it
  is important that both mating individuals are
  ready for reproduction or fertilization will not
  occur.
• In mammals, the distances gametes must travel are
  short and are within a protected environment
  (within the body of individuals). It is actually
  only sperm that must travel to meet the egg
  during mating. The great majority of sperm
  released dont ever reach the egg.
• 3 reasons for the large number of sperm produced
  (as compared to eggs)
• 1- Fertilization must occur quickly
• 2- Distance that sperm must travel
• 3- Enzymes on sperm are required to penetrate
  the egg membrane. Many sperm fail to penetrate.
  Once one penetrates, other sperm are prevented
  from entering.
• In some animals, particularly many insects,
  females are capable of storing sperm for long
  periods of time, and fertilize eggs at various
  times without the need for mating or additional
  male intervention. Some fish species can do this
  as well.
• In a few species, particularly some fish and some
  insects, the role of the male of the species has
  become exclusively that of producing sperm. In
  fact, in some species the male is dwarf, and
  lives a parasitic life style attached to the
  larger female body, just for gamete production.

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Types of development in vertebrates

• I- Egg laying vertebrates
• - Groups Fish, Amphibians, Reptilians, Birds,
  and only 3 species of mammals (Platypus, and 2
  spp. of Echidna)
• - Various layers of tissue exist within the
  eggs, which protect, nourish, and allow material
  exchange between embryo and environment.
• - All nutrition comes from material from inside
  the egg (yolk, etc.), not from the mothers body.
• - Embryo develops outside of the mothers body,
  and hatch.
• - In some cases, the eggs are retained inside
  the body for various lengths of time, but they
  are eggs just the same.
• II- Non-egg laying vertebrates
• - Groups Mammals except Platypus and Echidna
• - Tissue layers develop inside of the
  reproductive tissue of the mother and are
  maintained there through the gestation process.
  One of this tissues is the placenta.
• - Nutrition comes from the mothers body.
• - Embryo develops inside the mothers body, thus
  ?s give birth
• - 3 Types of mammals Platypus and Echidna (lay
  eggs) non-placental (Marsupials possums and
  kangaroos) Placental (all other mammals)
• - Non-placental mammals (marsupials) actually do
  have a placenta but differ in that they are borne
  very under-developed (as an embryo) and are
  maintained in the mothers pouch (the marsupium).
  Placental have a much longer gestation time and
  are borne at advanced state of development.

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Mammal examples
Egg-laying
Non-placental (Marsupials)
Placental (All other mammals)
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Human reproduction-female anatomy

• Eggs produced by meiosis (oogenesis) travel from
  ovaries, thru oviducts, to the uterus. Typically,
  a single egg at a time.
• If fertilization occurs, the fertilized egg
  (actually already an embryo) attaches to the
  internal lining of the uterus (implantation).
• The egg-producing cycle (menstrual cycle) lasts
  about 28 days (approx.)
• If no fertilization occurs, the egg disintegrates
  and is expelled from the body, along with blood
  and tissue from the endometrium (internal tissue
  layer of uterus). The endometrium becomes
  enlarged, and sloughs-off regularly, as part of
  the menstrual cycle.
• The menstrual cycle is under hormonal control.
  Hormonal production is regulated by negative
  feedback mechanism (their concentration in the
  bloodstream signals glands when to
  increase/decrease production).

Menstrual cycle occurs in primate mammals only
(monkeys, apes, humans). Other mammals dont
menstruate, they have an estrus (heat) cycles
of different durations, during which they ovulate
and mate.
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Path of egg and sperm within ?

• Eggs produced in ovary, move down through
  oviduct.
• Sperm from male moves up towards egg

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Hormonal control of menstrual cycle

• Most control is done by the hypothalamus (gland
  at base of brain).
• Ovaries release 2 hormones (estrogen and
  progesterone). At the beginning of the cycle
  their low levels ? hypothalamus secretes GnRH
  (gonadotropin releasing hormone) ? pituitary
  gland (below hypothalamus) releases 2
  gonadotropin hormones, LH (luteinizing hormone)
  and FSH (follicle stimulating hormone).
• FSH causes an egg to mature in a follicle of
  ovary.
• Both LH FSH stimulate the follicle to secrete
  estrogen into the blood ? endometrium of uterus
  enlarges and grows blood vessels.
• By day 14 an increase in LH in blood ? follicle
  bursts releasing egg (ovulation). The burst
  follicle becomes the corpus luteum and continues
  releasing estrogen and progesterone ? endometrium
  continues thickening.
• High levels of estrogen progesterone ?
  hypothalamus slows LH and FSH release by
  pituitary.

• No fertilization ?corpus luteum degenerates
• estrogen progesterone levels decline
• endometrium breaks down ? menstrual
• flow ? beginning of another cycle.

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Hormonal changes during pregnancy

• If egg is fertilized, it will grow by mitotic
  divisions and become an embryo and attaches to
  internal lining of uterus ? placenta is formed
  (fluid-filled bag)
• The placenta releases HCG (human chorionic
  gonadotropin) ? corpus luteum continues releasing
  estrogen and progesterone ? prevent another
  menstrual cycle.
• After 3 months of gestation (development of
  embryo in uterus), the placenta stops releasing
  HCG but continues releasing estrogen and
  progesterone.
• At birth, oxytocin is released
• ?uterus muscles contract, expelling the baby.

• Oxytocin also stimulates milk production by
• mammary glands, and contraction/shrinking
• of uterus.
• The placenta comes out after birth of baby
• (called afterbirth) ? estrogen progesterone
• levels drop ? hypothalamys secretes GnRH
• again ? menstrual cycle begins again.

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Human reproduction- Male anatomy

• Testes ( testicles) are held outside of the
  body, in the scrotum (bag), to maintain cool
  temperature.
• Sperm are produced by meiosis (spermatogenesis)
  in seminiferous tubules of testes.
• Seminal fluid made in prostate gland and seminal
  vesicles, along with sperm (altogether called
  semen travel outside of the body thru the vas
  deferens (sperm duct) during ejaculation.
• Hypothalamus produces GnRH ? pituitary releases
  gonadotropins (LH, FSH) ? androgen hormones are
  produced in tubules of testes.
• FSH stimulate sperm production.

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Secondary sex characteristics

• Both ? and ? produce androgens and estrogen
  hormones. Males produce more androgens, female
  produce more estrogens.
• Both hormones are responsible for secondary
  sexual characteristics, most of which are first
  expressed at puberty.
• Androgens ? facial hair, changes in voice
• Estrogens ? breast development, body shape.

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Surgical methods of birth control(in addition to
mechanical and hormonal)
? vasectomy- cutting vas deferens
? tubal ligation