Detailed Notes on Reproduction & Development

The Evolution of Reproduction

  • Asexual Reproduction

    • Produces clones
    • Methods:
    • Binary fission
    • Budding
    • Fragmentation
    • Parthenogenesis

  • Sexual Reproduction

    • Involves meiotic cell division and fertilization
    • Meiosis:
    • Halves the number of chromosomes
    • Produces gametes (egg and sperm)
    • Fertilization:
    • Combines two gametes to create a diploid zygote

  • Many species produce two types of gametes:

    • Spermatozoa (Sperm)
    • Ova (Eggs)
    • Fusion of gametes results in a zygote that develops into an embryo.
Factors Influencing Reproduction Strategy
  • Many animals utilize both sexual and asexual reproduction. Factors for choosing one over the other include:
    • Favorable Conditions:
    • Abundant food supply
    • Warm temperatures
    • Unfavorable Conditions:
    • Scarcity of food
    • Cool temperatures
    • Overcrowding
    • Accumulation of wastes
Costs of Sexual Reproduction
  • Two-fold cost of sex:
    • Time and energy required to find a mate
    • Fewer offspring produced per reproductive cycle
Hypotheses for Asexual Reproduction Limitations
  • Possible reasons for the rarity of asexual reproduction in animals:
    • Beneficial mutations can spread through sexual reproduction.
    • Harmful mutations can be eliminated more effectively through sexual reproduction.
    • Red Queen Hypothesis: suggests that sexual reproduction provides advantages in a constantly changing environment.
Vertebrate Reproduction Strategies

  • Aquatic Vertebrates (Fish & Amphibians):

    • Utilize external fertilization
    • May broadcast spawn or come close to increase fertilization chances.

  • Terrestrial Vertebrates (Reptiles, Birds, Mammals):

    • Use internal fertilization
    • May also be used by some aquatic species
    • Adaptation includes the concept of the amniotic egg, allowing for reproduction outside water and improving fertilization chances.
Reproductive Strategies: r-strategists vs. K-strategists

  • r-strategists:

    • Produce many offspring (e.g., 8000/year).
    • Minimal parental investment.

  • K-strategists:

    • Produce fewer offspring (e.g., 2/year).
    • Higher parental investment.
Amniotic Egg Evolution
  • Amniotic Egg:
    • Evolved to allow terrestrial animals to lay eggs on land.
    • Reproductive methods include:
    • Oviparity: lay eggs
    • Ovoviviparity: internal egg hatching
    • Viviparity: live birth (common in mammals)
Male and Female Gametes

  • Male Gametes (Sperm Production):

    • Produced in the testes
    • Anatomy components: head (acrosome for penetrating the egg), neck (contains mitochondria), tail (flagellum for locomotion).

  • Female Gametes (Egg Production):

    • Produced in the ovaries
    • Eggs travel through oviduct (fallopian tube) for fertilization and implantation in the uterus for development.
Hormonal Control of Reproduction

  • The hypothalamus releases gonadotropin-releasing hormone (GnRH), triggering the release of:

    • Luteinizing hormone (LH)
    • Follicle-stimulating hormone (FSH)

  • LH and FSH stimulate the release of testosterone and estrogen.

  • Hormonal control impacts the development of secondary sexual characteristics and regulates the menstrual cycle in females.

Ovarian and Uterine Cycles

  • Ovarian Cycle Phases:

    • Follicular phase (follicle grows)
    • Ovulation (follicle released)
    • Luteal phase (corpus luteum forms and degenerates)

  • Uterine Cycle Phases:

    • Menstrual flow
    • Proliferative phase
    • Secretory phase
Menopause and Estrus Cycle
  • Menopause in women occurs due to decreased estradiol and progesterone post-reproductive age.
  • In many non-human mammals, the estrus cycle indicates reproductive receptiveness occurring less frequently than in primates.
Gametogenesis
  • Spermatogenesis: Formation of sperm
  • Oogenesis: Formation of ova:
    • Primary oocyte halts in prophase I before birth, resumes after puberty.
    • Meiosis yields secondary oocyte, which halts in metaphase II until fertilization.
Fertilization and Early Development

  • Sperm introduction in female tract leads to capacitation:

    • Increased motility
    • Loss of membrane proteins
    • Changes in plasma membrane fluidity

  • After fertilization, the oocyte completes meiosis II and changes prevent polyspermy.

  • The zygote undergoes mitosis (cleavage) increasing cell number without size growth.

Embryonic Development Stages
  • Morula: Solid ball of cells
  • Blastula: Fluid-filled structure leading to implantation.
  • As development progresses, the blastocyst forms and undergoes gastrulation producing three germ layers:
    • Ectoderm, Mesoderm, Endoderm.
  • Organogenesis follows, developing major organs from germ layers.
Trimesters of Pregnancy
  • Second and third trimesters primarily focus on fetal growth, doubling in size in the final two months.
  • Childbirth is followed by placenta delivery.