Vertebrate Reproduction 1

Vertebrate Reproduction

General features

  • Timing of sexual maturity
  • Environmental factors
    • Photoperiod, temperature, breeding sites etc
    • Act on hypothalamic-pituitary system
      • Hormones control breeding, courtship + gonadal development

Reproduction definitions

  • Semelparous - breed once then die 
  • Iteroparous - 2 or more reproductive cycles in lifetime (most common)
  • Associated reproductive pattern - male androgens highest at mating (most common)
  • Dissociated reproductive pattern - mating occurs when androgens reduced
  • Oviparous - eggs
  • Viviparous - live young
    • necessitates internal fertilisation

Sex determination

  • 3 types
    • Genotypic
      • Differences in 1 pair chromosomes
      • Homogametic sex - chromosomes identical
        • 'default'
      • Heterogametic sex - chromosomes different
        • Requires gonadal steroid hormones to overcome default
          • SRY gene - Y chromosome, initiates testosterone release
      • Sex chromosomes evolved separatelyin birds + mammals
    • Temperature-dependant
      • High/Low temperature determines sex
      • Intermediate temperature = ratio of both sexes
      • Oestrogens can reverse 'masculinising temperatures'
      • Androgens cannot
    • Behavioural
      • Hermaphroditism
        • Sequential and simultaneous
      • EXAMPLE: female saddleback wrasse changes sex if there are too few males present

Hypothalamic-pituitary axis

  • Ultimate control of reproduction 
  • Ventral brain
  • Hypothalamus - neuro-endocrine organ
  • Interface - nervous + endocrine systems
  • Receives nerve impulses
  • Transduces signal - chemical stimuli
  • 2 kinds neuropeptide hormones act on pituitary
    • Gonadotropin-releasing hormones (GnRHs)
    • Gonadotropin-inhibiting hormones (GnIHs)
  • 2 main gonadotropins
    • Follicle Stimulating Hormone (FSH)
      • Males
        • Spermatogenesis
      • Females
        • Maturation ovarian follicles
        • Inhibited by corpus luteum
        • If implantation fails, CL degenerates, FSH inhibition stops, follice development restarts
    • Luteinizing hormone (LH)
      • Males 
        • Interstitial cells
        • Testosterone inhibits overproduction
      • Females
        • causes ovulation

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