CIE A Level Biology复习笔记18.3.4 Assisted Reproduction

• Endangered mammals tend to have small and isolated populations
• Small populations are prone to inbreeding and inbreeding depression
  • Inbreeding depression is the reduction in fitness of a population due to breeding between closely related individuals and the resulting increased homozygosity
• When populations are isolated it can also be difficult for individuals to find suitable reproductive mates
• Previously large mammals were transported between zoos in captive breeding programs
  • Advantage: Humans were able to monitor the health of the mother and foetus
  • Disadvantage: It was highly expensive and unreliable as sometimes individuals would refuse to mate
• Science has come up with several solutions for inbreeding and the lack of reproductive mates in endangered mammals

IVF

• In vitro fertilization involves the fertilization of an egg outside of the female body
  • For example in a test tube or petri dish
• Method:
  • A needle is inserted into the female’s ovaries and eggs are extracted
  • The eggs are kept in a culture medium for a short amount of time
  • Male semen is mixed with the eggs so fertilization can occur
  • Several zygotes form and develop into embryos
  • The embryos are placed in a culture for several days
  • The embryos are transferred either into the mother, or another female
• IVF allows is advantageous over natural mating as it allows humans to control and confirm fertilization of the embryo

Embryo transfer

• Pregnancies are high risk for females; complications can arise which in some cases prove fatal
• Since the population numbers of an endangered species are already very low each reproductive female is of very high value and importance
• Embryo transfer can be used to avoid the risks of pregnancy for the vulnerable female so that she can provide many eggs for multiple offspring
• Method:
  • An egg belonging to a female of the vulnerable species is fertilized by the sperm belonging to a male of the same species
  • A zygote forms which develops into an embryo
  • After fertilization, the embryo is taken from the uterus of the female and transferred to a surrogate female
  • The embryo develops to full term and the offspring is born
• The surrogate mother can be from another non-vulnerable species
• This technique has been used to try and conserve populations of several different species of African antelope

Surrogacy

• A surrogate is any female that becomes pregnant with the embryo from another female and carries the embryo to full term
• Surrogate mothers require hormone treatment before they receive an embryo
  • The hormones ensure that her uterus is in the right condition for the embryo to embed
• There are multiple ways in which the embryo might have been conceived:
  • Naturally
  • Artificial insemination (semen from the male is injected into the uterus of the female)
  • IVF
• A surrogate female can be the same or different species to the biological mother of the embryo
  • If it is a different species it needs to be closely related to ensure compatibility of the embryo and uterus