section_path: Reproductive systems/Self-fertilization
section_name: Self-fertilization
paragraph_text: Self-fertilization
  
  Two killifish species, the mangrove killifish (Kryptolebias marmoratus) and Kryptolebias hermaphroditus, are the only known vertebrates to self-fertilize. They produce eggs and sperm by meiosis and routinely reproduce by self-fertilisation. This capacity has apparently persisted for at least several hundred thousand years. Each individual hermaphrodite normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ. In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another. Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of fertilization assurance (reproductive assurance) at each generation.