<P> Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization . </P> <P> Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair . However, various species, including the Colombian Rainbow boa (Epicrates maurus), Agkistrodon contortrix (copperhead snake) and Agkistrodon piscivorus (cotton mouth snake) can also reproduce by facultative parthenogenesis - that is, they are capable of switching from a sexual mode of reproduction to an asexual mode - resulting in production of WW female progeny . The WW females are likely produced by terminal automixis . </P> <P> Mole salamanders are an ancient (2.4 - 3.8 million year - old) unisexual vertebrate lineage . In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes . As a result, the mature eggs produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the female salamander . Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes . Thus little, if any, genetic variation is produced . Recombination between homeologous chromosomes occurs only rarely, if at all . Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long - term maintenance of meiosis in these organisms . However, meiosis may have been maintained during evolution by the efficient recombinational repair of DNA damages that meiosis provides, an advantage that could be realized at each generation . </P> <P> The mangrove killifish (Kryptolebias marmoratus) produces both eggs and sperm by meiosis and routinely reproduces by self - fertilisation . The capacity for selfing in these fishes has apparently persisted for at least several hundred thousand years . Each individual hermaphrodite normally fertilizes itself when an egg and sperm that it has produced by an internal organ unite inside the fish's body . 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 . </P>

Which of the following is unique about vertebrates in terms of the phylum chordata