Abortive development in the crested newt Triturus cristatus

Crested newts exhibit a balanced lethal system which causes development to abort in 50% of tailbud embryos. The main features of arrested embryos are described, shown to be constant in a range of culture conditions and found to be unchanged by parabiosis or extirpation of the central nervous system or heart. Two categories of abortive embryos can be distinguished in some spawnings. A corresponding segregation of an easily identified marker chromosome confirms previous reports that chromosome 1 carries the recessive lethal factors. The marker chromosome is tentatively associated with the blistered form of abortive embryo. Two possible modes of origin for the balanced lethal system are discussed, either from ancestral autosomes or from sex chromosomes. The latter is favoured because it involves fewer assumptions. The subsequent evolution of the system should inevitably lead to progressively earlier embryonic mortality by an accumulation of multiple recessive lethal factors on each chromosome variant.