Genetically identical avian twins

We report on the first established case of identical twinning in birds. The occurrence, genetic identity and subsequent development of identical emu ( Dromaius novaehollandiae) twins is described, and comparisons drawn with si ngle chicks hatched in the same season. The twin-bearing egg, significantly larger than any other egg in the same clutch, was artificially incubated a nd hatching assistance was necessary. At hatching the female twins were sma ll, but their combined weight approximated that of a single chick from an e gg of equivalent weight, and by 18 months of age they were near average in size. DNA analysis of blood collected from the twins showed a complete matc h with 22 bands per sample larger than 3.8 kb. The estimated probability of this occurring by chance was 1.0 x 10(-13) in two unrelated emu and 1.0 x 10(-6) in siblings, indicating a high probability that the twins were ident ical.