The phenomenon of multiple embryos in birds and other oviparous vertebrates is very rare and thus poorly studied. Some populations of wild birds or lines of poultry, including the zatorska goose, may be particularly predisposed to developing monovular twin embryos (MTE). The aim of the study was to investigate the frequency of single yolk twin embryos in the zatorska goose and to establish whether these embryos are generated as a result of polyembryony or other developmental anomalies.Embryopathological analysis, which included identification of the developmental stages of all MTEs at the time of death, was carried out on 1,955 unhatched eggs obtained from 20 pedigree breeding flocks of the zatorska goose. Post-mortem examinations revealed as many as 13 MTEs (0.66% compared to embryonated eggs) at varying developmental stages.The results of genotyping of the microsatellite loci of 7 pairs of twins demonstrated genetic similarity in 6 of them; this suggests that they developed from 1 blastoderm, while differences in 2 loci of 1 pair show that the MTE originates from 2 blastodiscs. Pedigree analysis of the MTEs does not provide evidence that predisposition toward twinning results from the genetic background. However, more frequent MTE occurrence could be seen in eggs stored for more than 8 d. It seems that embryos of the zatorska goose are more susceptible to duplication under special conditions, such as prolonged egg storage.

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