First wild XXY house mice

Laboratory house mice (Mus musculus) with the XXY condition can be generated with ease and have been used as a biomedical model. However, although the XXY constitution has been described in humans and many domestic and wild mammal species, and a very large number of wild house mice have been karyotyped previously, no wild individuals of M. musculus with an XXY karyotype have ever been reported. Therefore, it is rather extraordinary that two wild XXY house mice were caught by us on two different farms in northern Italy in 2008. Except for the extra X chromosome, one male had a standard karyotype (2n = 40) and the other, the karyotype of the Cremona metacentric population (2n = 22). In this paper, the phenotype of these two individuals is described. Observations for both of these wild males agree with those of laboratory XXY mice, i.e., they had a normal body mass and appearance, but significantly smaller testes than normal, and no visible germ cells. The incidence of the XXY chromosome anomaly in wild mice (two among 5,123 wild mice surveyed by us and our colleagues, i.e., approximately 0.08% among wild-caught males) is intermediate between that found in male laboratory mice (approximately 0.04%) and that found in male humans (0.2%).