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%).