ORIGINAL INVESTIGATION First reports of XXY aneuploidy in natural populations of Thomas’ pine vole Microtus thomasi (Rodentia: Arvicolidae) from Greece Michail Th. Rovatsos à , George P. Mitsainas, Costas Stamatopoulos, Eva B. Giagia-Athanasopoulou Department of Biology, Laboratory of Zoology, Section of Animal Biology, University of Patras, GR-26500 Rio, Greece Received 17 January 2007; accepted 25 October 2007 Abstract As part of an extensive cytological study in natural populations of Thomas’ pine vole Microtus thomasi, two cases of XXY aneuploidy (2n ¼ 45, FN ¼ 47) from different localities of NW Peloponnesus, Greece were recently recorded. These constitute the first report of sex chromosome aneuploidy for this species, in a sample of 366 individuals from 89 localities of Greece. Both individuals appeared with a male phenotype and normal morphology; however, measurements and histological examination in the testes of one of the two individuals imply a severely impaired fertility. The above findings translate to a relatively high rate of sex chromosome aneuploidy (0.55%) (upper 95% CI 1.96%) for the natural populations of Microtus thomasi and possible reasons for this are discussed. r 2007 Deutsche Gesellschaft fu¨r Sa¨ugetierkunde. Published by Elsevier GmbH. All rights reserved. keywords: Microtus thomasi; Kleinefelter’s syndrome; asynaptic; male sterility; sex chromosomes Introduction The natural populations of Thomas’ pine vole Microtus thomasi are characterized by a remarkable karyotypic variation, resulting until today in the occurrence of four different parapatric chromosomal variants, i.e. karyomorphs: ‘‘thomasi’’ (2n ¼ 44, FN ¼ 44), ‘‘atticus’’ (2n ¼ 44, FN ¼ 46), ‘‘subalpine’’ (2n ¼ 42, FN ¼ 42) and ‘‘Rb-subalpine’’ (2n ¼ 40, FN ¼ 42), the first of which is considered to be the ancestral form. All the chromosomal rearrangements responsible for this variation involve autosomal chro- mosomes, with the exception of the ‘‘atticus’’ karyo- morph, which had been considered to have derived from the ‘‘thomasi’’ karyomorph, through a pericentric inversion on the X chromosome. This remarkable chromosomal variation among Microtus thomasi natural populations of the SW Balkan Peninsula has been under elaborate investigation for more than 30 years (Giagia and Ondrias 1973; Giagia 1985; Giagia-Athanasopou- lou et al. 1995; Giagia-Athanasopoulou and Stamato- poulos 1997) and reinforces the belief that chromosomal rearrangements occur commonly in many mammalian species, especially rodents (Redi and Capanna 1988; Zima 2000; Zima 2004). In contrast, sex chromosome aneuploidy has been recorded in relatively few species of mammals. As expected, such aneuploidies have been more extensively studied in humans, often in relation to their phenotypic consequences (e.g. Hamerton et al. 1975; De Braekeleer and Dao 1991; Mandoki et al. 1991; Sayli et al. 1994; Mark et al. 1999). Other mammalian species, have been scantily studied from this aspect and the majority of the gathered data are either related to domestic animals (dog, cat, pig, horse, goat, etc.) (Breeuwsma 1968; ARTICLE IN PRESS www.elsevier.de/mambio 1616-5047/$ - see front matter r 2007 Deutsche Gesellschaft fu¨r Sa¨ugetierkunde. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.mambio.2007.10.010 Mamm. biol. 73 (2008) 342–349 à Corresponding author. E-mail address: mirovatsos@upnet.gr (M.T. Rovatsos).