A repeat DNA sequence from the Y chromosome in species of the genus Microtus J. A. Marchal 1 , M. J. Acosta 1 , M. Bullejos 1 , R. Dı´ az de la Guardia 2 & A. Sa ´nchez 1* 1 Departamento de Biolog {a Experimental, Facultad de Ciencias Experimentales y de la Salud, Universidad de Jae ´n, E-23071 Jae ´n, Spain; Tel: 34-953-002528; Fax: 34-953-012141; E-mail: abaca@ujaen.es; 2 Departamento de Gene ´tica, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain *Correspondence Received 1 April 2004. Received in revised form and accepted for publication by Jennifer Marshall Graves 3 August 2004 Key words: heterochromatin, Microtus, repeat DNA sequences, sex chromosomes Abstract In most mammals, the Y chromosome is composed of a large amount of constitutive heterochromatin. In some Microtus species, this feature is also extended to the X chromosome, resulting in enlarged (giant) sex chromosomes. Several repeated DNA sequences have been described in the gonosomal heterochromatin of these species, indicating that it has heterogeneous and species-speci¢c composition and distribution. We have cloned an AT-rich, 851-bp long, repeated DNA sequence speci¢c for M. cabrerae Ychromosomeheterochromatin.Theanalysisofotherspeciesofthegenus Microtus indicatedthat this sequence is also located on the Y chromosome (male-speci¢c) in three species (M. agrestis, M. oeconomus and M. nivalis),presentonbothYandXchromosomesandonsomeautosomesin M. arvalis and absentinthegenomeof M. guentheri. Our data also suggest that the mechanism of heterochromatin ampli- ¢cation operating on the sex chromosomes could have been different in each species since the repeated sequences of the gonosomal heterochromatic blocks in M. cabrerae and M. agrestis are different. The absence of this sequence in the mouse genome indicates that its evolutionary origin could be recent. Future analysis of the species distribution, localization and sequence of this repeat DNA family in arvicolid rodent species could help to establish the unsolved phylogenetic relationships in this rodent group. Introduction Mammalian Y chromosome is small, largely het- erochromatic and exceptionally gene poor. In fact, the majority of human and mouse Y chro- mosomes consist of repeated DNA sequences, located mainly in the heterochromatin of the long arm. Similarly, the Y chromosome of most species of the genus Microtus is mainly formed by constitutive heterochromatin. This feature is extended also to the X chromosome in some species of this genus (M. agrestis, M. cabrerae, M. chrotorrhinus, M. epiroticus and M. transcas- picus), showing very enlarged (giant) sex chromo- somes due to the presence of large blocks of constitutive heterochromatin (Modi 1987, Burgos et al. 1988b, 1988c, Singh et al. 2000). Both cytogenetic and molecular analyses have demonstrated that the gonosomal constitutive heterochromatin in the genus Microtus is highly Chromosome Research 12: 757–765, 2004. 757 # 2004 Kluwer Academic Publishers. Printed in the Netherlands