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Aim This study aimed to investigate if and how environmental characteristics (physical factors of the natural environment and the human impact on the landscape) influence the position and structure of a contact zone between two chromosomal races of the house mouse (Mus musculus domesticus Rutty 1712) from the island of Madeira. Location The western part of Madeira, a volcanic island in the North Atlantic. Methods Mice were sampled along a south/north-western transect following the main road, in human-modified outdoor habitats. Karyotypes of mice were determined using the yeast-stimulated bone marrow cell method. Trapping sites were characterized in terms of their physical (altitude, temperature, precipitation and soil type) and habitat (human landscape use and occupancy) features. Demographic parameters of mouse populations, based on trapping-with-removal techniques, were also analysed (relative abundance, sex-ratio, juvenile ratio and female fertility ratio), as well as body size (weight and length). Results Four chromosomal zones were identified on the basis of the frequency of two diagnostic rearrangements (Rb(6.7) in race E. Calheta and Rb(7.15) in race A. Cruz). E. Calheta was present in the two southern-most zones, followed by the contact zone characterized by the presence of two inter-racial hybrids and the co-occurrence of mice belonging to the two races. The northern-most part of the transect was occupied by A. Cruz. Environmental features differed leading us to split the transect into two parts. The southern part is characterized by lower altitude and precipitation, milder temperature, better soil quality supporting vegetable crops and vineyards, and more abundant and evenly distributed human habitats. This southern part is occupied by E. Calheta mice. The north-western part presents characteristics opposite to those described above with cereals as the main cultivated crop, and it includes the contact zone as well as the zone inhabited by A. Cruz mice. The demographic parameters evaluated in this study did not differ significantly between chromosomal zones. Main conclusions This ecological survey highlights differences in climatic and edaphic features that have moulded the agricultural activities of humans, contributing to a differentiation of their spatial development, and hence the structure of potential habitats for mice. Results are interpreted within the source–sink framework of population dynamics, following which E. Calheta may function as a source and the areas where A. Cruz and the contact zone are located may function as a sink. Our study suggests that the position and chromosomal composition of the contact zone is influenced by the human component underlying broader environmental features. Similar characteristics were most likely present during the historical settlement of Madeira. They may have favoured the independent divergence of the two races and influenced the dynamics of the contact zone.

Introduction The genus Arvicanthis (Lesson 1842) (Rodentia: Murinae), usually referred to as the unstriped grass rat, is mainly distributed in savanna and grassland habitats of Sub-Saharan Africa. Among the four chromosomal forms of Arvicanthis recently differentiated in Western and Central Africa, the one with a diploid chromosomal number (2n) of 62 and an autosomal fundamental number (NFa) of 62 or 64 is ascribed to Arvicanthis niloticus (Demarest 1822), while the one with 2n = 62 and a NFa between 74 and 76 is referred to A. ansorgei (Thomas 1910). Despite the broad area of sympatry recently uncovered along the inner delta of the Niger river in Mali [details in Volobouev et al. (2002) Cytogenetics and Genome Research, 96, 250–260], the distribution of the two species is largely parapatric and follows the latitudinal patterns of the West-African biogeographical domains, which are related to the latitudinal patterns of annual rainfall in this region. Here, we analyse the suggestion that the two species show specific adaptations to differences in climate aridity. Methods Karyologically screened animals were sampled in 19 localities in seasonally flooded regions located along the ‘Niger’ river in Mali and extending from 1100 to 200 mm of mean annual rainfall. The analysis of trapping success (TS) data allowed us to investigate the respective effects of climate (i.e. annual rainfall) and local (i.e. duration of the green herbaceous vegetation) aridity on the latitudinal and habitat distribution of the two species. Conclusions The broad zone of sympatry was found to correspond to a northward expansion of the recognized distribution area of A. ansorgei. TS values indicated that the two species responded very differently to climatic and local conditions of aridity. Arvicanthis ansorgei decreased in TS as regional conditions became more arid; a similar trend was also observed within regions where habitat occupancy decreased with local aridity. The higher TS observed in the most humid habitat relative to the others persisted throughout the latitudinal rainfall gradient. In contrast, TS of A. niloticus increased with latitudinal aridity. This species was present in more arid habitats than A. ansorgei from 1000 mm down to 400 mm of mean annual rainfall where a shift to the most humid habitat occurred. These opposite trends in TS distribution between species suggest that A. ansorgei is less adapted than A. niloticus to arid environments at both a regional and habitat level; thus, A. ansorgei would be able to invade dry regions only along the extensive floodplains bordering the inner delta of the ‘Niger’ river. Several biological traits that may be involved in limiting the southward distribution of A. niloticus are discussed.

Although selection against hybridization is expected to generate prezygotic divergence in unimodal hybrid zones, such a pattern has been seldom described. This study aims to better understand how prezygotic mechanisms may evolve in such zones. We investigated prezygotic divergence between populations of two subspecies of mice (Mus musculus musculus and M. m. domesticus) located at the edges of their unimodal hybrid zone in Denmark, and we developed an original multiple-population choice-test design, which allows assessment of within and between subspecies variation. Our study demonstrates that a strong assortative preference characterises one of the two subspecies (musculus) and that urinary signals are involved in this subspecies recognition. Taking into account the specific genetic and geographical characteristics of the Danish hybrid zone, we discuss the influence of the above pattern on its fate and the mechanisms that could have favoured this prezygotic divergence, among which the role of recombined populations constituting the core of the zone.

119–127.Molecular, chromosomal and morphometric analyses of wild-caught mice of subgenus Musfrom the central plain of Thailand are presented. These specimens are distinct from all speciespreviously described in the literature. This has led to the characterization of Mus fragilicaudasp. n., a new member of the set of closely related species encompassed by the subgenus . Whilethis species may be considered as a sibling and sympatric species of the Asian M. cervicolor,M. fragilicauda sp. n. is phylogenetically closer to the M. musculus complex of species and tothe other European species of Mus.Jean-Christophe Auffray & Josette Catalan, Institut des Sciences de l’Evolution, UMR 5554CNRS, CC064, Universite Montpellier 2, 34095 Montpellier cedex, France. E-mail:auffray@isem.univ-montp2.fr.Annie Orth, Eric Desmarais & Francois Bonhomme, Laboratoire Genome, Populations, Interactions,UMR 5000 CNRS, CC063, Universite Montpellier 2, 34095 Montpellier cedex 05, France.Jean-Paul Gonzalez, IRD, Research Center for Emerging Viral Diseases, CVD, Institute of Sciencesand Technology for Development, Mahidol University at Salaya, 25/25 Phutthamonthon 4, Nakhon-pathom 73170, Thailand

To study the colonization history of the house mouse ( Mus musculus domesticus ) on the Madeiran archipelago, complete mitochondrial d- loop sequences were obtained for 44 individuals from Madeira, Porto Santo and Ilhas Desertas. Altogether, 19 d- loop haplotypes were identified which formed part of a single clade in a phylogeny incorporating haplotypes from elsewhere in the range of M. m. domesticus , indicating that the Madeiras were colonized from a single source. Similarities between the sequences found in the Madeiras and those in Scandinavia and northern Germany suggest that northern Europe was the source area, and there is the intriguing possibility that the Vikings may have accidentally brought house mice to the archipelago. However, there is no record of Vikings visiting the Madeiras; on historical grounds, Portugal is the most likely source area for Madeiran mice and further molecular data from Portugal are needed to rule out that possibility.

The introgression pattern of centromeric nucleolar organizer regions (NORs) was studied in house mice from the hybrid zone between Mus musculus musculus and Mus musculus domesticus in Denmark. In this region, the two subspecies are chromosomally differentiated: M. m domesticus carries three pairs of Robertsonian (Rb) fusion chromosomes (2n = 34), while M. m. musculus exhibits the ancestral karyotype of 2n = 40 acrocentric chromosomes. A previous chromosomal analysis showed that the Rb clines were staggered and nonconcordant, and that the Rb fusions did not introgress into the M. m. musculus genome due to incompatibilities involving only the centromeric regions. In the present study, the distribution of a centromeric NOR cluster located on an acrocentric chromosome not involved in the Rb fusions (chromosome 11; NOR' 1) was investigated to determine if the observed centromeric incompatibilities were limited to the chromosomal rearrangements or were related to centromeric differentiation between the subspecies. The cytogenetic study by silver staining documented the activity pattern of the five major NOR-bearing chromosome pairs (12, 15, 16, 18, and 19) common to both subspecies and confirmed presence of NOR' 1 in M. m. musculus and absence in M. m. domesticus. The NOR" I activity pattern showed a very narrow and off-centered clinal transition. An in situ hybridization analysis with rDNA probes indicated that the decrease in frequency of activity of NOR" through the hybrid zone was related to absence of ribosomal genes in this cluster and not to transcription repression due to competitive or dominance interactions. The center of the NOR" I cline was significantly different from that of the consensus allozymic markers, but coincided with that of the steepest Rb cline. Several arguments support the view that the selective processes involved in maintaining the NOR" cline are related to the centromeric region of these chromosomes. These include: (1) the similarity in shape and position of the Rb and NOR" I clines; (2) the absence of activity dysfunction involving NOR"; and (3) the tight linkage between NORs and centro- meres in house mice. This study indicates that the centromeric segments of acrocentric chromosomes not involved in chromosomal rearrangements show incompatibilities similar to those evidenced by the Rb fusions. These centromeric incompatibilities are thus more likely related to centromeric origin and subspecific differentiation (domesticus vs. musculus) than to centromeric rearrangement (Rb vs. non-Rb). In this case, it may be predicted that the selective processes maintaining the chromosomal clines are distributed over many, if not all, the centromeres and may combine to limit the gene flow between M. m. domesticus and M. m. musculus.