M. G. Ramalhinho

Chromosomal phylogeny of Robertsonian races of the house mouse on the island of Madeira: testing between alternative mutational processes.

The ancestral karyotype of the house mouse (Mus musculus) consists of 40 acrocentric chromosomes, but numerous races exist within the domesticus subspecies characterized by different metacentric chromosomes formed by the joining at the centromere of two acrocentrics. An exemplary case is present on the island of Madeira where six highly divergent chromosomal races have accumulated different combinations of 20 metacentrics in 500-1000 years. Chromosomal cladistic phylogenies were performed to test the relative performance of Robertsonian (Rb) fusions, Rb fissions and whole-arm reciprocal translocations (WARTs) in resolving relationships between the chromosomal races. The different trees yielded roughly similar topologies, but varied in the number of steps and branch support. The analyses using Rb fusions/fissions as characters resulted in poorly supported trees requiring six to eight homoplasious events. Allowance for WARTs considerably increased nodal support and yielded the most parsimonious trees since homoplasy was reduced to a single event. The WART-based trees required five to nine WARTs and 12 to 16 Rb fusions. These analyses provide support for the role of WARTs in generating the extensive chromosomal diversification observed in house mice. The repeated occurrence of Rb fusions and WARTs highlights the contribution of centromere-related rearrangements to accelerated rates of chromosomal change in the house mouse.

Molecular studies on the colonization of the Madeiran archipelago by house mice

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.

Induction of micronuclei and sister chromatid exchange in bone-marrow cells and abnormalities in sperm of Algerian mice (Mus spretus) exposed to cadmium, lead and zinc

As a consequence of human activities, large amounts of cadmium, lead and zinc are released in the environment, often simultaneously. The aim of this study was to investigate under experimental conditions the DNA damage induced in Algerian mice (Mus spretus) exposed to cadmium (Cd), lead (Pb) and zinc (Zn) separately, or in selected combinations. Three cytogenetic end points were considered: the frequencies of micronucleated cells (MN) and sister chromatid exchange (SCE) in the bone marrow and the frequency of sperm abnormalities. Mice were treated by intraperitoneal (i.p.) injections with 5 or 10 doses of aqueous solutions of cadmium acetate, lead acetate and zinc acetate in concentrations corresponding to 1/10 of the LD50, respectively, 21.5, 0.46 and 1.5 mg/kg bw. The control groups were injected in the same way with distilled water. With only one exception (Cd + Zn group treated with 5 doses), the results show a significant increase of MN in all groups for both treatments (5 and 10 doses). Similarly, the results concerning the SCE revealed a statistically significant increase in all treated animals, with the exception of the Zn group treated with 5 doses. The number of sperm abnormalities was significantly higher in animals treated with 5 doses, except in the group Pb + Zn. In animals treated with 10 doses the number of sperm abnormalities was always statistically higher compared with controls. This study indicates that cadmium, lead and zinc can induce MN, SCEs and sperm abnormalities in Algerian mice and that the clastogenic potential is dependent on the time of exposure and the interaction between the three elements, confirming the environmental damage that may result from the simultaneous action of several metals. Most relevant is the toxic potential for Zn, related with the dose, which may compromise its protective effect against other metal contaminations, such as cadmium.

The non-random occurrence of Robertsonian fusion in the house mouse

Chromosomal rearrangements such as Robertsonian (Rb) fusions constitute a major phenomenon in the evolution of genome organization in a wide range of organisms. Although proximate mechanisms for the formation of Rb fusion are now well identified, the evolutionary forces that drive chromosomal evolution remain poorly understood. In the house mouse, numerous chromosomal races occur in nature, each defined by a unique combination of Rb fusions. Among the 106 different Rb fusions that were reported from natural populations, the low involvement of chromosome 19 in Rb fusions is striking, prompting the question of the randomness of chromosomal involvement in Rb fusions. We uncover a significant quadratic relationship between chromosome size and probability of fusing, which has never previously been in this species. It appears that fusions involving chromosome 19 are not particularly infrequent, given the expected low fusion probability associated with the chromosomes size. The results are discussed, assessing selective processes or constraints that may operate on chromosome size.

Rodents and Leptospira transmission risk in Terceira island (Azores).

The role of rodents as Leptospira renal carriers in Terceira island was evaluated (1993–1995) through kidney culture and serology [microscopic aglutination test (MAT)] of 94 mice and rats. Fifty-nine animals were positive (n = 41 by serology + culturing; n = 11 serology; n = 7 culturing), presenting a wide distribution in man-made and natural areas. House mice had the highest bacteriological (82.9%) and serological (90.9%) rates, being strictly related to serovar arborea. Black rats were involved in the dispersion of all isolated L. interrogans sensu lato serovars (arborea,copenhageni and icterohaemorrhagiae). Logistic regression analysis and non-metric multi-dimensional scaling, relating Leptospira infection with biological and environmental variables, expressed that adult males Mus domesticus, sexually active and living in humid biotopes, mainly above 500 m, are the most likely reservoirs. This study emphasizes the role of house-mice in the epidemiology of leptospirosis in Terceira and the need of reducing the risk of Leptospira transmission through integrated control programmes, primarily focusing on adult house-mice in peri-domestic environments, before the breeding season.

Molecular insights into the colonization and chromosomal diversification of Madeiran house mice

The colonization history of Madeiran house mice was investigated by analysing the complete mitochondrial (mt) D‐loop sequences of 156 mice from the island of Madeira and mainland Portugal, extending on previous studies. The numbers of mtDNA haplotypes from Madeira and mainland Portugal were substantially increased (17 and 14 new haplotypes respectively), and phylogenetic analysis confirmed the previously reported link between the Madeiran archipelago and northern Europe. Sequence analysis revealed the presence of four mtDNA lineages in mainland Portugal, of which one was particularly common and widespread (termed the ‘Portugal Main Clade’). There was no support for population bottlenecks during the formation of the six Robertsonian chromosome races on the island of Madeira, and D‐loop sequence variation was not found to be structured according to karyotype. The colonization time of the Madeiran archipelago by Mus musculus domesticus was approached using two molecular dating methods (mismatch distribution and Bayesian skyline plot). Time estimates based on D‐loop sequence variation at mainland sites (including previously published data from France and Turkey) were evaluated in the context of the zooarchaeological record of M. m. domesticus. A range of values for mutation rate (μ) and number of mouse generations per year was considered in these analyses because of the uncertainty surrounding these two parameters. The colonization of Portugal and Madeira by house mice is discussed in the context of the best‐supported parameter values. In keeping with recent studies, our results suggest that mutation rate estimates based on interspecific divergence lead to gross overestimates concerning the timing of recent within‐species events.

Patterns of genic diversity and structure in a species undergoing rapid chromosomal radiation: an allozyme analysis of house mice from the Madeira archipelago

The chromosomal radiation of the house mouse in the island of Madeira most likely involved a human-mediated colonization event followed by within-island geographical isolation and recurrent episodes of genetic drift. The genetic signature of such processes was assessed by an allozyme analysis of the chromosomal races from Madeira. No trace of a decrease in diversity was observed suggesting the possibility of large founder or bottleneck sizes, multiple introductions and/or a high post-colonization expansion rate. The Madeira populations were more closely related to those of Portugal than to other continental regions, in agreement with the documented human colonization of the island. Such a Portuguese origin contrasts with a study indicating a north European source of the mitochondrial haplotypes present in the Madeira mice. This apparent discrepancy may be resolved if not one but two colonization events took place, an initial north European introduction followed by a later one from Portugal. Asymmetrical reproduction between these mice would have resulted in a maternal north European signature with a nuclear Portuguese genome. The extensive chromosomal divergence of the races in Madeira is expected to contribute to their genic divergence. However, there was no significant correlation between chromosomal and allozyme distances. This low apparent chromosomal impact on genic differentiation may be related to the short time since the onset of karyotypic divergence, as the strength of the chromosomal barrier will become significant only at later stages.

R2d2 Drives Selfish Sweeps in the House Mouse

A selective sweep is the result of strong positive selection driving newly occurring or standing genetic variants to fixation, and can dramatically alter the pattern and distribution of allelic diversity in a population. Population-level sequencing data have enabled discoveries of selective sweeps associated with genes involved in recent adaptations in many species. In contrast, much debate but little evidence addresses whether “selfish” genes are capable of fixation—thereby leaving signatures identical to classical selective sweeps—despite being neutral or deleterious to organismal fitness. We previously described R2d2, a large copy-number variant that causes nonrandom segregation of mouse Chromosome 2 in females due to meiotic drive. Here we show population-genetic data consistent with a selfish sweep driven by alleles of R2d2 with high copy number (R2d2HC) in natural populations. We replicate this finding in multiple closed breeding populations from six outbred backgrounds segregating for R2d2 alleles. We find that R2d2HC rapidly increases in frequency, and in most cases becomes fixed in significantly fewer generations than can be explained by genetic drift. R2d2HC is also associated with significantly reduced litter sizes in heterozygous mothers, making it a true selfish allele. Our data provide direct evidence of populations actively undergoing selfish sweeps, and demonstrate that meiotic drive can rapidly alter the genomic landscape in favor of mutations with neutral or even negative effects on overall Darwinian fitness. Further study will reveal the incidence of selfish sweeps, and will elucidate the relative contributions of selfish genes, adaptation and genetic drift to evolution.

Karyotype of dormice Eliomys quercinus from Tirol (Austria)

A karyotype of 2n = 52 chromosomes was found in twoEliomys quercinus (Linnaeus, 1766) specimens from two different localities of Tirol (Austria). The karyotype is similar to the one described in the Italian Alps, suggesting that these mountains were not a barrier to the northern expansion of this chromosomal race.

Detection of antibodies against Anaplasma phagocytophilum in Algerian mice (Mus spretus), Portugal.

The recent detection of Anaplasma phagocytophilum in Portugal stimulated further research on the agents enzootic cycle, which usually involves rodents. Thus a total 322 rodents belonging to five species, including 30 Apodemus sylvaticus (wood mouse), 65 Mus musculus (house mouse), 194 M. spretus (algerian mouse), 5 Rattus norvegicus (brown rat) and 28 R. rattus (black rat), were studied by indirect immunofluorescent assay (IFA) and/or polymerase chain reaction (PCR) for A. phagocytophilum exposure in four sampling areas of mainland and two areas of Madeira Island, Portugal. Overall, 3.6% (7/194) of M. spretus presented with IFA-positive results. Seropositive mice were detected in all three mainland sampling areas where this species was captured, with prevalence of 5.2% (5/96) and 5.0% (1/20) for the Ixodes-areas of Arrábida and Mafra, and 1.3% (1/78) for Mértola, a difference that was not statistically significant (p > 0.05). The majority of IFA-positive mice were detected in spring when considering either Arrábida alone (p = 0.026) or all M. spretus sampling areas together (p = 0.021), although the significance of this association was not evident after Bonferroni correction. Nevertheless, neither the seropositive M. spretus, nor additional samples of 10% seronegative rodents from mainland, and 16% of rodents collected in Madeira Island showed evidence of A. phagocytophilum active infections when spleen and/or lung samples were tested by PCR. Either the M. spretus results represents residual antibodies from past A. phagocytophilum infections, present infections with limited bacteremia, or cross-reactions with closely related agents deserves more investigation.