Genetic diversity and population size: island populations of the common shrew, Sorex araneus

Populations of many species are currently being fragmented and reduced by human interactions. These processes will tend to reduce genetic diversity within populations and reduce individual heterozygosities because of genetic drift, inbreeding and reduced migration. Conservation biologists need to know the effect of population size on genetic diversity, as this is likely to influence a population's ability to persist. Island populations represent an ideal natural experiment with which to study this problem. In a study of common shrews (Sorex araneus) on offshore Scottish islands, 497 individuals from 13 islands of different sizes and 6 regions on the mainland were trapped and genotyped at eight microsatellite loci. Previous genetic work had revealed that most of the islands in this study were highly genetically divergent from one another and the mainland. We found that most of the islands exhibited lower genetic diversity than the mainland populations. In the island populations, mean expected heterozygosity, mean observed heterozygosity and mean allelic richness were significantly positively correlated with log island size and log population size, which were estimated using habitat population density data and application of a Geographic Information System.     

Dental arch asymmetry in an isolated Adriatic community

Developmental stability reflects the ability of a genotype to develop in the same way under varying environmental conditions. Deviations from developmental stability, arising from disruptive effects of environmental and genetic stresses, can be measured in terms of fluctuating asymmetry, a particularly sensitive indicator of the ability to cope with these stresses during ontogeny. In an inbred Adriatic island population, we expected dental arch fluctuating asymmetry 1) to be higher than in an outbred sample from the same island, and 2) within this population, to increase with the level of inbreeding. Due to environmental stress, we also expected to find higher fluctuating asymmetry in the outbred island population than in an urban reference group from the same country. The material consisted of 506 dental casts of 253 children from 1) the island of Hvar, and 2) Zagreb, Croatia. Three-dimensional coordinates of 26 landmarks spanning the arches were digitized. The analysis partitioned the asymmetry of arch forms into components for directional and fluctuating bilateral asymmetry, using the appropriate Procrustes method (geometric morphometrics). The results corroborated the hypotheses. Fluctuating asymmetry was found to be higher on the island than in Zagreb in all groups and in both jaws, and increased significantly with endogamy level in the lower jaw. There was no significant directional asymmetry in the Zagreb sample and likewise none in the upper jaws of the outbred island group, but significant directional asymmetry in both jaws of the inbred population and also in the lower jaws of the outbred island group. These results suggest an environmental as well as a genetic influence on dental arch asymmetry. Although the lower jaws expressed these two stresses almost additively, the upper jaws appeared to be better buffered. The role of directional asymmetry as a potential indicator of craniofacial developmental instability clearly merits further attention.     

Ecomorphometric variation and sexual dimorphism in the common shrew (Sorex araneus)

We investigated the evolution of the biomechanics of the mandible in island and mainland populations of the common shrew on the west coast of Scotland. We predicted that climatic differences between populations should cause differences in prey composition leading to changes in the mechanical potential (MP) of the mandible. In females, MP was correlated with climate, with greater MP in warmer and drier habitats. In males, MP was significantly greater than in females but there was no relationship between male MP and climate. This led to increased sexual dimorphism in colder and wetter climates. The same pattern was found after a phylogenetic least squares analysis was conducted to account for shared phylogenetic history. We discuss possible reasons for this pattern, including male–male combat and the greater necessity of females to feed as efficiently as possible to meet their extremely high energy requirements during lactation.     

Evidence for echolocation in the common shrew, Sorex araneus

In this laboratory experiment it is shown that, like four North American soricid shrew species, the European common shrew Sorex araneus L. is able to use echolocation to identify open and closed tubes at a distance of 200 mm.
Three common shrews captured in Sweden were used for the experiments, which were carried out in darkness and within a sound-proof box. The experimental set-up eliminated orientation using sight, sound or scent from outside the experimental cage. Echolocation calls consisted of broadband ultrasonic clicks at low sound pressure. These were recorded using an ultrasound detector.
The ecological significance of echolocation in shrews is discussed. It is proposed that common shrews use echolocation to locate protective cover, thus minimizing the risk to be taken by, e.g. owls.
Echolocation may also be used for detecting obstacles in subterranean tunnels. Hence, echolocation could be of certain importance when abandoned burrows in the periphery of the tunnel system are restored during periods of increasing population densities. Since density peaks in most populations occur regularly each summer, and may reach extreme magnitudes in cyclic populations, the ecological significance of echolocation in shrews may be considerabl.     

Nondisjunction frequencies in Robertsonian heterozygotes from natural populations of the common shrew, Sorex araneus L

Pregnant female common shrews were collected from an area of Robertsonian polymorphism, involving five different arm combinations, around Oxford (England). The females and their fetuses were karyotyped, and the karyotypes of the sires were deduced. Ten pregnancies where at least one parent was known to be either a single or double Robertsonian heterozygote were available for analysis. From these pregnancies, upper and lower estimates of anaphase I nondisjunction arising from Robertsonian heterozygosity were calculated to be 1.0-2.5% per heterozygous arm combination and 1.5-3.7% per heterozygous parent. One trisomic fetus with retarded development was identified. This trisomy can be attributed to anaphase I nondisjunction of a trivalent in a female Robertsonian heterozygote.     

Meiotic studies of Robertsonian heterozygotes from natural populations of the common shrew, Sorex araneus L

Adult male common shrews, both Robertsonian heterozygotes and homozygotes, were collected from Oxford and elsewhere in Britain. In both simple Robertsonian heterozygotes and Robertsonian heterozygotes with monobrachial homology, regular chain configurations were observed at meiosis I; only 1-2% were incomplete such that univalents were observed. On the average, there was one chiasma per chromosome arm among those that displayed Robertsonian variation, including both chain configurations and bivalents. According to one hypothesis, a single chiasma per chromosome arm may facilitate proper disjunction of chain trivalents of simple Robertsonian heterozygotes. Based on metaphase II counts, anaphase I nondisjunction frequency can be estimated as 1.0% per heterozygous individual and 0.7% per heterozygous arm combination.     

Impact of a population bottleneck on symmetry and genetic diversity in the northern elephant seal

Abstract The northern elephant seal (NES) suffered a severe population bottleneck towards the end of the nineteenth century. Theoretical expectations for the impact of population bottlenecks include the loss of genetic diversity and a loss of fitness (e.g. through a disruption of developmental stability); however, there are few direct demonstrations in natural populations. Here, we report on the comparison of archive samples collected prior to and following the NES population bottleneck. Measures of genetic diversity show a loss of variation consistent with expectations and suggest a strong disruption in the pattern of allele frequencies following the bottleneck. Measures of bilateral characters show an increase in fluctuating asymmetry.     

Allozyme variability in common shrew Sorex araneus of Western Siberia

The allozyme variability of the common shrew Sorex araneus of Western Siberia is studied. The small level of genetic variability and lack of reliable differences in indices of polymorphism for taiga, subtaiga, and forest-steppe populations are revealed. A significant level of genetic differentiation in populations belonging to adjacent chromosomal races—Novosibirsk and Serov (F _st = 0.3) — has been shown.     

Spermatogenesis in heterozygotes for Robertsonian chromosomal rearrangements from natural populations of the common shrew, Sorex araneus

Twenty-two adult male common shrews were collected from 5 sites in the vicinity of Oxford (UK) close to the zone of hybridization between two karyotypic races. The shrews were subdivided into 3 karyotypic categories: homozygotes, simple Robertsonian heterozygotes (which form one or more trivalents at prophase I of meiosis) and complex Robertsonian heterozygotes (which form a quadrivalent). The ratio of primary spermatocytes to round spermatids was determined from transverse sections of seminiferous tubules, to provide an indication of germ cell death. In no individual was there severe germ cells loss. Homozygotes had the highest mean spermatocyte: spermatid ratio and complex heterozygotes the lowest, but there was substantial individual variation and the differences were not significant. Complex heterozygotes also had a higher proportion of defective seminiferous tubules and lower testis weights than did other categories and it is reasonable to propose that, as a population, complex heterozygotes had reduced fitness relative to other categories on the basis of spermatogenic performance. However, there is no evidence from studies of spermatogenesis that simple Robertsonian heterozygotes are less fit than homozygotes.     

Cases of coat colour anomalies in the common shrew, Sorex araneus L

Coat colour anomalies in the common shrew, Sorex araneus L., in the geographical range of this species, including Poland, are extremely rare. This study describes atypically coloured common shrews. Light colouration of the coat is a result of lack ofpigment in the entire hair or hair fragments. It appears that atypically coloured shrews occur more often in isolated populations whose gene transfer with neighboring populations is limited.     

Finger ridge-count asymmetry and diversity in Andean Indians and interpopulation comparisons

A separate analysis of ulnar and radial finger ridge-counts, obtained from 115 Aymara Indians (55 males and 60 females) of northern Chile, was performed. From these variables, directional asymmetry, fluctuating asymmetry, indices of bilateral asymmetry (√A²), and intraindividual diversity (s/√5) were calculated for each sex. The results show that most bimanual differences for the ridge-counts are not statistically significant in the Aymara, except for radial counts in female first and second fingers (right hand means are larger), while most ulnar-radial differences are highly significant in both sexes (radial values exceed ulnar ones). Most sex differences do not reach statistical significance, although males have more ridge-counts, lower directional asymmetry, somewhat lower fluctuating asymmetry, and lower indices of asymmetry and diversity than females. As fluctuating asymmetry is not larger in males, the dermatoglyphic findings do not indicate support for the hypothesis that males are less canalized than females. In accordance with the findings of other authors, interpopulation comparisons in the indices of asymmetry and diversity show ethnic differences. Both indices tend to be low in samples of African ancestry, high in samples of European origin, and intermediate in the Aymara, while Indian groups are characterized by high asymmetry and low diversity values. Moreover, the data reveal a geographical trend in that asymmetry and diversity values tend to decrease from the northern to the southern hemisphere in populations of Europe, the Middle East, and Africa, thus indicating greater ridge-count variability and heterogeneity among fingers in northern populations. It is assumed that this gradient primarily reflects different degrees of miscegenation and heterozygosity. Am J Phys Anthropol 105:377–393, 1998. © 1998 Wiley-Liss, Inc.     

Factors responsible for a karyotypic polymorphism in the common shrew, Sorex araneus

A Robertsonian karyotypic polymorphism in the common shrew in the Oxford area, first described in the 1950s, was re-examined. The polymorphism involves chromosome arm combinations kq, no and pr (characteristic of the Oxford karyotypic race), ko (characteristic of the Hermitage karyotypic race) and jl (found in both races). The polymorphism for jl was sporadic along a north-south transect through the Oxford area, with the frequency of the twin-acrocentric morph never exceeding 10%. The frequency of the Oxford race-specific metacentrics decreased and the frequency of the Hermitage race-specific metacentric ko increased from north to south along the transect. At a latitudinal grid reference of about 180 km, there was a high frequency of individuals with chromosome arms k, n, o and q in the ancestral acrocentric state. This was coincident with the area of occurrence of ko-kq and ko-no Oxford-Hermitage hybrids. Such hybrids are double Robertsonian heterozygotes with monobrachial homology and are likely to suffer reduced fertility in consequence. It is proposed that this is a source of selection against the monobrachial hybrids and hence results in an increase in frequency of the acrocentric morphs. This scheme goes some way to explain the clines of polymorphism for arm combinations kq, no and ko, but it is suggested that other selective factors are involved. It cannot explain the cline of polymorphism for pr, which is in general terms similar to that for kq and no, but is more shallow and centred further north.     

Fixation of metacentric chromosomes in populations of the common shrew Sorex araneus L. from populations of Eastern Europe

In this review, we discuss the processes of fixation of Robertsonian chromosome fusions in populations of the common shrew Sorex araneus L. Various Robertsonian fusions, accumulating in populations, create an illusion of large chromosomal rearrangements, reciprocal translocations of complete chromosome arms. The use of these rearrangements for phylogenetic reconstructions results in false conclusions. Robertsonian fusions accumulate in populations at such stages of the species evolution, when large open or subdivided populations prevail (populations of warm periods of Pleistocene and many present-day populations) and are fixed in small isolated populations and glacial refugia. The formation of monomorphic chromosome races requires a long time, several glaciation epochs during the whole Pleistocene.     

Karyological and allozyme survey of the Common shrew, Sorex araneus, from Macedonia

Karyotype and genetic variation of the common shrew (Sorex araneus) from Mt. Pelister in southern Macedonia has been studied. Whereas all autosomes in the chromosomal set (2 n_a, = 28, the only present metacentrics being af, bc, jl, and tu) were of the standard type as well as the sex chromosomes X and Y₂ in males, the Y₁ chromosome was a small metacentric. This chromosomal feature is unique among the common shrew populations studied cytogenetically so far. Three out of 33 loci analysed (Sdh, 6Pgd, Mdh-1) were discriminant between the Mt. Pelister population and Sorex araneus from Slovenia and two loci (Est-3, Ada) were partially discriminant. A relatively high value of Nei's genetic distance (D = 0.137) confirms unique character of the Pelister population.     

Synaptonemal complexes of the common shrew, Sorex araneus L., in spermatocyte spreads

This paper presents an electron microscopic analysis of the behavior and morphology of the sex chromosomes and autosomes during prophase I in the common shrew, Sorex araneus L. The pairing patterns of the X/Y1Y2 and autosomal Robertsonian trivalents are described.     

Recombination map of the common shrew, Sorex araneus (Eulipotyphla, Mammalia)

The Eurasian common shrew (Sorex araneus L.) is characterized by spectacular chromosomal variation, both autosomal variation of the Robertsonian type and an XX/XY(1)Y(2) system of sex determination. It is an important mammalian model of chromosomal and genome evolution as it is one of the few species with a complete genome sequence. Here we generate a high-precision cytological recombination map for the species, the third such map produced in mammals, following those for humans and house mice. We prepared synaptonemal complex (SC) spreads of meiotic chromosomes from 638 spermatocytes of 22 males of nine different Robertsonian karyotypes, identifying each autosome arm by differential DAPI staining. Altogether we mapped 13,983 recombination sites along 7095 individual autosomes, using immunolocalization of MLH1, a mismatch repair protein marking recombination sites. We estimated the total recombination length of the shrew genome as 1145 cM. The majority of bivalents showed a high recombination frequency near the telomeres and a low frequency near the centromeres. The distances between MLH1 foci were consistent with crossover interference both within chromosome arms and across the centromere in metacentric bivalents. The pattern of recombination along a chromosome arm was a function of its length, interference, and centromere and telomere effects. The specific DNA sequence must also be important because chromosome arms of the same length differed substantially in their recombination pattern. These features of recombination show great similarity with humans and mice and suggest generality among mammals. However, contrary to a widespread perception, the metacentric bivalent tu usually lacked an MLH1 focus on one of its chromosome arms, arguing against a minimum requirement of one chiasma per chromosome arm for correct segregation. With regard to autosomal chromosomal variation, the chromosomes showing Robertsonian polymorphism display MLH1 foci that become increasingly distal when comparing acrocentric homozygotes, heterozygotes, and metacentric homozygotes. Within the sex trivalent XY(1)Y(2), the autosomal part of the complex behaves similarly to other autosomes.