Myopus Research Articles

Gonadal development and birth weight in X*X and X*Y females of the wood lemming, Myopus schisticolor

A quantitative histological analysis of ovaries from 8- to 10-day-old wood lemmings revealed significant differences between females with X*Y and X*X sex chromosome constitutions. The ovarian volume of X*Y females was on average 57% of X*X, and the number of oocytes was less than half in X*Y compared to X*X. However, the frequency of growing oocytes in relation to the total number was 6.5% for X*Y compared to 3.0% for X*X. Oogenesis in X*Y wood lemmings resembles in many respects that of mice heterozygous for certain translocations and with tertiary trisomy (Ts31H), and those with X0 monosomy. The fertility in X*Y wood lemmings is not reduced. On the contrary, X*Y females have a higher reproductive fitness than X*X and XX. This is discussed in relation to the present findings. The body weight at birth was 8% higher in X*Y than in X*X.

Colonization history in Fennoscandian rodents

Fennoscandia probably constitutes one of the best places on earth to study faunal history. During the height of the most recent glacial period Fennoscandia was completely covered with ice. Thus, the majority of extant species must originate from ancestors who survived the latest glaciation in non-glaciated areas outside Fennoscandia. Moreover, the geography and geological history of Fennoscandia suggests that post-glacial recolonization by land mammals must have been restricted to specific routes in time and space. Phylogeographic surveys of mitochondrial DNA (mtDNA) variation in Fennoscandian rodents have demonstrated that glacial history and patterns of post-glacial colonization have played a major role in shaping present day patterns of genetic differentiation within species. Thus, the genetic imprints of historical demographic conditions and vicariant geographic events have been retained within species and can be used to infer the history of populations. The field vole (Microtus agrestis) is used to illustrate these data and processes. Comparisons are made with phylogeographic surveys of the bank vole (Clethrionomys glareolus), the eastern house mouse (Mus musculus) and the wood lemming (Myopus schisticolor) as well as a few other species for which less extensive studies have been performed. The main patterns of post-glacial colonization of Fennoscandia by rodents are described. The effects of timing and patterns of colonization on contemporary population genetic structure and levels of genetic variation are discussed. Specifically, the effects of hybridization and introgression as well as founder events and bottlenecks are explored.

Colonization history in Fennoscandian rodents

Fennoscandia probably constitutes one of the best places on earth to study faunal history. During the height of the most recent glacial period Fennoscandia was completely covered with ice. Thus, the majority of extant species must originate from ancestors who survived the latest glaciation in non-glaciated areas outside Fennoscandia. Moreover, the geography and geological history of Fennoscandia suggests that post-glacial recolonization by land mammals must have been restricted to specific routes in time and space. Phylogeographic surveys of mitochondrial DNA (mtDNA) variation in Fennoscandian rodents have demonstrated that glacial history and patterns of post-glacial colonization have played a major role in shaping present day patterns of genetic differentiation within species. Thus, the genetic imprints of historical demographic conditions and vicariant geographic events have been retained within species and can be used to infer the history of populations. The field vole (Microtus agrestis) is used to illustrate these data and processes. Comparisons are made with phylogeographic surveys of the bank vole (Clethrionomys glareolus), the eastern house mouse (Mus musculus) and the wood lemming (Myopus schisticolor) as well as a few other species for which less extensive studies have been performed. The main patterns of post-glacial colonization of Fennoscandia by rodents are described. The effects of timing and patterns of colonization on contemporary population genetic structure and levels of genetic variation are discussed. Specifically, the effects of hybridization and introgression as well as founder events and bottlenecks are explored.

Telomeric (TTAGGG)n sequences are associated with nucleolus organizer regions (NORs) in the wood lemming.

The distribution of the (TTAGGG)n telomeric sequence was studied in chromosomes of the wood lemming, Myopus schisticolor, by fluorescence in-situ hybridization. As expected, the hybridization signals were observed at telomeres of all chromosomes. However, quite a number of interstitial telomeric sites were present in the pericentric heterochromatic regions. Consistent strong hybridization signals were also seen at one terminus of chromosomes 5, 7 and 12--15. By post-hybridization G-banding and silver-staining, the large blocks of the telomeric sequences on chromosomes 5 and 12 were localized to nucleolus organizer regions (NORs).

XY sex reversal in the wood lemming is associated with deletion of Xp21-23 as revealed by chromosome microdissection and fluorescence in situ hybridization.

In the wood lemming (Myopus schisticolor), XY sex reversal occurs naturally because of the presence of an X chromosome variant designated X*. The two types of X chromosome, X and X*, can be distinguished by G-banding, and analyses have demonstrated complex rearrangements of the short arm of X*. Here, chromosomal microdissection, degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR) and fluorescence in situ hybridization (FISH) techniques have been used to generate and map DNA probes for different parts of the X and X* chromosomes. The results showed that the region of Xp21-23 is deleted from the X* and some of the deleted DNA sequences are homologous to the mouse gamma-satellite. The deletion must be associated with the sex reversal in this species. FISH experiments with dissected probes of X and distal half of Xq provided evidence for presence of homologous sequences between large regions of the X and Y chromosomes, including euchromatic and heterochromatic parts of the sex chromosomes. The findings of this study will be of significance for further cloning of important candidate gene(s) responsible for the XY sex reversal.

X-linked sex-reversing genes

Rearrangement of the X chromosome generates fertile XY females in the wood lemming, and a duplication has been discovered in Xp21 in some XY females in the human. This has enabled identification and mapping of a novel sex-reversing gene in the human, double dosage of which blocks development of the testis. Whether or not the human gene is related to the gene that causes sex reversal in the wood lemming remains to be seen. Yet similar genes must exist in all eutherian mammals, according to Ohno’s Law.

Quantification of cranial convergences in arvicolids (Rodentia)

Cranial convergence in nine species of arvicolids is quantified phenetically using geometric morphometry. In a preliminary step, a hypothesis about phyletic relationships is proposed as a framework against which to examine morphological comparisons. The cranial morphology is then depicted in three series of 37, 20 and 13 landmarks characterizing the lower, upper and lateral sides of the skull respectively. Superimposition (Procrustes) methods are used to quantify shape differences and establish phenograms for the three sides of the skull. The phenogram obtained for lateral sides reveals a strong connection between skull profile and mode of life: surface dwelling forms have elongated skulls whereas burrowers have angular skulls. The phenogram of the lower sides show no clear correlation between shape and mode of life. Analysis of the upper cranial sides revealed a substantial difference between hard-substrate burrowers and other ecological groups. The results of the morphological analyses are compared with the phyletic hypothesis and with ecological data to explore how convergences take place in the evolution of arvicolids. Analyses pointed out three instances of morphological convergence: one between Synaptomys cooperi and voles, one between Lagurus lagurus and lemmings, and one between Myopus schisticolor and some voles. Four main ecological events are discussed.

Low mitochondrial DNA variation and recent colonization of Scandinavia by the wood lemming Myopus schisticolor

The population genetic structure of wood lemmings (Myopus schisticolor) from Scandinavia, Finland and western Siberia was examined by restriction fragment length polymorphism of mitochondrial DNA (mtDNA) in 45 individuals from six localities. The 12 observed mtDNA haplotypes demonstrated a distinct phylogeographic pattern, suggesting that the postglacial colonization of Scandinavia by wood lemmings occurred from north‐east. However, a very low level of haplotype and nucleotide diversity and a lack of geographical structure were found within Scandinavia. The limited mtDNA diversity in the Scandinavian populations probably reflects recent divergence in situ after colonization by a limited number of founders. Allozyme data support this scenario.

Low mitochondrial DNA variation and recent colonization of Scandinavia by the wood lemming Myopus schisticolor

AbstractThe population genetic structure of wood lemmings (Myopus schisticolor) from Scandinavia, Finland and western Siberia was examined by restriction fragment length polymorphism of mitochondrial DNA (mtDNA) in 45 individuals from six localities. The 12 observed mtDNA haplotypes demonstrated a distinct phylogeographic pattern, suggesting that the postglacial colonization of Scandinavia by wood lemmings occurred from north‐east. However, a very low level of haplotype and nucleotide diversity and a lack of geographical structure were found within Scandinavia. The limited mtDNA diversity in the Scandinavian populations probably reflects recent divergence in situ after colonization by a limited number of founders. Allozyme data support this scenario.

Genetic Conflicts and the Paradox of Sex Determination: Three Paths to the Evolution of Female Intersexuality in a Mammal

That sex determining systems ever change is paradoxical but can be explained by noting that conflict between selfish elements and their modifiers will often cause a shift in sex determining strategy. The evolution of the novel sex determining system of moles ( Talpa europaeaand T. occidentalis, may, we argue, be an example of just such a process. Three different models for the evolution of female intersexuality are presented. These all attempt to account for (1) the fact that a few years ago populations of moles had high frequencies of sterile XX individuals that were either morphologically male or intersex (other XX individuals were normal females) and (2) that presently, the XX individuals in the same population are exclusively fertile intersexes that are functionally female; i.e. have follicle producing ovotestes. This case history is compared to that of the wood lemming and two similarities are discussed. First, in both cases it is noted that one end product could be approached from different routes. Second, selfish elements may be involved in the evolution of both systems. In general, it is suggested that XY sex determination, far from being resilient to evolutionary change, is vulnerable to take-over by selfish elements. This is particularly the case in mammals in which transplacental interactions could allow manipulation of sex determination in one foetus by another. This, we also suggest, is a good candidate explanation for the evolution of novel sex determination in Talpa.

Pneumocystis cariniiin arvicoline rodents: seasonal, interspecific, and geographic differences

We studied the seasonal prevalence of Pneumocystis carinii in Microtus agrestis (n = 289, two locations), Clethrionomys glareolus (n = 37), and Myopus schisticolor (n = 31) in southern and central Finland, in C. glareolus (n = 406) and Microtus oeconomus (n = 167) in Finnish Lapland, and in Lemmus lemmus (n = 35) in a fall sample in northern Norway. All material came from populations at peak and declining densities. The highest prevalence of P. carinii in M. agrestis in both locations in southern and central Finland was observed in November (30 and 10%). Two M. oeconomus were infected with P. carinii. No individual of the other species examined was positive for P. carinii. The prevalence and intensity of P. carinii infection in M. agrestis fed experimental low-protein diets were similar to those found in the field material.

Genetic differentiation among populations of Myopus schisticolor (the wood lemming)--isozyme variation.

To investigate genetic differentiation among populations of the wood lemming Myopus schisticolor (Muridae, Rodentia) isozyme variation in 10 populations from three regions, Fennoscandia, Western and Eastern Siberia, was examined. From 20 loci examined, the two most polymorphic ones, Idh-1 and Pgi-1, were used in the complete survey. The results of hierarchical F-statistics analysis showed significant differentiation among all populations; single locus FPT values agreed well (Idh-1, FPT = 0.20; Pgi-1, FPT = 0.20). However, allele frequencies at each locus demonstrated different patterns of subdivision among populations within regions and between regions. Thus, significant values of F-statistics within (FPR = 0.10) and between (FRT = 0.10) regions were found at Idh-1. Pgi-1 showed no significant differentiation among populations within regions (FPR = 0.01). The subdivision among populations at Pgi-1 was based on differentiation between regions (FRT = 0.19). The lack of differentiation among populations within a geographical region at the Pgi-1 locus is surprising for any rodent species, and the discrepancy between single-locus patterns of subdivision seems likely to result from selection at the Pgi-1 locus.

Unusual distribution of Zfy and Zfx sequences on the sex chromosomes of the wood lemming, a species exhibiting XY sex reversal.

Sex reversal occurs naturally in the wood lemming (Myopus schisticolor) due to the presence in populations of this species of a variant (mutated) X chromosome, designated X*. Thus, X*Y animals develop into females, whereas XY animals develop into normal males. Chromosome mapping by in situ hybridization of DNA sequences homologous to the human ZFY gene localized the wood lemming Zfx sequences to region p12----p11 on both the wild-type X and the mutated X* chromosomes, at or proximal to a presumed breakpoint (Xp12) involved in the generation of the X* chromosome from the normal X, and Zfy sequences along the entire short arm of the Y chromosome. Differences between Zfx and Zfx* were readily detected by Southern blot analysis. However, both the Zfx and Zfx* genes expressed similarly sized transcripts in all adult somatic tissues investigated. Although the precise molecular difference between the Zfx and Zfx* genes is still unknown, their chromosomal location suggests that either Zfx or some other closely linked gene(s) on the X chromosome may be a major X-linked sex-determining gene, Tdx, which in the X* chromosome fails to interact properly with the Y-linked testis-determining gene, Tdy, thus causing X*Y embryos to develop into females. At least 15 copies of wood lemming Zfy sequences are distributed along the short arm of the Y chromosome. Northern hybridization analyses of adult tissues and somatic cell lines indicated that these Zfy repeats were transcriptionally inactive. Normally, 3-kb Zfy (ZFY) transcripts are readily detected in mouse and human testes, especially in the germ cells. It has therefore been postulated that expression of the Zfy (ZFY) gene may be important for spermatogenesis. Whether the lack of sufficient Zfy transcripts in the testis of the adult wood lemming has any impact on spermatogenesis in this species is still to be elucidated by further studies.

Sex ratio, XY females and absence of inbreeding in a population of the Wood Lemming, Myopus schisticolor Lilljeborg, 1844

Sex ratio, XY females and absence of inbreeding in a population of the Wood Lemming, Myopus schisticolor Lilljeborg, 1844

Home range size and activity of the wood lemming, Myopus schisticolor

We present the results from a radiotelemetric study on space use and activity in a natural population of the wood lemming Myopus schisticolor. Male home ranges were larger than females, and the ratio female home range to male home range was smaller than expected compared to other small rodents. Males moved distances 4‐12 times longer than females. We argue that this difference in mobility gives a higher probability of capturing males than females in snap trap studies. If there is a sex biased trappability, this might explain the increasing female biased sex ratio during the snow free season (1:1 in spring, 3:1 [female:male| in autumn), as a higher mortality of males during the reproductive season.

Is the “mammalian” brown fat-specific mitochondrial uncoupling protein present in adipose tissues of birds?

1. 1. Mitochondria were isolated from the furcular, subcutaneous, abdominal, nape and lateral adipose tissue depots of five species of bird (pheasant, Japanese quail, pigeon, house sparrow and great tit) acclimatized to the Northern winter 2. 2. Mitochondrial proteins were separated by SDS-polyacrylamide gel electrophoresis, blotted onto nitrocellulose membranes, and probed for the presence of the 32,000–33,000 M r uncoupling protein characteristic of “mammalian” brown adipose tissue, using an anti-(ground squirrel uncoupling protein)serum. 3. 3. Immunoreactivity consistent with uncoupling protein was not detected in mitochondria from any of the avian adipose tissues. Immunoreactivity was, however, evident in mitochondria from perirenal or interscapular adipose tissue from a range of mammals—rats, mice, golden hamsters, Orkney voles, wood mice, pipistrelle bats, wood lemmings, and newborn lambs, cattle, reindeer and red deer. 4. 4. These results provide biochemical evidence that “mammalian-like” thermogenic brown adipose tissue is absent from avian species; adipose tissues in birds appear to be functionally “white”.

Meiotic behavior of gonosomically variant females of Akodon azarae (Rodentia, Cricetidae).

The meiotic behavior of sex chromosomes has been investigated in variant females of Akodon azarae, both in pachytene oocytes and metaphase I. In somatic cells, these females have a heteromorphic sex pair, in which the minor chromosome has been previously interpreted as a major deletion of the long arm of the X chromosome (dX). After microspreading for synaptonemal complex analysis, pachytene oocytes show two axes of very different lengths (100:17.1), which correspond to the sex chromosomes X and dX. True synapsis is abnormally restricted (43.3%) between these sex chromosomes; on the other hand, self-synapsis of both the X and dX chromosomes is frequent (60%). Single, nonsynapsed axes or axial segments are thickened. Strong chromatin condensation occurs around nonsynapsed axes or axial segments, giving many of these sex pairs an appearance similar to an XY body ("sex vesicle"). The minor gonosome axis differs from that of the Y chromosome of male meiosis, as the former is shorter (relative to the X) and has a different synaptic behavior. In 17 metaphases I from XdX variant females, only heteromorphic, end-to-end joined sex pairs were observed. These variant females differ from the variant females of the wood lemming Myopus schisticolor in several respects, but a similar mechanism seems to be prevalent in other species of the genus Akodon. Self-synapsis of unequal gonosomes in oocytes is assumed as an escape from functional deterioration, following the hypothesis put forward by others.

Aberrant chromosomal sex-determining mechanisms in mammals, with special reference to species with XY females.

Both mouse and man have the common XX/XY sex chromosome mechanism. The X chromosome is of original size (5-6% of female haploid set) and the Y is one of the smallest chromosomes of the complement. But there are species, belonging to a variety of orders, with composite sex chromosomes and multiple sex chromosome systems: XX/XY1Y2 and X1X1X2X2/X1X2Y. The original X or the Y, respectively, have been translocated on to an autosome. The sex chromosomes of these species segregate regularly at meiosis; two kinds of sperm and one kind of egg are produced and the sex ratio is the normal 1:1. Individuals with deviating sex chromosome constitutions (XXY, XYY, XO or XXX) have been found in at least 16 mammalian species other than man. The phenotypic manifestations of these deviating constitutions are briefly discussed. In the dog, pig, goat and mouse exceptional XX males and in the horse XY females attract attention. Certain rodents have complicated mechanisms for sex determination: Ellobius lutescens and Tokudaia osimensis have XO males and females. Both sexes of Microtus oregoni are gonosomic mosaics (male OY/XY, female XX/XO). The wood lemming, Myopus schisticolor, the collared lemming, Dirostonyx torquatus, and perhaps also one or two species of the genus Akodon have XX and XY females and XY males. The XX, X*X and X*Y females of Myopus and Dicrostonyx are discussed in some detail. The wood lemming has proved to be a favourable natural model for studies in sex determination, because a large variety of sex chromosome aneuploids are born relatively frequently. The dosage model for sex determination is not supported by the wood lemming data. For male development, genes on both the X and the Y chromosomes are necessary.

Demography during a population crash of the wood lemming, Myopus schisticolor

Four-year microtine population density cycles are extreme in northern Fennoscandia. We studied populations of wood lemmings (Myopus schisticolor) and bank voles (Clethrionomys glareolus) in the peak phase and subsequent crash phase at two localities, Varaldskogen and Risberget, Norway. Bank voles at Varaldskogen and bank voles and wood lemmings at Risberget crashed during the nonreproductive period in winter when we were not trapping animals. Wood lemmings at Varaldskogen, however, crashed during the reproductive period in summer. Reproduction in the wood lemming population was as intense during the summer crash as in the peak phase. Further, survival of overwintered animals was the same in peak populations and in the population undergoing the crash. However, lack of recruitment was the cause of the summer crash of the wood lemming population. Poor nutrition is the most likely cause of the lack of recruitment.

Relationship between nucleoli and sex chromosomes during meiosis of the male wood lemming Myopus schisticolor: a fine‐structure study

Electron microscopy of ultrathin serial sections has been used to study the origin and fate of a mass of fibrillar material (FM) during spermatogenesis in the wood lemming Myopus schisticolor. In the course of early pachytene, one of the two nucleoli completely disappears. The remaining nucleolus loses its granular portion and acquires a "round body" encased by the fibrillar moiety, and the restructuring is accompanied by the appearance of FM in the close vicinity of this nucleolus. During diakinesis, the FM increases in volume and density and selectively infiltrates the chromatin of the XY pair. The intermingling of sex chromosomes and FM is at its maximum in metaphase I, giving the XY chromatin a patchy appearance. The FM separates along with the chromatin during the ensuing anaphase I and is shed from the chromosomes during early telophase I. By the time the nuclear envelope is reconstituted, the FM is completely separated from the chromatin. It disintegrates in the spermatids. The FM could not be stained using the Ag-NOR technique. In the wood lemming, X and Y chromosomes show an end-to-end association without a detectable synaptonemal complex. The FM may contribute to the attachment of the two sex chromosomes to each other. Thus, the FM is considered to be a substitute for a chiasma, which normally guarantees proper segregation in anaphase I.