Chromosome numbers, genome sizes, cell volumes and evolution of snake-head fish (family Channidae)

Chromosome number, C-value and cell volume studies were carried out on three species of the genus Channa , viz., C. punctatus, C. striatus and C. gachua . The chromosome number, karyotypic structure and DNA content per cell along with cell volume are reported and described. A series of chromosomal rearrangements are established in three different karyotypes along with polyploidy. Both pericentric inversion and Robertsonian fusion played a major role in chromosome rearrangements. The nuclear DNA content of these three species is within 19-29% of the present-day placental mammals, and is thus lower than the median amount for fishes in general and teleosts in particular. Their lower DNA content suggests that the three species of the family Channidae are highly specialized, and this is supported by their known morphologic, reproductive, behavioural and ecological characteristics.
The evolutionary significance of these chromosomal rearrangements, their origin and their mode of establishment are discussed. A probable phylogenetic model based on karyotype, C-value and chromosomal rearrangements of the genus is presented.     

Impact of Geese on the Limnology of Lakes and Ponds from Bylot Island (Nunavut,Canada)

Arctic freshwater ecosystems are important habitats for northern wildlife. Arctic climate impact studies suggest that global change could result in major modifications and perturbations of lakes, ponds and wildlife. Most studies focus either on freshwater ecosystems or on animal populations, but few have investigated the links that exist between them. Animal populations have the potential to alter the nutrient inputs in lakes and ponds via faeces. The present study is the first to reveal the impact of an expanding Greater Snow Goose (Chen caerulescens atlantica) population on the limnology of arctic lakes and ponds. A survey of 27 freshwater ecosystems was performed on Bylot Island (Nunavut, Canada) in order to identify patterns in limnological conditions. Using a multivariate statistical approach, our study shows that the presence of birds in the catchment of lakes and ponds has an impact on their nutrient status. Concentrations of major ions that were related to the distance from the sea were the main environmental variable explaining the limnological differences observed among lakes and ponds. Nutrient variables that were mostly related to the presence of Snow Geese played a secondary but significant role. N and P concentrations were different among impacted and non‐impacted sites, underlining the impact of animal populations on northern freshwater ecosystems. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparative cytogenetics among populations of Astyanax altiparanae (Characiformes, Characidae, Incertae sedis)

Cytogenetic data are presented for Astyanax altiparanae populations from three Brazilian hydrographic systems. The chromosomal data obtained in A. altiparanae support the hypothesis of diploid number conservation. However, small differences in the karyotype formula and number of nucleolar organizer regions were observed in these populations. The apparent karyotypical similarity among the studied populations strongly suggests a close relationship among them with some chromosomal divergences due to gene flow restriction.     

Diversification, Sympatric Speciation, and Trophic Polymorphism of Arctic Charr, Salvelinus Alpinus Complex, in Transbaikalia

In northern Transbaikalia, independently evolving landlocked populations of Arctic charr are found in mountain lakes. To assess the diversity of charr in this region, speciation modes involved in the evolution of charr forms, and the role of trophic polymorphism in their divergence, we studied the morphology and feeding of dwarf, small, and large forms of Arctic charr from a number of Transbaikalian lakes. Meristic data on charr from five lakes support the earlier conclusion that the three forms do not represent separate lineages but have independently diverged in sympatry in each of the lakes. In 10 lakes, the dwarf form showed varying degrees of differentiation from normal (small and large) charr in meristic characters (up to morphologically distinct and presumably reproductively isolated groupings), which is viewed as various levels of sympatric divergence. Indexes of gill raker length in fish from 20 lakes vary among populations of both dwarf and normal charr, with forms having short and long rakers being sympatric in some of these lakes. However, the index can be used only for comparing charr of different forms up to about 32cm fork length (FL) because it is strongly negatively correlated with size in larger fish. The study of charr diets in 21 lakes indicates that large charr are piscivorous whereas dwarf and small charr feed on a wide range of invertebrates, partitioning these resources in different ways. Planktivores, including very specialized ones, and non-planktivores (benthic feeders, insectivores) can be identified within the small and dwarf forms. The proportion of plankton in the diets of dwarf and small charr is positively correlated with the number and length of gill rakers while the proportion of benthos is negatively correlated. Allopatric planktivorous and non-planktivorous small charr differ in body proportions; parallel emergence of such morphotypes in different parts of the range is a characteristic feature of the Salvelinus alpinus complex.     

Chromosomal variability in natural populations of Chironomus cingulatus meigen (Diptera, Chironomidae)

Chromosomal polymorphism has been studied in seven natural populations of Chironomus cingulatus from Western Europe, Western Siberia, and the Republic of Sakha (Yakutia). The banding sequences pool of the species includes 15 banding sequences. Chromosomal polymorphism was revealed in five out of seven chromosomal arms. Arm B is the most polymorphic with four banding sequences. There are three banding sequences in arm A. Arms D, E, and G have two banding sequences. None of the chromosome rearrangements were revealed in arms C and F. The populations of C. cingulatus differ clearly in their number and frequency of banding sequences, which indicates that different gene sequences are adaptive in different populations.     

Comparative cytogenetics and molecular phylogeography in the group astyanax altiparanae--Astyanax aff. bimaculatus (Teleostei, Characidae)

The genus Astyanax comprises small characin fish of the neotropical region. The so-called 'yellow-tailed characins' compose one of the most widely distributed Astyanax groups. A. altiparanae and A. aff. bimaculatus, are evolutionarily closely related and commonly found in several Brazilian hydrographic basins. In the present work, chromosomal data of specimens of A. altiparanae and A. aff. bimaculatus from 4 hydrographic basins in the states of S?o Paulo (Upper Tietê, Paranapanema, Ribeira de Iguape) and Rio de Janeiro (Guapimirim) are shown. All the populations showed 50 chromosomes, with different karyotypic formula. Although only a single Ag-NOR bearing chromosome pair was observed, all populations possess multiple cistrons of 18S rDNA. FISH with the 5S rDNA probe showed single signals at the interstitial position of one metacentric chromosome pair. C-bands are distributed in the terminal and interstitial regions of several chromosomes. However, the As-51 satDNA are frugally located in a few chromosomes of fishes from Upper Tietê, Paranapanema and Guapimirim Rivers, being absent in individuals of A. aff. bimaculatus from Ribeira de Iguape River basin. Beside these 4 populations, molecular phylogeography studies were also performed in individuals from Middle and Lower Tietê River basin and from 2 additional collection sites in the Paranapanema and Ribeira de Iguape River basins. The phylogeographic analysis using 2 mtDNA regions (totalizing 1.314 bp of ND2 and ATPase6/8 genes) of 8 populations of the group of 'yellow-tailed characins' from 3 major hydrographic basins showed structuring of populations, suggesting a correlation between chromosomal (nuclear) and molecular (mitochondrial) data.     

B chromosomes of the Podisma sapporensis Shir. (Orthoptera, Acrididae) analysed by chromosome microdissection and FISH

The analysis of the distribution of repetitive DNA of the B chromosomes of Podisma sapporensis in the A and B chromosomes of the natural populations and in A chromosomes of three other species of the Podismini grasshoppers were made. DNA-libraries of the B chromosome and the euchromatic segment of the A chromosome of P. sapporensis were generated by meiotic chromosome microdissection followed by degenerated oligonucleotide primed polymerase chain reaction (DOP-PCR). Paints based on these DNA-libraries were used for FISH analysis to detect localization of homologous sequences in A and B chromosomes of P. sapporensis from different natural populations. On the basis of the FISH analysis the authors suggest that evolution of the B chromosomes in Podisma sapporensis was associated mainly with the insertions of "alien DNA sequences" into ancestral A chromosome and their further amplification. The number of initial sites of amplifications differed in the different Bs, the distance between these sites also varying. Karyotype evolution in P. sapporensis was associated partly with the insertion of "alien DNA sequences" into pericentromeric chromosomal regions. Insertion into the small short arms of the acrocentric chromosomes followed, with the DNA amplification leading to the formation of the additional C-heterochromatic arms or euchromatic-like regions of different size.     

KARYOTYPES AND REPRODUCTIVE SEASONALITY OF THE GENUS BANGIA (RHODOPHYTA) IN BRITISH COLUMBIA,CANADA1

Chromosome number and morphology as well as reproductive seasonally were examined in Bangia populations collected from 88 sites along the British Columbia coasts. Sexual filaments were recorded at 35 sites and asexual filaments at 58. Four different types of Bangia were determined. These were characterized by reproductive state and chromosome number, as well as by their seasonally and range of distribution. A summer-fall three-chromosome sexual type occurred from approximately 50° N northward and a winter-spring four-chromosome sexual type had a northerly limit of approximately 54° N, extending to the southernmost areas of British Columbia. Two asexual types occurred, exhibiting little seasonally, six-chromosome populations throughout the coast and three-chromosome populations at three southerly sites only. Karyograms were prepared for the four chromosomal types, assigning tentative centromere locations. Somatic chromosome pairing was reported for the first time in the Rhodophyta.     

Karyological and taxonomic notes on Genista L. (Papilionoideae, Leguminosae) from the Iberian Peninsula

Karyological information about Iberian Genista species is limited because of their wide geographical distribution. It is a major factor in the understanding of a genus whose genetic evolution is mostly related to aneuploidy and euploidy. The chromosome numbers of the following taxa have been counted: G. carpetana subsp. carpetana (n = 20), G. cinerascens (n = 12), G. micrantha (n = 18), G. mugronensis subsp. rigidissima (n = 18), G. ramosissima (2n = 48) and G. tinctoria (n = 24). The previously-assumed uniformity of chromosome number (n = 24) for all the subspecies of G. cinerea is supported here by data from subspp. murcica and speciosa. In G. florida, a new chromosome number, n = 23, has been found for both subspecies (florida and polygaliphylla). A discussion of the discrepancies between these data and previous ones is included. A new level of ploidy has been found in G. tuurnefortii subsp. tournefortii (n = 32). The data stress the necessity of obtaining chromosomal information from a number of plants of a taxon before its characteristic number is clear, and indicate that more chromosomal data will help the understanding of the cytological diversity within taxa in Genista.     

Chromosome Numbers and Karyotypes of Six Oxytropis Species (Fabaceae) from the Qinghai Tibetan Plateau,China

Chromosome numbers and karyotypes of 13 populations of six Oxytropis species (Fabaceae) from the Qinghai Tibetan Plateau, China, were presented. The chromosome numbers and karyotypes in O.ochrocephala, O.tatarica, O.kansuensis and O.humifusa (2n=16) were reported for the first time. B chromosomes were found from O.stracheyana (2n=48). The basic chromosome number of x=8 is confirmed for the genus. The available chromosomal data indicate that polyploidy may have played an important role in the evolution of the genus, with the incidence of polyploidy in the genus reaching 58%. However, our results indicated that among the populations here examined only one was a hexaploid with 2n=48. Such a chromosomal pattern indicates that the karyotypic repatterning at the diploid level seems to be the predominant feature of chromosomal evolution in the Oxytropis species from the Qinghai Tibetan Plateau, and that sympatric speciation via hybridization and polyploidization has played a minor role in the species diversification of the genus from this area.     

Biodemographic and molecular analysis of an isolated Alpine population (Postua)

Isolated populations have been the object of several genetic and anthropological studies, since endogamy and inbreeding often lead to the acquisition of a particular gene pool. In this context, we studied the small, ancient population of Postua in the north-western Italian Alps. We used biodemographic and molecular techniques to analyse the population structure in order to evaluate the relationship between geographical and genetic isolation. We examined about 26,000 certificates kept in the town and parish archives, concerning the period from 1640 to 1999. High rates of endogamy and isonymy, short marriage distances and a low ratio between the number of surnames and the number of individuals were inferred. In the molecular analysis, we compared the distribution of Y chromosome SNPs (single nucleotide polymorphisms) with those of mitochondrial variations and Y chromosomal microsatellites (short tandem repeat polymorphisms) in 102 healthy individuals originating from Postua. A control sample (94 individuals) was collected from a plain area, 50 km away. We examined 23 SNPs and an Alu repeat, located in the nonrecombinant portion of the Y chromosome. To further delineate Y chromosome lineages, the biallelic haplogroups were further resolved using Y microsatellite markers (DYS19, DYS391, DYS392, DYS393). Mitochondrial HVS-I and HVS-II regions were sequenced, and RFLP screening with the six classical enzymes was performed. Postua is similar to other populations living in northern Italy, but it shows a lower number of haplotypes. The samples were compared with other European populations. We calculated genetic distances according to Reynold and Nei and we carried out a phylogenetic analysis by phylogenetic trees and reduced median networks construction. Postua clusters with other samples from northern Italy but in a separate position, probably indicating drift phenomena. These relationships are supported by AMOVA (analysis of molecular variance). Our results suggest that the influence of neighbouring populations on the gene pool of Postua has been very low through both females and males.     

Walleye, Stizostedion vitreum, and northern pike, Esox lucius, populations in three Alberta lakes

Results are presented from a survey of walleye and northern pike in Ethel, Marie and Wolf Lakes, Alberta conducted May-September, 1983. Three types of experimental gill net were fished, mainly inshore. Northern pike were abundant in all three lakes but white sucker and yellow perch also were common. Walleye were caught in largest numbers in Wolf Lake. Age and growth of both walleye and northern pike were determined from examination of the opercular bones. The walleye in the three lakes were represented by a few strong year classes but there was no synchronism between the lakes. The strong 1975 year class in Ethel and Wolf Lakes may have been correlated with high spring precipitation and heavy run-off. Although there were dominant year classes in the northern pike populations (especially the 1979 year class) the variability was not so marked as in the walleye. There were no significant differences between length-at-age for year classes of walleye in Ethel and Marie or northern pike in all three lakes. In Wolf Lake the 1978 and 1979 year classes grew faster than in previous years. Walleye and northern pike growth was well described by the von Bertalanffy growth model. Females of both species grew to a greater ultimate length and had a longer lifespan than the males. Fecundity data are presented for walleye from Wolf Lake (log₁₀ absolute fecundity = 0.856 + 2.441 log₁₀ length) and northern pike from Marie Lake (log₁₀ absolute fecundity =?2.671+4.052 log₁₀ length). The stomach contents of both species were examined during May and June. The majority of walleye had empty stomachs. Walleye and northern pike in Ethel and Wolf Lakes fed on a variety of fish and invertebrate species but both fed mainly on fish (in particular whitefish) in Marie Lake. The similarity of diet suggests competition for prey species. A walleye yield to commercial fishermen of c. 0.69 kg ha^?1 year^?1 in Wolf Lake has made up 8% of the total catch. Since 1970/71 when mesh size of nets was increased, the yield has been reduced to c. 0.30 kg ha^?1 year^?1. Anglers remove an additional c. 0.48 kg ha^?1 year^?1 walleye from Wolf Lake. Northern pike yields to commercial fishermen have been c. 2.00 kg ha^?1 year^?1 in Wolf Lake. Ethel and Marie Lakes have yielded only poor catches of walleye and northern pike. A yield model was used to illustrate that faster-growing northern pike have higher potential yields than walleye. Walleye produce higher yields in Wolf Lake than in Marie Lake, the reverse being true for northern pike. It is suggested that northern pike could be cropped at a higher rate in Marie and Wolf Lakes with a possible improvement in walleye stocks.     

Comparative Cytogenetics Among Allopatric Populations of the Fish, Hoplias Malabaricus. Cytotypes with 2n = 42 Chromosomes

The available chromosomal data on Hoplias malabaricus make possible the identification of three major karyotypic forms in this fish group, all of them bearing 2n = 42 chromosomes, and named as Cytotypes A, B and E in previous studies. While Cytotype A and B share a general macrokaryotypic feature, Cytotype E is well differentiated concerning the morphology and size of some chromosome pairs. On the other hand, Cytotype B presents an exclusive XX/XY sex chromosome system. Six allopatric populations, belonging to Cytotype A, were subjected to cytogenetic analysis in the present study. Despite their basic karyotypic similarity, some differences in the chromosome formulae, as well as in the heterochromatin and Ag-NORs locations, were observed among populations indicating that they no more correspond to a unit, at least in the cytogenetical level. This revised version was published online in July 2006 with corrections to the Cover Date.     

Strait of Gibraltar: an effective gene-flow barrier for wind-pollinated Carex helodes (Cyperaceae) as revealed by DNA sequences, AFLP, and cytogenetic variation

The Strait of Gibraltar is the most important barrier disconnecting the landmasses of Europe and Africa on the western Mediterranean extreme. Carex helodes is a wind-pollinated species endemic to the western Mediterranean. Because molecular and cytogenetic data allow the inference of its evolutionary history, we analyzed variations in chromosome number, including meiotic chromosome behavior, amplified fragment length polymorphism (AFLP) fingerprints, and nucleotide substitutions in plastid and nuclear DNA sequences. Cytogeographic results showed that the African populations have stabilized at a single chromosome number of 2n = 74, whereas the most frequent cytotype in Iberia is 2n = 72. Phylogenetic reconstructions of 17 sequences from nine closely related species revealed that C. helodes is monophyletic and that the Moroccan populations are embedded in the Iberian lineages. The haplotype network is also consistent with a European origin of the northern African haplotype. AFLP analysis also revealed hierarchical levels of genetic variation compatible with a founder effect process responsible for the African populations. All sources of evidence support the hypothesis that the Strait of Gibraltar has been an effective gene-flow barrier, generating two isolated evolutionary lineages after their dispersal. Recent connections between the two lineages appear unlikely, whereas active gene flow occurs among populations within the two lineages.     

Chromosomal distribution of the transposable elements Osvaldo and blanco in original and colonizer populations of Drosophila buzzatii

Chromosomal distribution of transposable elements (TEs) Osvaldo and blanco in D. buzzatii was studied in three original natural populations from Argentina (Berna, Puerto Tirol and La Nostalgia) and a colonizer population from the Iberian Peninsula (Carboneras). The Spanish population showed significant differences for Osvaldo and blanco copy numbers when we compared the X chromosome and the autosomes; but it is mainly the accumulation of copies in chromosome 2, where most sites with high insertion frequency were located, that causes the discrepancy with the negative selection model. We found no significant differences in TE frequency between chromosomal regions with different exchange rates, and no evident accumulation of TE was detected within chromosomal inversions where recombination rate is reduced. The Carboneras population shows euchromatic sites of Osvaldo and blanco with high occupancy and others with low copy number. On the contrary, Argentinian populations show only a generalized low occupancy per insertion site. Moreover, the mean copy number of both elements is higher in Spain than in Argentina. All these results suggest an important role of the colonization process in the distribution of TEs. The increase in the copy number of the TEs analysed and their elevated frequency in some chromosomal sites in Carboneras is, most probably, a sequel of the founder event and drift that took place at the time of the colonization of the Old World by D. buzzatii from the New World some 300 years ago.     

Gene mapping of 28S and 5S rDNA sites in the spined loach Cobitis taenia (Pisces, Cobitidae) from a diploid population and a diploid–tetraploid population

We compare the chromosomal 28S and 5S rDNA patterns of the spined loach C. taenia (2n = 48) from an exclusively diploid population and from a diploid–polyploid population using 28S and 5S rDNA probe preparation and labelling, and fluorescence in situ hybridization (FISH). The 5S rDNA was located in two to three chromosome pairs, and separated from the 28S loci for the males and one female (F1) from the diploid population. Loaches from a diploid–polyploid population, and one female (F2) from the diploid population were characterized by at least one chromosome pair with 5S and 28S overlapping signals. The fishes differed mainly in their number of 28S rDNA loci, located on 3–6 chromosomes. All individuals from both populations were characterized by one acrocentric chromosome bearing a 28S rDNA signal on the telomeres of its long arm. The number of major ribosomal DNA in the karyotype of C. taenia by FISH was always higher than the number of Ag-NORs. Our data confirm the extensive polymorphism of NORs in both populations, as already has been observed in closely related Cobitis species, and less polymorphic 5S rDNA pattern. However, this preliminary result highlights the need for a wider scale study.     

Studies in the Melica ciliata – complex: 1. Distribution of allozyme variation within and among individuals, populations and geographic regions

Allozyme variation in 83 European populations of the Melica ciliata/ transsilvanica species complex has been investigated. In spite of a diploid chromosome number, most loci were duplicated and patterns of within-population variation were not explainable unless apomixis or uncommon patterns of chromosomal segregation were assumed. Allelic richness and genetic diversity were very high but most of the variation (55-98%) was due to population differentiation. Geographic patterns were analysed both using phenetic, based on allele frequencies within populations, and cladistic, based on multi-locus genotypes, methods. Both methods congruently recognised three large groups of populations with discrete geographic distributions, in addition to a number of smaller groups. Of the three main groups, one was distributed in eastern and central Europe, one was restricted to western Europe north of the Alps and one was distributed in Iberia, Brittany and along the western shores of the Mediterranean. These groups are likely to have had separate histories but there appears to have been significant gene-flow among them. Among the less distinct groups, it was suggestive that some, e.g. the group containing all populations from the disjunct distribution in the Baltic basin, have been formed by introgression between the three major groups, whereas other deviating populations may represent additional distinct entities.     

On the Origin of the Lacustrine Charr <Emphasis Type="Italic">Salvelinus alpinus</Emphasis> Complex from the Kolyma and Sea of Okhotsk Basins

Morphological and genetic characteristics are used for the analysis of the origin of charrs from the lakes of the Kolyma basin (Chuk, Gek, Gulyaevskoe-4, Gulyaevskoe-6, and Lenkovoe) and the Sea of Okhotsk basin (Lake Chistoe). In addition to these samples, the fragments of the control region of mitochondrial DNA (CR mtDNA, 550-bp) and exon 2 of recombination activating gene 1 (RAG1, 899-bp) are sequenced in the charrs from lakes Dzhul’etta and Cherechen’ (Kolyma basin) and lakes Elikchanskie, Bol’shoi Mak-Mak, Ueginskoe (Sea of Okhotsk basin), Ulakhan-Silyan-Kyuel’ (Yana River basin) as well as in northern Dolly Varden from the Kamchatka River. The charr from Lake Chistoe is represented by northern Dolly Varden. In the charrs from lakes Chuk, Gek, Gulyaevskoe-4, Gulyaevskoe-6, Lenkovoe, Cherechen’, Elikchanskie, Bol’shoi Mak-Mak, and Ueginskoe, the haplotypes of mtDNA of the Bering group (a haplogroup of northern Dolly Varden) are revealed. Based on the morphological and RAG1 data, populations of charrs from these lakes belong to the Salvelinus alpinus complex. A transfer of mtDNA of northern Dolly Varden to the charrs from the Kolyma and Sea of Okhotsk basins occurred during the last postglacial expansion and subsequent hybridization. Based on the results of morphological and genetic analysis, the charrs from the Kolyma and Sea of Okhotsk basins cannot be unambiguously referred to the phylogenetic groups of Eurasian Arctic charr or Taranets charr. The presence of mtDNA haplotypes of the Arctic group (a haplogroup of Taranets charr) in the populations of lakes Dzhul’etta, Maksi, and Ueginskoe shows a possibility of their belonging to the group of Taranets charr.     

The tobacco mouse and its relatives: a "tail" of coat colors, chromosomes, hybridization and speciation

The article reviews over 30 years' study of the chromosomal variation of the western house mice (Mus musculus domesticus) from the neighboring valleys of Poschiavo and Valtellina on the Swiss-Italian border. This is done in the context of the social and political history of this area, on the grounds that mice, as commensals, are influenced by human history. The chromosomal study of mice in this area was initiated because their unusual black coat color led a 19th century naturalist to describe the "tobacco mice" from Val Poschiavo as a separate species (Mus poschiavinus). The special coloration of the Val Poschiavo mice is matched by their chromosomes: they have 26 chromosomes instead of the usual 40. The Val Poschiavo mice are not a separate species according to the Biological Species Concept; instead they constitute a chromosome race (the "Poschiavo", POS) that is related to other races with reduced chromosome numbers that occur in N Italy (of which only those races in Val Poschiavo and Upper Valtellina have black coats). A phylogenetic analysis of mitochondrial DNA sequences suggests that the lineage of chromosome races found in N Italy was not formed during an extreme population bottleneck, although such bottlenecks have apparently occurred during the origin of individual races and certainly have influenced single populations. In one small, isolated population in Valtellina (Migiondo), two chromosome races (the POS and the "Upper Valtellina", UV, 2n = 24) became reproductively isolated from each other. In another small population (Sernio) bottlenecking led to fixation of a hybrid form with the UV karyotype and coat color, but with allozyme and microsatellite alleles characteristic of mice with the standard 40-chromosome karyotype. Two of the chromosome races in Valtellina (the UV and the "Mid Valtellina", MV, 2n = 24) also appear to be the product of hybridization. The dynamic history and patchy distribution of the house mouse chromosome races in Val Poschiavo and Valtellina in part reflects extinction-recolonization events; the formation of the UV and MV races and the introduction of the pale brown Standard race mice are believed to reflect such events. Dynamism in the chromosomal constitution of single populations is also evident from 25 years of data on the population in Migiondo. Due to change in agricultural practices, house mice in Valtellina and Val Poschiavo are becoming rarer, which is likely to have further impacts on the distribution and characteristics of the chromosome races in this area.     

Integrative analysis of chromosome banding,telomere localization and molecular genetics in the highly variable <Emphasis Type="Italic">Ctenomys</Emphasis> of the Corrientes group (Rodentia; Ctenomyidae)

The genus Ctenomys comprises about 70 species with great chromosome diversity. The Corrientes group is one of the most chromosomally variable lineages in the genus, where the diploid number (2n) varies from 41 to 70. In this group, three nominal species and numerous polymorphic and polytypic populations have been described. In order to get insight into the chromosomal evolution of this species complex, we applied different banding and molecular cytogenetic techniques. The results were interpreted in an evolutionary context, based on mitochondrial cytochrome b analyses. Studied samples are representative of the broad chromosomal variability in the group, including specimens with 2n?=?42 to 2n?=?70. Heterochromatin was scarce but concentrated in a few chromosomes. Centromeric DAPI-negative heterochromatin was observed in some autosomal pairs, which differed among populations. Location and amount of DAPI-neutral heterochromatin within the Y chromosome varied among populations. The variable distribution of heterochromatin indicates its dynamic behavior. NORs were detected in one pair of autosomes, which also differed among some populations. Telomeric FISH signals were observed in all complements only at the chromosome ends. The Corrientes group belongs to a clade that also includes C. pearsoni, C. lami, C. minutus, C. ibicuiensis and C. torquatus. Almost all of these species are variable at the chromosomal level, suggesting that this is the ancestral condition of the clade. Within the Corrientes group, the observed low genetic divergence, in contrast with its high chromosomal variability, is indicative of decoupling between the rates of chromosomal and mitochondrial evolution.