Ernesto Capanna

Robertsonian metacentrics of the house mouse lose telomeric sequences but retain some minor satellite DNA in the pericentromeric area

A combination of cytogenetic and molecular biology techniques were used to study the molecular composition and organisation of the pericentromeric regions of house mouse metacentric chromosomes, the products of Robertsonian (Rb) translocations between telocentrics. Regardless of whether mitotic or meiotic preparations were used, in situ hybridisation failed to reveal pericentromeric telomeric sequences on any of the Rb chromosomes, while all metacentrics retained detectable, although reduced (average 50 kb), amounts of minor satellite DNA in the vicinity of their centromeres. These results were supported by slot blot hybridisation which indicated that mice with 2n=22 Rb chromosomes have 65% of telomeric sequences (which are allocated to the distal telomeres of both Rb and telocentric chromosomes and to the proximal telomeres of telocentrics) and 15% the amount of minor satellite, compared with mice with 2n=40 all-telocentric chromosomes. Pulsed field gel electrophoresis and Southern analysis of DNA from Rb mice showed that the size of the telomeric arrays is similar to that of mice with all-telocentric chromosomes and that the minor satellite sequences were hybridising to larger fragments incorporating major satellite DNA. Since the telomeric sequences are closer to the physical end of the chromosome than the minor satellite sequences, the absence of telomeric sequences and the reduced amount of minor satellite sequences at the pericentromeric region of the Rb metacentrics suggest that the breakpoints for the Rb translocation occur very close to the minor satellite-major satellite border. Moreover, it is likely that the minor satellite is required for centromeric function, 50–67 kb being enough DNA to organise one centromere with a functionally active kinetochore.

The other chromatin

Abstract. Current understanding of heterochromatin, thanks to molecular data, focuses on its performing several functions in evolution and development. Heterochromatin shows characteristic distribution patterns in karyotypes and contributes to the broad scattering of genome sizes through biological taxa. Heterochromatin remains compacted and thus different from properly stained euchromatin during somatic interphase. A minimum amount of heterochromatin, however, is required for it to be visible in light microscopy. It may further escape notice during the dynamic processes of embryogenesis and gametogenesis. Present-day biology is in search of specific proteins and DNA sequences that comprise heterochromatin. The data that result from overcoming the threshold of visibility will support understanding of interference by heterochromatin in ontogeny and evolution. The contributions of Sigrid and Wolfgang Beermann to the study of heterochromation diminution (DNA elimination) are recalled, and we also discuss the functions and effects of heterochromatin on differential DNA endoreplication and in speciation.

New developments in vertebrate cytotaxonomy III. Karyology of bony fishes: A review

Several years after the introduction of modern cytological techniques in the karyological study of the bony fishes, an attempt may be made to take stock of the results obtained, the problems raised and the difficulties encountered by workers engaged in this type of investigation. The first limitation is set by the value of the karyotype itself. Because it is purely morphological and its immediate adaptive value is unknown, karyotype transformations are difficult to interpret in analyses of populations of the same species or of related species. When related species are being compared, similar karyotypes are considered indicative of a relatively recent separation, whereas widely differing karyotypes are believed to indicate an earlier separation.However, this assumption is not always valid. In the bony fishes a karyotype composed of 48 acrocentric chromosomes occurs not only in related species but also in species that are phylogenetically further apart. In some taxa the karyotype therefore appears ‘neutral’ with respect to both speciation and phyletic evolution (Fig. 1) and it is therefore often impossible to establish a definite correlation between karyotype and karyotype transformations and to identify the processes involved. Although this situation diminishes the immediate value of the karyological evidence, it raises a series of questions that, when set in the right problematical context, make karyological analyses extremely interesting .

Contact zone between chromosomal races of Mus musculus domesticus . 2. Fertility and segregation in laboratory-reared and wild mice heterozygous for multiple Robertsonian rearrangements

Litter size, anaphase I nondisjunction and X–Y dissociation at metaphase I were studied in homozygous and heterozygous house mice from a central Italian chromosomal hybrid zone between the CD (2n=22) race and the standard race (2n=40). We also observed the segregation of the two chromosomal forms (Robertsonian and non-Robertsonian) in male and female multiple heterozygotes from the karyotype of their offspring and chromosomal arm counts of metaphase II. Litter size was significantly reduced in the F1 hybrids, but there was no difference in litter size between male and female F1s. Fertility in wild mice decreased with increasing numbers of structural heterozygosities (0–5). Some metacentrics appear to be under meiotic drive but there was no rule as to which of the two forms was favoured in backcrosses. An original observation of a negative correlation between the length of metacentrics and transmission rate was described in hybrids. Slight cosegregation of chromosomes with a similar morphology was present in the progeny of males and females. These observations are discussed in relation to the stability of this hybrid zone through time.

REDUCED GENE FLOW AT PERICENTROMERIC LOCI IN A HYBRID ZONE INVOLVING CHROMOSOMAL RACES OF THE HOUSE MOUSE MUS MUSCULUS DOMESTICUS

The West European house mouse, Mus musculus domesticus, is a particularly suitable model to investigate the role of chromosomal rearrangements in reproductive isolation. In fact, it exhibits a broad range of chromosomal polymorphism due to Robertsonian (Rb) fusions leading to various types of contact zones between different chromosomal races. In the present study, we analyzed a parapatric contact in central Italy between the Cittaducale chromosomal race (CD: 2n= 22) and the surrounding populations with standard karyotype (2n= 40) to understand if Rb fusions play a causative role in speciation. One hundred forty‐seven mice from 17 localities were genotyped by means of 12 microsatellite loci. A telomeric and a pericentromeric locus situated on six chromosome arms (four Rbs and one telocentric) were selected to detect differences in the amount of gene flow for each locus in different chromosomal positions. The analyses performed on the two subsets of loci show differences in the level of gene flow, which is more restricted near the centromeres of Rb chromosomes. This effect is less pronounced in the homozygotes populations settled at the border of the hybrid zone. We discuss the possible cause of the differential porosity of gene flow in Rbs considering “hybrid dysfunctions” and “suppressed recombination” models.

Allozyme variation and systematics of European moles of the genus Talpa (Mammalia, Insectivora)

Genetic variation and divergence of several populations of moles of the genus Talpa from Spain, Austria, Italy, and Greece were investigated by means of the electrophoretic analysis of 38 enzyme loci. All samples tested showed a low level of genetic variability, most loci being monomorphic. The expected mean heterozygosity per locus ( H e ) ranged from 0 to 0.024 (0.061 in the population of T. romana from Calabria). Each of the 230 specimens tested falls within one of five distinct gene pools, differentiated from each other by from 7 to 13 diagnostic loci. No evidence of hybridization or gene flow was found among the five groups. The data confirm specific status of T. europaea, T. caeca , and T. romana (by some authors considered as a subspecies of T. europaea ); they also suggest specific rank for stankovici (until now treated as a subspecies of T. romana ) and for occidentalis (generally considered as a subspecies of T. caeca ). Further support is provided by the analysis of two pairs of sympatric and syntopic populations, respectively of T. romana and T. caeca , and of T. romana and T. europaea . In both cases, neither hybrids nor introgressed individuals were observed. The values of Neis genetic distance found among the mole species considered range from 0.22 (between T. europaea and T. caeca ) to 0.46 (between T. occidentalis and T. stankovici ). From these values, times of evolutionary divergence of 1 to 3 million years were estimated. These figures appear consistent with those from fossil records. The validity of a number of subspecific taxa is discussed on the basis of genetic data. T. europaea velessiensis from Greece was found virtually identical to the Italian populations of T. europaea ; the validity of T. romana brachycrania appears also not supported by genetic data; genetic differentiation of T. r. major and T. r. adamoi from T. r. romana is low ( D ≅ 0.03), as is that of T. caeca augustana from T. c. caeca . Some discrepancies found between morphological and genetic data and their consequences at the taxonomic level are discussed.

The chromosomes of 11 species of cyprinidae and one cobitidae from Italy, with some remarks on the problem of polyploidy in the cypriniformes

The present paper gives a detailed description of the morphology of the karyotype of 11 Cyprinidae and 1 Cobitidae indigenous to Italian inland waters. The data relating to Chondrostoma toxostoma, C. soetta, Phoxinus phoxinus, Rutilus rubilio, Barbus meridionalis and Cobitis taenia are new from a karyological point of view; as regards Leuciscus cephalus, L. souffia, Alburnus alburnus, Scardinius erythrophtalmus, Tinca tinca and Barbus barbus the already known diploid numbers are confirmed and the morphology of the karyotypes is specified. The data on the morphology of the karyotype are used in discussing the problem of polyploidization of the chromosomal complement in both the Cyprinidae and the whole order Cypriniformes.

Chiasma repatterning across a chromosomal hybrid zone between chromosomal races of Mus musculus domesticus.

Chiasma number and distribution were analysed in male house mice from a karyotypic hybrid zone between the CD race (2n = 22) and the standard race (2n = 40) located in central Italy. Chiasma repatterning occurs across the transect. The overall trend produces a diminution of chiasmata in the mice with CD chromosomal background. The progressive reduction of chiasmata indicates that genes could pass from one race to another in an asymmetrical way: from metacentric races to the standard population.

Karyotype and intermale aggression in wild house mice: Ecology and speciation

Social aggression shown by male mice from two races from upper Valtellina in Italy with 2n=24 and 26 chromosomes, respectively, was contrasted in dyadic encounters. In encounters with mice of the same karyotype 2n=26 were uniformly more aggressive than their 2n=24 counterparts. When resident mice from both these races were tested against intruders of the other karyotype, 2n=26 mice again were more likely to prove aggressive and to eventually dominate their partners even when they were the intruder. Perhaps those behavioral differences account for the apparent replacing of the 2n=24 chromosome race by the 2n=26 variety in geographical areas where they overlap.

Genome distribution, chromosomal allocation, and organization of the major and minor satellite DNAs in 11 species and subspecies of the genus Mus

We compared the genome distribution, chromosomal allocation, and organization of the major and minor satellite DNAs (satDNAs) in 11 species and subspecies of the genus Mus. Southern blot analysis of the major and minor satDNAs showed similar fragment profiles in all 11 species, with the exception of M. cervicolor and M. cookii for the major satDNAs and M. caroli, M. cervicolor, and M. cookii for the minor satDNAs, where these sequences could not be detected by the probes used. In situ hybridization of the major and minor satDNA probes revealed chromosome-specific allocations of these sequences with quantitative species-specific patterns. Fluorometric analysis of the organization of the satellite sequences suggested that in the M. domesticus genome satDNA sequences are clustered in tandem repeats that are longer than those present in other Mus genomes. When compared with the other Mus genomes so far studied, the domesticus genome shows the highest quantity of satDNA sequences with a long-range organization of satDNA sequences.