Comparative chromosome studies in Cetacea

A comparative analysis of the karyotypes of three dolphins - Tursiops truncatus Montagu, 1821, Tursiops australis Charlton-Robb et al., 2011, and Grampus griseus Cuvier, 1812

The aim of this study is to produce G-banded karyotypes of three dolphin species, Tursiops truncatus Montagu, 1821, Tursiops australisCharlton-Robb et al., 2011, and Grampus griseus Cuvier, 1812, and to determine if any differences between the species can be observed. Monolayer skin cultures were established and processed for chromosome study by trypsin banding. The results indicate that the three species here investigated have the same diploid number (2n = 44) and very similar gross chromosome morphology, however G-banding allows distinction between each species. Chromosome 1 in G. griseus is significantly different from the other 2 species, and chromosome 2 in T. australis is subtly different from the other 2 species. This result is of potential significance in taxonomic studies, and can provide an unequivocal answer in the assessment of suspected hybrids between these species.

Karyotype Evolution in 10 Pinniped Species: Variability of Heterochromatin versus High Conservatism of Euchromatin as Revealed by Comparative Molecular Cytogenetics

Pinnipedia karyotype evolution was studied here using human, domestic dog, and stone marten whole-chromosome painting probes to obtain comparative chromosome maps among species of Odobenidae (Odobenus rosmarus), Phocidae (Phoca vitulina, Phoca largha, Phoca hispida, Pusa sibirica, Erignathus barbatus), and Otariidae (Eumetopias jubatus, Callorhinus ursinus, Phocarctos hookeri, and Arctocephalus forsteri). Structural and functional chromosomal features were assessed with telomere repeat and ribosomal-DNA probes and by CBG (C-bands revealed by barium hydroxide treatment followed by Giemsa staining) and CDAG (Chromomycin A3-DAPI after G-banding) methods. We demonstrated diversity of heterochromatin among pinniped karyotypes in terms of localization, size, and nucleotide composition. For the first time, an intrachromosomal rearrangement common for Otariidae and Odobenidae was revealed. We postulate that the order of evolutionarily conserved segments in the analyzed pinnipeds is the same as the order proposed for the ancestral Carnivora karyotype (2n = 38). The evolution of conserved genomes of pinnipeds has been accompanied by few fusion events (less than one rearrangement per 10 million years) and by novel intrachromosomal changes including the emergence of new centromeres and pericentric inversion/centromere repositioning. The observed interspecific diversity of pinniped karyotypes driven by constitutive heterochromatin variation likely has played an important role in karyotype evolution of pinnipeds, thereby contributing to the differences of pinnipeds’ chromosome sets.

An Indo-Pacific Humpback Dolphin Genome Reveals Insights into Chromosome Evolution and the Demography of a Vulnerable Species

The Indo-Pacific humpback dolphin (Sousa chinensis) is a small inshore species of odontocete cetacean listed as Vulnerable on the IUCN Red List. Here, we report on the evolution of S. chinensis chromosomes from its cetruminant ancestor and elucidate the evolutionary history and population genetics of two neighboring S. chinensis populations. We found that breakpoints in ancestral chromosomes leading to S. chinensis could have affected the function of genes related to kidney filtration, body development, and immunity. Resequencing of individuals from two neighboring populations in the northwestern South China Sea, Leizhou Bay and Sanniang Bay, revealed genetic differentiation, low diversity, and small contemporary effective population sizes. Demographic analyses showed a marked decrease in the population size of the two investigated populations over the last ~4,000 years, possibly related to climatic oscillations. This study implies a high risk of extinction and strong conservation requirement for the Indo-Pacific humpback dolphin.

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Deducing chromosome evolution from ancestral Cetruminantia and ancestral Odontoceti

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Reconstructing the demographic history of Sousa chinensis

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Implying high risk of extinction and strong conservation requirement for S. chinensis

Description of the karyotypes of Stejneger's beaked whale (Mesoplodon stejnegeri) and Hubbs' beaked whale (M. carlhubbsi)

The genus Mesoplodon (Cetacea: Odontoceti: Ziphiidae) is one of the few cetacean genera with the karyotype 2n = 42. The 2n = 42 karyotype of M. europaeus and M. carlhubbsi is largely consistent with the general cetacean karyotype 2n = 44, although other 2n = 42 karyotypes do not exhibit clear homologies with the general cetacean karyotype. Therefore, the chromosomes of Mesoplodon species may be the key to understanding cetacean karyological evolution. In the present study, the male karyotypes of M. stejnegeri and M. carlhubbsi were examined. In both species, the diploid number of the male karyotype was 42. Both species had the following characteristics: 1) a huge subtelocentric X chromosome with a large C-block; 2) a small metacentric Y chromosome; 3) nucleolus organizer regions (NORs) in the terminal regions of a large autosome and one or two small metacentric autosomes; 4) small metacentric autosomes; 5) large submetacentric and subtelocentric autosomes; 6) less accumulated C-heterochromatin in the centromeric region; and 7) heteromorphism in C-heterochromatin accumulation between homologues. Characteristics 1 and 3 are peculiar to only the karyotypes of Mesoplodon species, whereas characteristics 4, 5, 6, and 7 are also found in the species with the general cetacean karyotype 2n = 44.

X Chromosome Evolution in Cetartiodactyla

The phenomenon of a remarkable conservation of the X chromosome in eutherian mammals has been first described by Susumu Ohno in 1964. A notable exception is the cetartiodactyl X chromosome, which varies widely in morphology and G-banding pattern between species. It is hypothesized that this sex chromosome has undergone multiple rearrangements that changed the centromere position and the order of syntenic segments over the last 80 million years of Cetartiodactyla speciation. To investigate its evolution we have selected 26 evolutionarily conserved bacterial artificial chromosome (BAC) clones from the cattle CHORI-240 library evenly distributed along the cattle X chromosome. High-resolution BAC maps of the X chromosome on a representative range of cetartiodactyl species from different branches: pig (Suidae), alpaca (Camelidae), gray whale (Cetacea), hippopotamus (Hippopotamidae), Java mouse-deer (Tragulidae), pronghorn (Antilocapridae), Siberian musk deer (Moschidae), and giraffe (Giraffidae) were obtained by fluorescent in situ hybridization. To trace the X chromosome evolution during fast radiation in specious families, we performed mapping in several cervids (moose, Siberian roe deer, fallow deer, and Pere David's deer) and bovid (muskox, goat, sheep, sable antelope, and cattle) species. We have identified three major conserved synteny blocks and rearrangements in different cetartiodactyl lineages and found that the recently described phenomenon of the evolutionary new centromere emergence has taken place in the X chromosome evolution of Cetartiodactyla at least five times. We propose the structure of the putative ancestral cetartiodactyl X chromosome by reconstructing the order of syntenic segments and centromere position for key groups.

Comparative Chromosome Map and Heterochromatin Features of the Gray Whale Karyotype (Cetacea)

Cetacean karyotypes possess exceptionally stable diploid numbers and highly conserved chromosomes. To date, only toothed whales (Odontoceti) have been analyzed by comparative chromosome painting. Here, we studied the karyotype of a representative of baleen whales, the gray whale (Eschrichtius robustus, Mysticeti), by Zoo-FISH with dromedary camel and human chromosome-specific probes. We confirmed a high degree of karyotype conservation and found an identical order of syntenic segments in both branches of cetaceans. Yet, whale chromosomes harbor variable heterochromatic regions constituting up to a third of the genome due to the presence of several types of repeats. To investigate the cause of this variability, several classes of repeated DNA sequences were mapped onto chromosomes of whale species from both Mysticeti and Odontoceti. We uncovered extensive intrapopulation variability in the size of heterochromatic blocks present in homologous chromosomes among 3 individuals of the gray whale by 2-step differential chromosome staining. We show that some of the heteromorphisms observed in the gray whale karyotype are due to distinct amplification of a complex of common cetacean repeat and heavy satellite repeat on homologous autosomes. Furthermore, we demonstrate localization of the telomeric repeat in the heterochromatin of both gray and pilot whale (Globicephala melas, Odontoceti). Heterochromatic blocks in the pilot whale represent a composite of telomeric and common repeats, while heavy satellite repeat is lacking in the toothed whale consistent with previous studies.

Molecular cytogenetic characterization of the Amazon River dolphin Inia geoffrensis

Classical and molecular cytogenetic (18S rDNA, telomeric sequence, and LINE-1 retrotransposon probes) studies were carried out to contribute to an understanding of the organization of repeated DNA elements in the Amazon River dolphin (boto, Inia geoffrensis). Twenty-seven specimens were examined, each presenting 2n = 44 chromosomes, the karyotype formula 12m + 14sm + 6st + 10t + XX/XY, and fundamental number (FN) = 74. C-positive heterochromatin was observed in terminal and interstitial positions, with the occurrence of polymorphism. Interstitial telomeric sequences were not observed. The nucleolar organizer region (NOR) was located at a single site on a smallest autosomal pair. LINE-1 was preferentially distributed in the euchromatin regions, with the greatest accumulation on the X chromosome. Although the karyotype structure in cetaceans is considered to be conserved, the boto karyotype demonstrated significant variations in its formula, heterochromatin distribution, and the location of the NOR compared to other cetacean species. These results contribute to knowledge of the chromosome organization in boto and to a better understanding of karyoevolution in cetaceans.

Big and slow: phylogenetic estimates of molecular evolution in baleen whales (suborder mysticeti)

Baleen whales are the largest animals that have ever lived. To develop an improved estimation of substitution rate for nuclear and mitochondrial DNA for this taxon, we implemented a relaxed-clock phylogenetic approach using three fossil calibration dates: the divergence between odontocetes and mysticetes approximately 34 million years ago (Ma), between the balaenids and balaenopterids approximately 28 Ma, and the time to most recent common ancestor within the Balaenopteridae approximately 12 Ma. We examined seven mitochondrial genomes, a large number of mitochondrial control region sequences (219 haplotypes for 465 bp) and nine nuclear introns representing five species of whales, within which multiple species-specific alleles were sequenced to account for within-species diversity (1-15 for each locus). The total data set represents >1.65 Mbp of mitogenome and nuclear genomic sequence. The estimated substitution rate for the humpback whale control region (3.9%/million years, My) was higher than previous estimates for baleen whales but slow relative to other mammal species with similar generation times (e.g., human-chimp mean rate > 20%/My). The mitogenomic third codon position rate was also slow relative to other mammals (mean estimate 1%/My compared with a mammalian average of 9.8%/My for the cytochrome b gene). The mean nuclear genomic substitution rate (0.05%/My) was substantially slower than average synonymous estimates for other mammals (0.21-0.37%/My across a range of studies). The nuclear and mitogenome rate estimates for baleen whales were thus roughly consistent with an 8- to 10-fold slowing due to a combination of large body size and long generation times. Surprisingly, despite the large data set of nuclear intron sequences, there was only weak and conflicting support for alternate hypotheses about the phylogeny of balaenopterid whales, suggesting that interspecies introgressions or a rapid radiation has obscured species relationships in the nuclear genome.

Chromosomal change and rectangular evolution in North American cyprinid fishes

An important question in evolutionary biology concerns the manner and tempo in which organismal and/or genetic changes that promote evolutionary divergence occur. One recent hypothesis, termed rectangular evolution, holds that most significant evolutionary change occurs during occasional or periodic speciation episodes, with long periods of evolutionary stability in the interim. An alternative view, termed phyletic gradualism, holds that evolutionary divergences proceed by the slow and even accumulation of genetic differences within populations of established species. Two brief tests of rectangular evolution are presented using chromosomal data from North American cyprinid fishes (minnows), a group known to have experienced heterogeneous rates of splitting. Within the rapidly speciated genus Notropis, rates of chromosomal evolution appear slower relative to other, less rapidly speciated confamilial genera. Species of Notropis also are less divergent chromosomally, on the average, than are species from other cyprinid genera. These results are in compatible with a rectangular mode of chromosomal divergence these fishes. The results also reveal inconsistencies with a gradual mode chromosomal divergence, but at present this hypothesis cannot be falsified. Consideration of these and other data suggests that different levels of the cyprinid genome may follow independent evolutionary paths.

Cross-species chromosome painting in Cetartiodactyla: reconstructing the karyotype evolution in key phylogenetic lineages

Recent molecular and morphological studies place Artiodactyla and Cetacea into the order Cetartiodactyla. Within the Cetartiodactyla such families as Bovidae, Cervidae, and Suidae are well studied by comparative chromosome painting, but many taxa that are crucial for understanding cetartiodactyl phylogeny remain poorly studied. Here we present the genome-wide comparative maps of five cetartiodactyl species obtained by chromosome painting with human and dromedary paint probes from four taxa: Cetacea, Hippopotamidae, Giraffidae, and Moschidae. This is the first molecular cytogenetic report on pilot whale, hippopotamus, okapi, and Siberian musk deer. Our results, when integrated with previously published comparative chromosome maps allow us to reconstruct the evolutionary pathway and rates of chromosomal rearrangements in Cetartiodactyla. We hypothesize that the putative cetartiodactyl ancestral karyotype (CAK) contained 25-26 pairs of autosomes, 2n = 52-54, and that the association of human chromosomes 8/9 could be a cytogenetic signature that unites non-camelid cetartiodactyls. There are no unambiguous cytogenetic landmarks that unite Hippopotamidae and Cetacea. If we superimpose chromosome rearrangements on the supertree generated by Price and colleagues, several homoplasy events are needed to explain cetartiodactyl karyotype evolution. Our results apparently favour a model of non-random breakpoints in chromosome evolution. Cetariodactyl karyotype evolution is characterized by alternating periods of low and fast rates in various lineages. The highest rates are found in Suina (Suidae+Tayasuidae) lineage (1.76 rearrangements per million years (R/My)) and the lowest in Cetaceans (0.07 R/My). Our study demonstrates that the combined use of human and camel paints is highly informative for revealing evolutionary karyotypic rearrangements among cetartiodactyl species.

G-banded karyotype and ideogram for the North Atlantic right whale (Eubalaena glacialis)

Published cytogenetic data for extant cetacean species remain incomplete. In a review of the literature, we found karyotypic information for 6 of the 13 tentatively recognized species of the suborder Mysticeti (baleen whales). Among those yet to be described is the critically endangered North Atlantic right whale (Eubalaena glacialis). Herein, we describe and propose a first-generation G-banded karyotype and ideogram for this species (2n = 42), obtained from peripheral blood chromosome preparations from a stranded male calf. This information may prove useful for future genetic mapping projects and for interspecific and intraspecific genomic comparisons by techniques such as zoo-FISH.

Natural hybridization between Dall's porpoises (Phocoenoides dalli) and harbour porpoises (Phocoena phocoena)

Natural hybridization occurs rarely in mammals compared with other taxonomic groups of animals. Cetaceans appear unique among mammals in exhibiting striking karyological uniformity, which suggests that they have the potential to produce hybrid offspring more readily than other mammals. However, the detection and accurate identification of wild mammalian hybrids is difficult, and molecular evidence for wild cetacean hybrids is extremely limited. Here, we present molecular and morphological evidence of frequent hybridization between free-ranging Dall's, Phocoenoides dalli (True, 1885), and harbour, Phocoena phocoena (L., 1758), porpoises. The study describes a temporally and geographically concentrated case of natural hybridization in large mammals. Molecular analyses of mitochondrial and nuclear DNA revealed the species identity, sex, and direction of cross of several hybrid individuals. In concert with morphological and behavioural observations, these data confirmed the hybrid status of putative crosses in the field, including reproductive females. All crosses examined had Dall's porpoise as the maternal parent. This directionality may reflect the indiscriminate pursuit of female porpoises by male harbour porpoises. Our finding of extensive localized hybridization, despite apparently strong isolation elsewhere in their range, suggests that ecological influences on mating behaviour may be of primary importance in the reproductive isolation of these, and possibly other, cetacean species.

Mitogenomic analyses provide new insights into cetacean origin and evolution

The evolution of the order Cetacea (whales, dolphins, porpoises) has, for a long time, attracted the attention of evolutionary biologists. Here we examine cetacean phylogenetic relationships on the basis of analyses of complete mitochondrial genomes that represent all extant cetacean families. The results suggest that the ancestors of recent cetaceans had an explosive evolutionary radiation 30-35 million years before present. During this period, extant cetaceans divided into the two primary groups, Mysticeti (baleen whales) and Odontoceti (toothed whales). Soon after this basal split, the Odontoceti diverged into the four extant lineages, sperm whales, beaked whales, Indian river dolphins and delphinoids (iniid river dolphins, narwhals/belugas, porpoises and true dolphins). The current data set has allowed test of two recent morphological hypotheses on cetacean origin. One of these hypotheses posits that Artiodactyla and Cetacea originated from the extinct group Mesonychia, and the other that Mesonychia/Cetacea constitutes a sister group to Artiodactyla. The current results are inconsistent with both these hypotheses. The findings suggest that the claimed morphological similarities between Mesonychia and Cetacea are the result of evolutionary convergence rather than common ancestry.

Molecular identification of hybrids between the two largest whale species, the blue whale (Balaenoptera musculus) and the fin whale (B. physalus)

Three anomalous balaenopterid whales, one pregnant female and two sterile males, were investigated by applying molecular approaches in order to establish their identity. The analysis showed that the whales were species hybrids between the blue and the fin whales. The female and one of the males had a blue whale mother and a fin whale father. The other male had a fin whale mother and a blue whale father. The difference between the mitochondrial cytochrome b gene of the two species suggests that they separated greater than or equal to 3.5 million years ago. The sequences of the mitochondrial control region of the blue and the fin whales differ by 7%. The difference in the mtDNA control region between three blue whale mtDNA haplotypes was less than or equal to 1%, about one tenth of the difference between the two species.

Intragenomic movement, sequence amplification and concerted evolution in satellite DNA in harvest mice, Reithrodontomys: evidence from in situ hybridization

Three DNA probes isolated from three species of Reithrodontomys (R. montanus, R. megalotis, R. fulvescens) were used to examine within and among species variation in the chromosomal location of satellite DNA and constitutive heterochromatin. These probes hybridized to the centromeric regions on all chromosomes in six species of the subgenus Reithrodontomys. Additionally, nearly all extra-centromeric C-band positive regions (with the exception of some heterochromatic material on the X and Y) hybridized to these probes. Within the subgenus Reithrodontomys both the chromosomal distribution and organization of satellite DNA has changed throughout evolution. The evolutionary transition has been from a totally centromeric position in R. fulvescens to centromeric and non-centromeric regions in other species that have undergone extensive chromosomal rearrangements from the primitive karyotype for peromyscine rodents. In addition, the monomer repeat of the satellite sequence differs between R. fulvescens (monomer defined by PstI) and the remaining species in the subgenus Reithrodontomys (monomer defined by EcoRI). These results suggest at least two amplification events for this satellite DNA sequence. Models and mechanisms concerned with the homogenization and spread of satellite sequences in complex genomes are evaluated in light of the Reithrodontomys data. From a phylogenetic standpoint, the satellite sequences composing heterochromatic regions were restricted to the subgenus Reithrodontomys, which supports morphological differences used to recognize two subgenera, Reithrodontomys and Aporodon. Probes failed to hybridize to any part of the karyotype of R. mexicanus (subgenus Aporodon) or to seven species from other closely related genera (Baiomys, Neotoma, Nyctomys, Ochrotomys, Onychomys, Peromyscus, Xenomys), some of which are considered as potential sister taxa for Reithrodontomys.

Composition and chromosomal localization of cetacean highly repetitive DNA with special reference to the blue whale, Balaenoptera musculus

Three highly repetitive DNA components--the common cetacean component, the heavy (GC-rich) satellite and the light (AT-rich) satellite--were were studied in the blue whale. Consensus sequences of the common component and the heavy satellite were determined on the basis of three repeats of the common component and eight repeats of the heavy satellite. The tandemly organized common cetacean component, which comprises a large portion of all cetacean--both odontocete (toothed whale) and mysticete (whalebone whale)--genomes has a repeat length of 1,760 bp and the three clones analysed showed a high degree of conformity. The repeat contains a 72 bp sequence with dyad symmetry and striking intrastrand complementarity. The rest of the repeat comprises a unique sequence. The repeat unit of the heavy satellite of the blue whale is 422 bp. Also this component is tandemly organized. About half the length of the repeat constitutes a unique sequence and the other half is made up of subrepeats with TTAGGG as a frequent motif. The light satellite has not been sequenced and its basic repeat unit has not yet been identified. The chromosomal localization of the three components was determined by in situ hybridization using 3H-labelled cloned fragments as probes. The common cetacean component was located in most interstitial and terminal C-bands. The heavy satellite occurred primarily in terminal C-bands. When the two components hybridized to the same terminal C-bands, the localization of the heavy satellite was distal to that of the common cetacean component. Neither component shared localization with the light satellite which is located in centromeric C-bands in just a few chromosome pairs.

Monoparametric models of flow cytometric karyotypes with spreadsheet software

Theoretical flow karyotypes from both plant and mammalian species have been simply modelled using computer spreadsheet software. The models are based upon published values of relative DNA content or relative lengths of each of the chromosomes. From such data, the histograms of chromosome distribution have been simulated for both linear and logarithmic modules of a flow cytometer, and as a function of the coefficient of variation. Simulated and experimental histograms are compard for Nicotiana plumbaginifolia. This readily accessible exercise facilitates the planning and execution of flow cytometric analysis and sorting of chromosomes.

Localization of NORs in spermatogonial metaphase chromosomes of six species of grasshoppers

The silver staining technique was employed to locate Nucleolar Organiser Regions (NORs) in six species of grasshoppers viz. Aiolopus thalassinus F. (Tryxalinae); Oeodaleus abruptus Thunb., Gastrimargus transversus Thunb., Heteropternis respondens Walk. (Oedipodinae); Parahieroglyphus biliniatus Bol. and Spathosternum prasiniferum Walk. (Catantopinae). Usually the NORs were located on the larger elements of the chromosomal complement. However, in O. abruptus NORs were found on autosomes S8 and S9. The salient observations were: (1) NORs were seen in only a few of the several spermatogonial metaphases examined; (2) Active NORs were mostly located either on one chromatid of the homologues or on the homologue depicting heteromorphism; (3) NORs showed either proximal, subproximal or interstitial locations. However, in O. abruptus and P. bilineatus NORs were located at two positions. Distribution of NORs in different species and their probable role in tracing the evolutionary pathways in Acridoidea are discussed.

Conservation of highly repetitive DNA in cetaceans

It is controversial whether odontocetes (toothed whales) and mysticetes (whalebone whales) have a common ancestry. Cetacean karyological uniformity, which is unique among mammalian orders, suggests a monophyletic origin; however, several anatomical authorities have maintained that odontocetes and mysticetes are diphyletic. We investigated the issue using Southern blot hybridization. Two labelled restriction fragment probes from the DNA of the sei whale (a mysticete) were hybridized to restricted DNA of cetacean species representing all extant families except the Eschrichtiidae, the gray whales. The probes hybridized to specific restriction fragments in all odontocete and mysticete materials. Hybridization showed preservation of hybridization homologies and a striking conservation of the length of highly repeated DNA sequences. The results are compatible with a common ancestry of odontocetes and mysticetes.

Chromosomal homology in southern Akodon

Differential staining (G and C) of southern South American Akodon are presented. A. olivaceus, A. longipilis and A. sanborni all have the same karyotype (2n = 52, NF = 58). A virtually identical band sequence is observed. This situation is interpreted using the canalization model of chromosomal evolution which stresses an optimum karyotype for each adaptive zone. Despite the high degree of conservation of the chromosome structures, the specific status of these species is supported by maintenance of distinctness when they occur in areas of sympatry.

Genome evolution in pocket gophers (genus Thomomys). I. Heterochromatin variation and speciation potential

A basic dichotomy exists in the amount and chromosomal position of constitutive heterochromatin (C-bands) in species of pocket gophers, genus Thomomys. Members of the "talpoides-group" of species (e.g., T. talpoides and T. monticola) have C-bands restricted to the centromeric regions. These taxa are characterized by Robertsonian patterns of karyotypic evolution. In contrast, species within the "bottae-group" are characterized by extensive amounts of heterochromatin, placed as whole-arm and apparent whole-chromosome (T. bottae) or as large interstitial blocks (T. umbrinus). These species are characterized by extensive non-Robertsonian variation in karyotype, variation which may be expressed from local population polymorphism to between population or species polytypy. Within T. bottae, the number of whole-arm heterochromatic autosomes is inversely proportional to the number of uniarmed chromosomes in the complement, which ranges from 0 to 36 across the species populations. In all-biarmed karyotypic populations, upward to 60 percent of the linear length of the genome is composed of heterochromatin. Populations with extensive heterochromatin variation and those with similar amounts meet and hybridize freely in nature. The implications of these date for current ideas on the function of heterochromatin, particularly as related to speciation models, are discussed.

Satellite DNA and heterochromatin variants: the case for unequal mitotic crossing over

Variations of constitutive heterochromatin (heteromorphisms) appear to be a general feature of eucaryotes. A variety of molecular and cytogenetic evidence supports the hypothesis that heteromorphisms result from unequal double-strand exchanges during mitotic DNA replication. Constitutive heterochromatin consists of highly repeated DNA sequences that are not transcribed. Thus, heteromorphisms are tolerated without overt phenotypic effect. Several of the highly repeated DNAs that comprise constitutive heterochromatin have been shown to contain site-specific endonuclease recognition sequences interspersed at regular intervals dependent upon nucleosome structure. These interspersed short repeated sequences could mediate unequal crossovers, resulting in quantitative variability of constitutive heterochromatin and satellite DNA. De novo variations of constitutive heterochromatin may be useful as markers of exposure to mutagens and/or carcinogens.

Conservation and chromosomal localization of DNA satellites in balenopterid whales

DNA satellites were isolated from three balenopterid species, viz. the minke, sei, and fine whales. In each of them at least two DNA satellites were recognizable with buoyant densities in neutral CsCl of rho = 1.702/1.703 and rho = 1.710/1;711, respectively. cRNAs from each satellite group were used for filter and in situ hybridisations. Homo-and heterologous DNA-cRNA hybrids within each satellite group yielded virtually identical melting curve profiles showing conservation of at least a considerable part of the DNA satellite sequences. There was no evident sequence homology between the rho = 1.702/1.703 and the rho = 1.710/1;711 satellites by filter hybridisation.--The in situ hybridisation showed that in each species the rho = 1.702/1.703 satellite was located in centromeric-paracentromeric C-bands in a few pairs, whereas the rho = 1.710/1.711 satellite was located in terminal C-bands throughout the karyotypes.--The data on the whale DNA satellites indicate that the quantitative evolution of the sateliite DNA sequences preceded species divergence of the balenopterids and that the satellite sequences have remained relatively unaltered since the divergence took place. The function of satellite DNA is considered to imply the introduction of both chromosomal and genic polymorphisms and thus being of great importance in speciation, Based upon these concepts a model is postulated for the function of satellite DNA. According to this model at meiotic pairing euchromatinheterochromatin overlapping between homologous chromosomes is considered to be of a general occurrence. This overlapping is presumed to be accentuated by the size heteromorphism frequently observed between homologous heterochromatic segments (C-bands). In the region of such euchromatinheterochromatin overlapping, cross-over would be excluded. The overlapping is suggested to be rectified progresssively in the chromosome arms, leaving unaffected crossing-over distant to the euchromatin-heterochromatin junctions. The consequence of this will be that genes in the proximity of the junctions are collectively inherited and selected, whereas genes distant to the the heterochromatin will be independently assorted and selected.