Chromosomal rearrangements, speciation and the theoretical approach

Chromosomal rearrangements played an important role in the speciation of rice rats of genus Cerradomys (Rodentia, Sigmodontinae, Oryzomyini)

Rodents of the genus Cerradomys belong to tribe Oryzomyini, one of the most diverse and speciose groups in Sigmodontinae (Rodentia, Cricetidae). The speciation process in Cerradomys is associated with chromosomal rearrangements and biogeographic dynamics in South America during the Pleistocene era. As the morphological, molecular and karyotypic aspects of Myomorpha rodents do not evolve at the same rate, we strategically employed karyotypic characters for the construction of chromosomal phylogeny to investigate whether phylogenetic relationships using chromosomal data corroborate the radiation of Cerradomys taxa recovered by molecular phylogeny. Comparative chromosome painting using Hylaeamys megacephalus (HME) whole chromosome probes in C. langguthi (CLA), Cerradomys scotii (CSC), C. subflavus (CSU) and C. vivoi (CVI) shows that karyotypic variability is due to 16 fusion events, 2 fission events, 10 pericentric inversions and 1 centromeric repositioning, plus amplification of constitutive heterochromatin in the short arms of the X chromosomes of CSC and CLA. The chromosomal phylogeny obtained by Maximum Parsimony analysis retrieved Cerradomys as a monophyletic group with 97% support (bootstrap), with CSC as the sister to the other species, followed by a ramification into two clades (69% of branch support), the first comprising CLA and the other branch including CVI and CSU. We integrated the chromosome painting analysis of Eumuroida rodents investigated by HME and Mus musculus (MMU) probes and identified several syntenic blocks shared among representatives of Cricetidae and Muridae. The Cerradomys genus underwent an extensive karyotypic evolutionary process, with multiple rearrangements that shaped extant karyotypes. The chromosomal phylogeny corroborates the phylogenetic relationships proposed by molecular analysis and indicates that karyotypic diversity is associated with species radiation. Three syntenic blocks were identified as part of the ancestral Eumuroida karyotype (AEK): MMU 7/19 (AEK 1), MMU 14 (AEK 10) and MMU 12 (AEK 11). Besides, MMU 5/10 (HME 18/2/24) and MMU 8/13 (HME 22/5/11) should be considered as signatures for Cricetidae, while MMU 5/9/14, 5/7/19, 5 and 8/17 for Sigmodontinae.

Genome structure and evolution in the cruciferous tribe Thlaspideae (Brassicaceae)

Whole-genome duplications (WGDs) and chromosome rearrangements (CRs) play the key role in driving the diversification and evolution of plant lineages. Although the direct link between WGDs and plant diversification is well documented, relatively few studies focus on the evolutionary significance of CRs. The cruciferous tribe Thlaspideae represents an ideal model system to address the role of large-scale chromosome alterations in genome evolution, as most Thlaspideae species share the same diploid chromosome number (2n = 2x = 14). Here we constructed the genome structure in 12 Thlaspideae species, including field pennycress (Thlaspi arvense) and garlic mustard (Alliaria petiolata). We detected and precisely characterized genus- and species-specific CRs, mostly pericentric inversions, as the main genome-diversifying drivers in the tribe. We reconstructed the structure of seven chromosomes of an ancestral Thlaspideae genome, identified evolutionary stable chromosomes versus chromosomes prone to CRs, estimated the rate of CRs, and uncovered an allohexaploid origin of garlic mustard from diploid taxa closely related to A. petiolata and Parlatoria cakiloidea. Furthermore, we performed detailed bioinformatic analysis of the Thlaspideae repeatomes, and identified repetitive elements applicable as unique species- and genus-specific barcodes and chromosome landmarks. This study deepens our general understanding of the evolutionary role of CRs, particularly pericentric inversions, in plant genome diversification, and provides a robust base for follow-up whole-genome sequencing efforts.

Karyotypic divergence reveals that diversity in the Oecomys paricola complex (Rodentia, Sigmodontinae) from eastern Amazonia is higher than previously thought

The genus Oecomys (Rodentia, Sigmodontinae) is distributed from southern Central America to southeastern Brazil in South America. It currently comprises 18 species, but multidisciplinary approaches such as karyotypic, morphological and molecular studies have shown that there is a greater diversity within some lineages than others. In particular, it has been proposed that O. paricola constitutes a species complex with three evolutionary units, which have been called the northern, eastern and western clades. Aiming to clarify the taxonomic status of O. paricola and determine the relevant chromosomal rearrangements, we investigated the karyotypes of samples from eastern Amazonia by chromosomal banding and FISH with Hylaeamys megacephalus (HME) whole-chromosome probes. We detected three cytotypes for O. paricola: A (OPA-A; 2n = 72, FN = 75), B (OPA-B; 2n = 70, FN = 75) and C (OPA-C; 2n = 70, FN = 72). Comparative chromosome painting showed that fusions/fissions, translocations and pericentric inversions or centromeric repositioning were responsible for the karyotypic divergence. We also detected exclusive chromosomal signatures that can be used as phylogenetic markers. Our analysis of karyotypic and distribution information indicates that OPA-A, OPA-B and OPA-C are three distinct species that belong to the eastern clade, with sympatry occurring between two of them, and that the “paricola group” is more diverse than was previously thought.

Ancestral Reconstruction of Karyotypes Reveals an Exceptional Rate of Nonrandom Chromosomal Evolution in Sunflower

Mapping the chromosomal rearrangements between species can inform our understanding of genome evolution, reproductive isolation, and speciation. Here, we present a novel algorithm for identifying regions of synteny in pairs of genetic maps, which is implemented in the accompanying R package syntR. The syntR algorithm performs as well as previous ad hoc methods while being systematic, repeatable, and applicable to mapping chromosomal rearrangements in any group of species. In addition, we present a systematic survey of chromosomal rearrangements in the annual sunflowers, which is a group known for extreme karyotypic diversity. We build high-density genetic maps for two subspecies of the prairie sunflower, Helianthus petiolaris ssp. petiolaris and H. petiolaris ssp. fallax Using syntR, we identify blocks of synteny between these two subspecies and previously published high-density genetic maps. We reconstruct ancestral karyotypes for annual sunflowers using those synteny blocks and conservatively estimate that there have been 7.9 chromosomal rearrangements per million years, a high rate of chromosomal evolution. Although the rate of inversion is even higher than the rate of translocation in this group, we further find that every extant karyotype is distinguished by between one and three translocations involving only 8 of the 17 chromosomes. This nonrandom exchange suggests that specific chromosomes are prone to translocation and may thus contribute disproportionately to widespread hybrid sterility in sunflowers. These data deepen our understanding of chromosome evolution and confirm that Helianthus has an exceptional rate of chromosomal rearrangement that may facilitate similarly rapid diversification.

Chromosomal phylogeny and comparative chromosome painting among Neacomys species (Rodentia, Sigmodontinae) from eastern Amazonia

BACKGROUND

The Neacomys genus is predominantly found in the Amazon region, and belongs to the most diverse tribe of the Sigmodontinae subfamily (Rodentia, Cricetidae, Oryzomyini). The systematics of this genus and questions about its diversity and range have been investigated by morphological, molecular (Cytb and COI sequences) and karyotype analysis (classic cytogenetics and chromosome painting), which have revealed candidate species and new distribution areas. Here we analyzed four species of Neacomys by chromosome painting with Hylaeamys megacephalus (HME) whole-chromosome probes, and compared the results with two previously studied Neacomys species and with other taxa from Oryzomyini and Akodontini tribes that have been hybridized with HME probes. Maximum Parsimony (MP) analyses were performed with the PAUP and T.N.T. software packages, using a non-additive (unordered) multi-state character matrix, based on chromosomal morphology, number and syntenic blocks. We also compared the chromosomal phylogeny obtained in this study with molecular topologies (Cytb and COI) that included eastern Amazonian species of Neacomys, to define the phylogenetic relationships of these taxa.

RESULTS

The comparative chromosome painting analysis of the seven karyotypes of the six species of Neacomys shows that their diversity is due to 17 fusion/fission events and one translocation, pericentric inversions in four syntenic blocks, and constitutive heterochromatin (CH) amplification/deletion of six syntenic autosomal blocks plus the X chromosome. The chromosomal phylogeny is consistent with the molecular relationships of species of Neacomys. We describe new karyotypes and expand the distribution area for species from eastern Amazonia and detect complex rearrangements by chromosome painting among the karyotypes.

CONCLUSIONS

Our phylogeny reflects the molecular relationships of the Akodontini and Oryzomyini taxa and supports the monophyly of Neacomys. This work presents new insights about the chromosomal evolution of this group, and we conclude that the karyotypic divergence is in accord with phylogenetic relationships.

Limited intrinsic postzygotic reproductive isolation despite chromosomal rearrangements between closely related sympatric species of small ermine moths (Lepidoptera: Yponomeutidae)

In evolutionarily young species and sympatric host races of phytophagous insects, postzygotic incompatibility is often not yet fully developed, but reduced fitness of hybrids is thought to facilitate further divergence. However, empirical evidence supporting this hypothesis is limited. To assess the role of reduced hybrid fitness, we studied meiosis and fertility in hybrids of two closely related small ermine moths, Yponomeuta padella and Yponomeuta cagnagella, and determined the extent of intrinsic postzygotic reproductive isolation. We found extensive rearrangements between the karyotypes of the two species and irregularities in meiotic chromosome pairing in their hybrids. The fertility of reciprocal F1 and, surprisingly, also of backcrosses with both parental species was not significantly decreased compared with intraspecific offspring. The results indicate that intrinsic postzygotic reproductive isolation between these closely related species is limited. We conclude that the observed chromosomal rearrangements are probably not the result of an accumulation of postzygotic incompatibilities preventing hybridization. Alternative explanations, such as adaptation to new host plants, are discussed.

Pooling strategy and chromosome painting characterize a living zebroid for the first time

We have investigated the complex karyotype of a living zebra-donkey hybrid for the first time using chromosome-specific painting probes produced from flow-sorted chromosomes from a zebra (Equus burchelli) and horse (Equus caballus). As the chromosomes proved difficult to distinguish from one another, a successful new strategy was devised to resolve the difficulty and characterize each chromosome. This was based on selecting five panels of whole chromosome painting probes that could differentiate zebra and donkey chromosomes by labelling the probes with either FITC or Cy3 fluorochromes. Each panel was hybridized sequentially to the same G-Q-banded metaphases and the results combined so that every zebra and donkey chromosome in each suitable metaphase could be identified. A diploid number of 2n = 53, XY was found, containing haploid sets of 22 chromosomes from the zebra and 31 chromosomes from the donkey, without evidence of chromosome rearrangement. This new strategy, developed for the first time, may have several applications in the resolution of other complex hybrid karyotypes and chromosomal aberrations.

Analysis of meiotic chromosome structure and behavior in Robertsonian heterozygotes of Ellobius tancrei (Rodentia, Cricetidae): a case of monobrachial homology

Synaptonemal complex (SC) chains were revealed in semisterile intraspecific F1 hybrids of Ellobius tancrei Blasius, 1884 (2n = 49, NF=56 and 2n=50, NF=56), heterozygous for Robertsonian (Rb) translocations. Chains were formed by Rb submetacentrics with monobrachial homology. Chromosome synapsis in spermatocytes of these hybrids was disturbed, apparently because of the problematic release of the chromosomes from the SC chains. These hybrids suffer from low fertility, and our data support the opinion that this is because a formation of Rb metacentrics with monobrachial homology within different races of the same species might be an initial event for the divergence of chromosomal forms.

Karyotypic variation in the Andean rodent Phyllotisxanthopygus (Waterhouse, 1837) (Rodentia, Cricetidae, Sigmodontinae)

Phyllotisxanthopygus (Waterhouse, 1837) is an Andean rodent endemic to South America. Despite its wide geographical distribution in Argentina, few individuals have been studied on the cytogenetic level and only through conventional staining. In this work, chromosome characterization of Argentine samples of this species was performed using solid staining, C-banding and base-specific fluorochromes. Twenty two specimens were analyzed, collected in the provinces of Jujuy, Catamarca, and the north and south of Mendoza. All studied specimens showed 2n=38, having mostly the bi-armed autosomes, metacentric or submetacentric. Fundamental Number varied between 70 and 72. These changes were due to the presence of chromosome heteromorphisms in individuals from southern Mendoza and Jujuy. C-banding revealed pericentromeric blocks of constitutive heterochromatin in most chromosomes. Acrocentric chromosomes involved in heteromorphisms showed high variation in the amount of heterochromatin within and among populations. Additionally, banding with fluorochromes (DAPI and chromomycin A3) revealed homologous localization of AT and GC rich regions among chromosomes of the different populations analyzed. Comparisons among heteromorphic pairs suggested, however, that the variation might be the result of complex chromosome rearrangements, involving possibly amplifications and/or deletions of heterochromatic segments. These results are in accordance with molecular studies that indicate genetic variability within and among the populations of this taxon.

Cryptic speciation within the Neotropical cichlid Geophagus brasiliensis (Quoy & Gaimard, 1824) (Teleostei Cichlidae): a new paradigm in karyotypical and molecular evolution

The family Cichlidae is one of the most species-rich taxa in the Neotropics. However, the factors that determine these high levels of biodiversity remain unexplored. We have analyzed the morphological, cytogenetic, and molecular data from 62 specimens of a widespread cichlid, Geophagus brasiliensis, from three adjacent basins in southeastern Brazil. Morphological analyses did not show differences among specimens. The cytogenetic data indicate the occurrence of multiple nucleolar organizer regions and four sympatric karyotypes that differ in the first pair of chromosome morphology, in the Doce River Basin; whereas the karyotype from the Paraíba do Sul Basin is widely divergent. The molecular data--616 bp fragment of cytochrome oxidase subunit I--revealed two haplogroups with the deepest genetic divergence (6.4%) ever reported within a nominal species in the Neotropical Region: One of the haplogroups is restricted to the quaternary lakes in the middle portion of the Doce Basin and the Mucuri River, whereas the other haplogroup is composed of haplotypes from elsewhere in the Doce Basin and the Paraíba do Sul Basin. These patterns suggest that G. brasiliensis undergoes a cryptic speciation process involving three major lineages that differ from the African explosive cichlid radiation.

Genetic and morphological variability in South American rodent Oecomys (Sigmodontinae, Rodentia): evidence for a complex of species

The rodent genus Oecomys (Sigmodontinae) comprises ~16 species that inhabit tropical and subtropical forests in Central America and South America. In this study specimens of Oecomys paricola Thomas, 1904 from Belém and Marajó island, northern Brazil, were investigated using cytogenetic, molecular and morphological analyses. Three karyotypes were found, two from Belém (2n = 68, fundamental number (FN) = 72 and 2n = 70, FN = 76) and a third from Marajó island (2n = 70, FN = 72). No molecular or morphological differences were found between the individuals with differing cytotypes from Belém, but differences were evident between the individuals from Belém and Marajó island. Specimens from Belém city region may represent two cryptic species because two different karyotypes are present in the absence of significant differences in morphology and molecular characteristics. The Marajó island and Belém populations may represent distinct species that have been separated for some time, and are in the process of morphological and molecular differentiation as a consequence of reproductive isolation at the geographic and chromosomal levels. Thus, the results suggest that O. paricola may be a complex of species.

Chromosomal rearrangements and the genetics of reproductive barriers in mimulus (monkey flowers)

Chromosomal rearrangements may directly cause hybrid sterility and can facilitate speciation by preserving local adaptation in the face of gene flow. We used comparative linkage mapping with shared gene-based markers to identify potential chromosomal rearrangements between the sister monkeyflowers Mimulus lewisii and Mimulus cardinalis, which are textbook examples of ecological speciation. We then remapped quantitative trait loci (QTLs) for floral traits and flowering time (premating isolation) and hybrid sterility (postzygotic isolation). We identified three major regions of recombination suppression in the M. lewisii × M. cardinalis hybrid map compared to a relatively collinear Mimulus parishii × M. lewisii map, consistent with a reciprocal translocation and two inversions specific to M. cardinalis. These inferences were supported by targeted intraspecific mapping, which also implied a M. lewisii-specific reciprocal translocation causing chromosomal pseudo-linkage in both hybrid mapping populations. Floral QTLs mapped in this study, along with previously mapped adaptive QTLs, were clustered in putatively rearranged regions. All QTLs for male sterility, including two underdominant loci, mapped to regions of recombination suppression. We argue that chromosomal rearrangements may have played an important role in generating and consolidating barriers to gene flow as natural selection drove the dramatic ecological and morphological divergence of these species.

A hybrid zone of the genus Ctenomys: A case study in southern Brazil

We describe variation at microsatellite loci and the chromosomal polymorphisms of a hybrid population, and hybridizing populations of Ctenomys minutus (the minor tuco-tuco) from the coastal plain of Rio Grande do Sul, southern Brazil. Cytogenetic analysis and a survey of six microsatellite loci included 101 specimens of C. minutus from the parental populations (2n/AN = 42/74 and 48a/76) and their contact zone. Cytogenetic analysis recorded 26 different karyotypes exhibited by 50 individuals from the hybrid population. Of the 26 karyotypes, only 14% presented a parental-like configuration, and none had the combinations of 2n and AN expected for an F1 hybrid. The remaining karyotypes were alternative hybrid forms, with 2n varying from 42 to 46 and AN from 68 to 80. These results suggest chromosomal rearrangements are only of minor significance in the establishment of reproductive barriers for this species.

Identification of distinct evolutionary units in allopatric populations of Hypostomus cf. wuchereri Günther, 1864 (Siluriformes: Loricariidae): karyotypic evidence

Few chromosomal reports are available for the endemic fish fauna from coastal basins in northeastern Brazil, and regional biodiversity remains partially or completely unknown. This is particularly true for Loricariidae, the most diverse family of armored catfishes. In the present work, allopatric populations of Hypostomus cf. wuchereri (Siluriformes: Loricariidae) from two basins in Bahia (northeastern Brazil) were cytogenetically analyzed. Both populations shared 2n = 76 chromosomes, a karyotype formula of 10m+18sm+48st/a (FN = 104) and single terminal GC-rich NORs on the second metacentric pair. Nevertheless, microstructural differences were detected by C-banding, fluorochrome staining and chromosomal digestion with restriction enzymes (Alu I, Bam HI, Hae III, and Dde I). The population from Una River (Recôncavo Sul basin) showed conspicuous heterochromatin blocks and a remarkable heterogeneity of base composition (presence of interspersed AT/GC-rich and exclusively AT- or GC-rich sites), while the population from Mutum river (Contas River basin) presented interstitial AT-rich C-bands and terminal GC/AT-rich heterochromatin. Each enzyme yielded a specific band profile per population which allowed us characterizing up to five heterochromatin families in each population. Based on the present data, we infer that these populations have been evolving independently, as favored by their geographic isolation, probably representing cryptic species.

Rate of Chromosome changes and Speciation in Reptiles

The chromosome changing rate (i.e. the number of chromosome rearrangements per million years) was studied in 1,329 reptile species in order to evaluate the karyological evolutionary trend and the existence of possible correlations between chromosome mutations and some aspects of the evolution of this class. The results obtained highlight the existence of a general direct correlation between chromosome changing rate and number of living species, although different trends can be observed in the different orders and suborders. In turtles, the separation of pleurodires from cryptodires was accompanied by a considerable karyological diversification. Among pleurodires, the evolution of the Chelidae and Pelomedusidae was also characterised by chromosome variation, while in cryptodires a marked karyological homogeneity is observed between and within infra-orders. Similarly there is no correlation between changing rate and species number in crocodiles, where the evolution of the families and genera has entailed few chromosome mutations. Chromosome variability was greater in lizards and snakes. In the formers variations in chromosome changing rate accompanied the separation of the infra-orders and the evolution of most of the families and of some genera. The origin of snakes has also been accompanied by a marked karyological diversification, while the subsequent evolution of the infra-orders and families has entailed a high level of chromosome variability only in colubroids. The karyological evolution in reptiles generally entailed a progressive reduction in chromosome changing rate, albeit with differences in the diverse orders and suborders. This trend seems to be consistent with the "canalization model" as originally proposed by Bickham and Baker in [Bickham, J.W. & R J. Baker, 1979. Bull. Carnegie Mus. Nat. Hist. 13: 70-84.] However, several inconsistencies have been found excluding that in this class the ultimate goal of chromosome variations was the achievement of a so-called "optimum karyotype'' as suggested by the above-mentioned theory. Other mechanisms could underpin chromosome variability in Reptiles. Among them a genomic composition more or less favourable to promoting chromosome rearrangements and factors favouring the fixation of a mutant karyotype in condition of homozygosis. Turtles and crocodiles would have a genome characterised by large chromosomes and a low level of chromosome compartmentalisation limiting the recombination and the frequency of rearrangements. A low rate of chromosome variability modifying little if at all the gene linkage groups would have favoured a conservative evolutionary strategy. In the course of evolution, lizards and snakes could have achieved a genome characterised by smaller chromosomes and a higher level of compartmentalisation. This would have raised the frequency of recombination and consequently an evolutionary strategy promoting a higher degree of variability and a greater level of speciation.

Phylogenetic relationships among some Ateles species: the use of chromosomic and molecular characters

As with most platyrrhines, the systematics of Ateles is under discussion. In order to help clarify its systematic, we employed chromosomic and molecular characters to analyze the phylogenetic relationship among some species of the genus Ateles. Chromosomic studies were conducted on 14 atelid specimens: eight Ateles from A. paniscus, A. chamek, A. belzebuth and A. geoffroyi, and six Alouatta caraya. Ateles paniscus showed 2N=32, whereas A. chamek, A. belzebuth and A. geoffroyi presented 2N=34, XX/XY (with a submetacentric X and a variable Y) corroborated by male meiosis. Nucleotide sequence variation at the mitochondrial cytochrome c oxidase subunit II gene (COII) was analyzed in ten New World monkey specimens. Parsimony trees showed consistent phylogenetic relationships using both chromosomic forms and mitochondrial COII gene sequences as characters. Particularly, chromosomic phylogenies showed A. hybridus as a divergent taxon from the remaining group, whereas A. chamek, A. belzebuth and A. marginatus form an unresolved clade with A. geoffroyi as sister group.

Rapid conversion of rDNA intergenic spacer of diploid mutants of rice derived from gamma-ray irradiated tetraploids

The organization of tandemly repeated sequences of ribosomal DNA (rDNA) in rice mutants derived from gamma-irradiated tetraploids was analyzed. Southern hybridization analysis of nuclear DNA revealed that most of the intergenic spacers (IGSs) in mutant rDNA are replaced concertedly by new molecular species. The new IGSs are produced by the amplification of a subrepeat of about 250 bp. Results obtained from sequence analyses indicate that various intermediate molecular species of the subrepeat were formed during structuring of the IGS region and that many rearrangements occurred between them. These findings demonstrate the effectiveness of recurrent irradiation of tetraploids for inducing artificial genome rearrangement, and also indicate the extreme plasticity and variability of genome structure in plants.

Meiotic synapsis of homogeneously staining regions (HSRs) in chromosome 1 of Mus musculus

About 50 copies of a long-range repeat DNA family with a repeat size of roughly 100 kb and with sequence homology to mRNAs are clustered in the G-light band D of chromosome 1 of the house mouse, Mus musculus. We studied amplified versions of the cluster which are found in many wild populations of M. musculus. They are cytogenetically conspicuous as one or two C-band positive homogeneously staining regions (single- and double band HSRs) which increase the mitotic length of chromosome 1. The double band HSR was phylogenetically derived from a single band HSR by a paracentric inversion. In homozygous condition, such HSRs contribute, albeit not as much as expected from their mitotic length, to the synaptonemal complex (SC) length of chromosome 1. In HSR heterozygous animals an elongation of the SCs was not noticeable. In single band HSR heterozygous males, synapsis proceeds regularly and continuously from the distal telomere towards the centromeric end without forming buckles. Thus, the single band HSR has no adverse effect on pairing. The same straight pairing behaviour was found in the majority of double band HSR heterozygous spermatocytes. This shows that extensive nonhomologous pairing can take place in the earliest phase of synapsis. Synapsis was discontinuous, leaving the central part of the bivalent 1 asynapsed, in only 14.3% of double band HSR heterozygous cells. In such cells the chromosome 1 SC is completed at a later stage of meiosis. The delay is presumably an effect of the inversion that includes one HSR band and the segment between the two HSR bands.

A dual level model for speciation by multiple pericentric inversions

A considerable body of evidence suggests that the deleterious meiotic effects of pericentric inversions in F1 hybrids can be overcome by changes in chiasma location and various means of non-homologous pairing. Such overriding mechanisms may render pericentric inversions benign and increase the likelihood of their fixation in population isolates. It has been argued that overriding mechanisms of this type negate the involvement of pericentric inversions as reproductive isolating mechanisms in speciation. It is suggested, however, that the involvement of pericentric inversions in speciation should be considered on two levels. First, that by reducing meiotic effects in F1 hybrids, overriding mechanisms facilitate the fixation of pericentric inversions. Secondly, when contact hybridization occurs between the chromosomally derived and parental populations second-level effects may be encountered. That is, the recombinational effects of pericentric inversion differences on coadapted gene complexes (sensu Brncic, 1954, Shaw & Coates, 1983) enforce profound inviability barriers in F2 and backcross matings. In this way, multiple pericentric inversions may act as significant post-mating isolating mechanisms, whereas individual inversions with less significant second-level effects may not.

The role of hybridization in the karyotype evolution of deer (Cervidae; Artiodactyla; Mammalia)

The karyotypes of 46 animals from a population ofCervus elaphus, Cervus nippon, and hybridsCervus elaphus xCervus nippon were studied using G- and C-banding techniques. It was found that four chromosome pairs known from the karyotype ofCervus elaphus are involved in two systems of Robertsonian translocations. Pedigree analysis supports the hypothesis of a simple Mendelian inheritance of each translocation system. With respect to these translocation systems, the population shows no significant deviation from the corresponding Hardy-Weinberg-structure. Thus, hybridization events can be accepted as a major factor in karyotype evolution of deer.

Exceptional chromosomal mutations in a rodent population are not strongly underdominant

The observation of karyotypic uniformity in most species has led to the widespread belief that selection limits chromosomal change. We report an unprecedented amount of chromosomal variation in a natural population of the South American marsh rat Holochilus brasiliensis. This variation consists of four distinct classes of chromosomal rearrangements: whole-arm translocations, pericentric inversions, variation in the amount of euchromatin, and variation in number and kind of supernumerary (B) chromosomes. Twenty-six karyotypes are present among 42 animals. Observations of the natural population over a 7-year period and breeding experiments with captive animals indicate that heterozygous individuals suffer no detectable reduction in fitness. This is at odds with a central assumption in current models of chromosomal speciation and provides a firm rejection of the view that selection necessarily restricts chromosomal change.