Chromosomal Repatterning Research Articles

Multiscale analysis of the randomization limits of the chromosomal gene organization between Lepidoptera and Diptera.

How chromosome gene organization and gene content evolve among distantly related and structurally malleable genomes remains unresolved. This is particularly the case when considering different insect orders. We have compared the highly contiguous genome assemblies of the lepidopteran Danaus plexippus and the dipteran Drosophila melanogaster, which shared a common ancestor around 290 Ma. The gene content of 23 out of 30 D. plexippus chromosomes was significantly associated with one or two of the six chromosomal elements of the Drosophila genome, denoting common ancestry. Despite the phylogenetic distance, 9.6% of the 1-to-1 orthologues still reside within the same ancestral genome neighbourhood. Furthermore, the comparison D. plexippus–Bombyx mori indicated that the rates of chromosome repatterning are lower in Lepidoptera than in Diptera, although still within the same order of magnitude. Concordantly, 14 developmental gene clusters showed a higher tendency to retain full or partial clustering in D. plexippus, further supporting that the physical association between the SuperHox and NK clusters existed in the ancestral bilaterian. Our results illuminate the scope and limits of the evolution of the gene organization and content of the ancestral chromosomes to the Lepidoptera and Diptera while helping reconstruct portions of the genome in their most recent common ancestor.

Multiple independent chromosomal fusions accompanied the radiation of the Antarctic teleost genus Trematomus (Notothenioidei:Nototheniidae)

BackgroundChromosomal rearrangements are thought to be an important driving force underlying lineage diversification, but their link to speciation continues to be debated. Antarctic teleost fish of the family Nototheniidae (Notothenioidei) diversified in a changing environmental context, which led to ecological, morphological, and genetic differentiation among populations. In addition, extensive chromosomal repatterning accompanied species divergence in several clades. The most striking karyotypic changes involved the recent species radiation (about 10 My) of the genus Trematomus, with chromosomal pair numbers ranging between 29 and 12. These dramatic reductions in chromosome number resulted mostly from large-scale chromosome fusions. Multiple centric and/or tandem fusions have been hypothesized in at least seven of the twelve recognized Trematomus species. To reconstruct their evolutionary history, we employed comparative cytogenomics (BAC-FISH and chromosome painting) to reveal patterns of interspecific chromosomal orthologies across several notothenioid clades.ResultsWe defined orthologous chromosomal segments of reference, termed Structural Units (SUs). SUs were identified in a total of 18 notothenioid species. We demonstrated for the first time that SUs were strongly conserved across every specimen examined, with chromosomal syntenies highlighting a paucity of intrachromosomal macro-rearrangements. Multiple independent fusions of these SUs were inferred in the Trematomus species, in contrast to the shared SU fusions in species of the sister lineage Notothenia.ConclusionsThe SU segments were defined units of chromosomal rearrangement in the entire family Nototheiidae, which diverged from the other notothenioid families 20 My ago. Some of the identified chromosomal syntenies within the SUs were even conserved in their closest relatives, the family Eleginopsidae. Comparing the timing of acquisition of the fusions in the closely related genera Notothenia and Trematomus of the nototheniid species family, we conclude that they exhibit distinct chromosomal evolutionary histories, which may be relevant to different speciation scenarios.

Multiple Origins and Nested Cycles of Hybridization Result in High Tetraploid Diversity in the Monocot Prospero.

Polyploidy is a major driving force in angiosperm evolution, but our understanding of establishment and early diversification processes following allo- vs. auto-polyploidy is limited. An excellent system to address such questions is the monocot plant Prospero autumnale, as it comprises several genomically and chromosomally distinct diploid cytotypes and their auto- and allotetraploid derivatives. To infer origins and evolutionary trajectories of the tetraploids, we use genome size data, in situ hybridization with parental genomic DNAs and specific probes (satDNA, rDNAs), as well as molecular-phylogenetic analyses. Thus, we demonstrate that an astounding range of allotetraploid lineages has been formed recurrently by chromosomal re-patterning, interactions of chromosomally variable parental genomes and nested cycles of extensive hybridization, whereas autotetraploids have originated at least twice and are cytologically stable. During the recurrent formation and establishment across wide geographic areas hybridization in some populations could have inhibited lineage diversification and nascent speciation of such a hybrid swarm. However, cytotypes that became fixed in populations enhanced the potential for species diversification, possibly exploiting the extended allelic base, and fixed heterozygosity that polyploidy confers. The time required for polyploid cytotype fixation may in part reflect the lag phase reported for polyploids between their formation and species diversification.

Co-localization of ribosomal and telomeric sequences in Leptynia (Insecta: Phasmatodea)

Stick insects have been studied mainly for non-conventional reproduction modes, such as parthenogenesis, hybridogenesis and androgenesis. Parallel karyological investigations have evidenced extensive numerical and structural chromosome re-patterning, particularly evident in hybrid parthenogenetic taxa. Chromosome sets of bisexual Leptynia (Pantel) species show an evolutionary trend from 40 to 36 chromosomes and are characterized by cytological satellites of variable size and localization. We performed fluorescence in situ hybridization (FISH) analysis using 45S ribosomal genes and pentameric (TTAGG)n telomere sequences as probes in two strictly related but karyotypically distinct species, L. montana Scali (2n = 38/37; XX/XO) and L. attenuata Pantel (2n = 36). L. attenuata has recently been split into three subspecies (L. attenuata attenuata, L. attenuata iberica and L. attenuata algarvica), and found to share an XX/XY sex chromosome formula, unusual for stick insects. FISH by 45S rDNA sequences consistently labelled the short arm of the 4th chromosome pair, often of a variable size. Silver staining showed that nucleolar organizer regions (NORs) are active. FISH of the telomeric repeats, besides ordinary telomeres, also labelled the short arm of this same pair. The use of both probes in double FISH analysis fully confirmed the co-localization of ribosomal and telomeric highly repeated sequences. Since it is increasingly emerging that the co-localization of NORs and telomeric sequences appears to be a feature shared by evolutionarily distant animals, its possible role is discussed.

Chromosome evolution in marginal populations of Aegilops speltoides: causes and consequences

Genome restructuring is an ongoing process in natural plant populations. The influence of environmental changes on the genome is crucial, especially during periods of extreme climatic fluctuations. Interactions between the environment and the organism manifest to the greatest extent at the limits of the species' ecological niche. Thus, marginal populations are expected to exhibit lower genetic diversity and higher genetic differentiation than central populations, and some models assume that marginal populations play an important role in the maintenance and generation of biological diversity. In this review, long-term data on the cytogenetic characteristics of diploid Aegilops speltoides Tauch populations are summarized and discussed. This species is distributed in and around the Fertile Crescent and is proposed to be the wild progenitor of a number of diploid and polyploid wheat species. In marginal populations of Ae. speltoides, numerical chromosomal aberrations, spontaneous aneuploidy, B-chromosomes, rDNA cluster repatterning and reduction in the species-specific and tribe-specific tandem repeats have been detected. Significant changes were observed and occurred in parallel with changes in plant morphology and physiology. Considerable genomic variation at the chromosomal level was found in the marginal populations of Ae. speltoides. It is likely that a specific combination of gene mutations and chromosomal repatterning has produced the evolutionary trend in each specific case, i.e. for a particular species or group of related species in a given period of time and in a certain habitat. The appearance of a new chromosomal pattern is considered an important factor in promoting the emergence of interbreeding barriers.

Biosystematics and evolutionary studies in Indian Drimia species

Abstract The taxonomic position and genetic relationship within Indian Drimia species is controversial due to their morphological similarities and genomic complexities. The present work gives an insight on the genetic relationship between Indian Drimia species on the basis of their karyotype, pollen morphology, flower opening characteristics, hybridization behavior, and by use of DNA sequence of two molecular markers (internal transcribed spacers [ITS] and maturase K [matK]). The karyotypic studies of Indian Drimia species revealed various polyploid forms making their identification and delimitation more difficult. The five species of Indian Drimia are grouped into two complexes, indica complex and wightii complex on the basis of their pollen morphology, karyotype, and hybridization behavior. These two groups were found to be evolving separately. The cytomorphological studies of wightii complex revealed that it is evolving through polyploid and chromosome repatterning, while indica complex have adapted polyploid as well as hybridization for evolution. Phylogeny obtained from DNA sequences of molecular markers (ITS and matK) confirmed that the indica complex and wightii complex are evolving parallely, by grouping them in two clusters. Thus, a combination of conventional and molecular methods proved to be of great use for delimiting a small but complex group of Indian Drimia species.

A dynamic evolution of chromosome in subgenus Potamogeton revealed by physical mapping of rDNA loci detection

In this study, we analyzed 12 species in the subgenus Potamogeton (8 tetraploids and 2 diploids, 2 putative tetraploid hybrids) at the chromosomal level, including counting the chromosome number and physically mapping the rDNA. The extent of variation in the chromosome number and rDNA loci was determined in the Potamogeton species on both the inter- and intra-specific level. Moreover, one of the hybrid sets (P. perfoliatus a (TM), x P. wrightii a (TM) Euro) was picked for performing artificial pollination for producing F1 generation adults and for sequential morphological character analysis and FISH detection. After comparing the parental species and natural hybrids from three different geographic locations, the intraspecific variations of rDNA loci were revealed in sampled plants; in addition, comparisons between artificial F1 and natural hybrids (P. x intortusifolius) showed a rapid change of 45S rDNA loci in response to the interspecific hybridization. We also compared our results concerning rDNA patterns with phylogenetic documents to derive complementary clues to karoytype evolution and interspecific relationships. Based on frequent hybridizations and active clonal reproduction with weak genetic selection, the rDNA chromosomal repatterning, such as the gain or loss of rDNA loci together with rDNA movements, might be one trend of chromosome evolution in this genus.

Revision of the stick insect genus Leptynia: description of new taxa, speciation mechanism and phylogeography

Leptynia specimens were analyzed by karyotype analysis, mitochondrial gene sequencing and SEM of bodies and eggs. Here we describe a new species, Leptynia annaepaulae, and three subspecies of L. attenuata Pantel (L. attenuata attenuata, L. attenuata iberica, L. attenuata algarvica). The phylogeny of the genus Leptynia is congruent with a karyotype trend toward a reduction of chromosome number and the shift from the shared XX/X0 sex chromosome formula to the unusual XX/XY one. Chromosome repatterning appears to occur ahead of genetic differentiation, following a chromosome model of cladogenesis. Chromosome and genetic differentiation, in turn, appears to precede morphological distinction, thus realizing a condition of incipient species for most of the Leptynia taxa. Actually, morphological analyses revealed that, only rarely clear cut differences exist among and between taxa, while, more often, just trends in the differentiating traits occur, since the investigated characters generally suffer from some overlapping: In this study, only the 10th:9th ratio value and the subanal vomer appear to be diagnostic for L. annaepaulae against all other Leptynia taxa. As a consequence, the subanal vomer as well as cercus tooth features with egg chorion traits are not sharply diagnostic for the remaining co-generic taxa; however, comparisons are quite helpful in reducing uncertainties. A likely phylogeographic scenario for the genus supports that Leptynia ancestors spread from Northern Africa into Southern Spain where an ancestral taxon originated L. annaepaulae (2n = 40/39, XX/X0, with 2 large dibrachial pairs). Later on, a northbound colonization, should have originated L. caprai (2n = 40/39, XX/X0, all acrocentrics), from which L. montana (2n = 38, XX/X0) and L. attenuata (2n = 36, XY/XX) originated; supporting instances of chromosome repatterning have been actually observed. In this connection we like to stress that, particularly in stick insects, androgenesis has been a preferential pathway to quickly make homozygous those odd chromosome rearrangements likely responsible for low fitness in the heterozygotes.

Cytogenetic analyses of eight species in the genus Leptodactylus Fitzinger, 1843 (Amphibia, Anura, Leptodactylidae), including a new diploid number and a karyotype with multiple translocations

BackgroundThe karyotypes of Leptodactylus species usually consist of 22 bi-armed chromosomes, but morphological variations in some chromosomes and even differences in the 2n have been reported. To better understand the mechanisms responsible for these differences, eight species were analysed using classical and molecular cytogenetic techniques, including replication banding with BrdU incorporation.ResultsDistinct chromosome numbers were found: 2n = 22 in Leptodactylus chaquensis, L. labyrinthicus, L. pentadactylus, L. petersii, L. podicipinus, and L. rhodomystax; 2n = 20 in Leptodactylus sp. (aff. podicipinus); and 2n = 24 in L. marmoratus. Among the species with 2n = 22, only three had the same basic karyotype. Leptodactylus pentadactylus presented multiple translocations, L. petersii displayed chromosome morphological discrepancy, and L. podicipinus had four pairs of telocentric chromosomes. Replication banding was crucial for characterising this variability and for explaining the reduced 2n in Leptodactylus sp. (aff. podicipinus). Leptodactylus marmoratus had few chromosomes with a similar banding patterns to the 2n = 22 karyotypes. The majority of the species presented a single NOR-bearing pair, which was confirmed using Ag-impregnation and FISH with an rDNA probe. In general, the NOR-bearing chromosomes corresponded to chromosome 8, but NORs were found on chromosome 3 or 4 in some species. Leptodactylus marmoratus had NORs on chromosome pairs 6 and 8. The data from C-banding, fluorochrome staining, and FISH using the telomeric probe helped in characterising the repetitive sequences. Even though hybridisation did occur on the chromosome ends, telomere-like repetitive sequences outside of the telomere region were identified. Metaphase I cells from L. pentadactylus confirmed its complex karyotype constitution because 12 chromosomes appeared as ring-shaped chain in addition to five bivalents.ConclusionsSpecies of Leptodactylus exhibited both major and minor karyotypic differences which were identified by classical and molecular cytogenetic techniques. Replication banding, which is a unique procedure that has been used to obtain longitudinal multiple band patterns in amphibian chromosomes, allowed us to outline the general mechanisms responsible for these karyotype differences. The findings also suggested that L. marmoratus, which was formerly included in the genus Adenomera, may have undergone great chromosomal repatterning.

Island species radiation and karyotypic stasis in Pachycladonallopolyploids

BackgroundPachycladon (Brassicaceae, tribe Camelineae) is a monophyletic genus of ten morphologically and ecogeographically differentiated, and presumably allopolyploid species occurring in the South Island of New Zealand and in Tasmania. All Pachycladon species possess ten chromosome pairs (2n = 20). The feasibility of comparative chromosome painting (CCP) in crucifer species allows the origin and genome evolution in this genus to be elucidated. We focus on the origin and genome evolution of Pachycladon as well as on its genomic relationship to other crucifer species, particularly to the allopolyploid Australian Camelineae taxa. As species radiation on islands is usually characterized by chromosomal stasis, i.e. uniformity of chromosome numbers/ploidy levels, the role of major karyotypic reshuffling during the island adaptive and species radiation in Pachycladon is investigated through whole-genome CCP analysis.ResultsThe four analyzed Pachycladon species possess an identical karyotype structure. The consensual ancestral karyotype is most likely common to all Pachycladon species and corroborates the monophyletic origin of the genus evidenced by previous phylogenetic analyses. The ancestral Pachycladon karyotype (n = 10) originated through an allopolyploidization event between two genomes structurally resembling the Ancestral Crucifer Karyotype (ACK, n = 8). The primary allopolyploid (apparently with n = 16) has undergone genome reshuffling by descending dysploidy toward n = 10. Chromosome "fusions" were mediated by inversions, translocations and centromere inactivation/loss. Pachycladon chromosome 3 (PC3) resulted from insertional fusion, described in grasses. The allopolyploid ancestor originated in Australia, from the same or closely related ACK-like parental species as the Australian Camelineae allopolyploids. However, the two whole-genome duplication (WGD) events were independent, with the Pachycladon WGD being significantly younger. The long-distance dispersal of the diploidized Pachycladon ancestor to New Zealand was followed by the Pleistocene species radiation in alpine habitats and characterized by karyotypic stasis.ConclusionsKaryotypic stasis in Pachycladon suggests that the insular species radiation in this genus proceeded through homoploid divergence rather than through species-specific gross chromosomal repatterning. The ancestral Pachycladon genome originated in Australia through an allopolyploidization event involving two closely related parental genomes, and spread to New Zealand by a long-distance dispersal. We argue that the chromosome number decrease mediated by inter-genomic reshuffling (diploidization) could provide the Pachycladon allopolyploid founder with an adaptive advantage to colonize montane/alpine habitats. The ancestral Pachycladon karyotype remained stable during the Pleistocene adaptive radiation into ten different species.

Karyology, mitochondrial DNA and the phylogeny of Australian termites

A comprehensive karyological characterization of 20 Australian and three European species of Isoptera, together with a mitochondrial gene analysis is presented. Higher termites appear karyotypically very uniform, while lower termites are highly variable. The differences in chromosome number are explained through Robertsonian changes or multiple translocation events. An ancestral acrocentric karyotype can be suggested as the most primitive one. In Kalotermitidae chromosomal repatterning has repeatedly arisen with the X0-male type possibly representing a XY-derived condition. This argues against a simple origin of termites from cockroaches. The fixed chromosome number of Rhinotermitidae and Termitidae (2n=42, XY/XX) may be explained with the non-random nature of chromosomal evolution. A sex-linked multivalent, either with a ring or a chain structure, is found in the majority of species. Phylogenetic analyses on COII sequences recognize Mastotermitidae as the basal lineage and define the Rhinotermitidae+Termitidae cluster with a good bootstrap support. Kalotermitidae fail to be joined in a single cluster in agreement with the detected chromosomal variability. On the other hand, the karyotypic conservation of the Termitidae family contrasts with the polytomy evidenced at the subfamily level.

Standard karyotypes of two populations of the Scincus scincus complex from Tunisia and Morocco (Reptilia: Scincidae)

A study on chromosomes of two Scincus populations attributed to S. scincus and S. albifasciatus was carried out by conventional Giemsa staining. Both samples showed the same diploid number of 2N = 32 and identical chromosome morphology. Therefore, these populations seem to be lacking in chromosome changes that could act as barriers to gene flow. However, because speciation in lizards is not always associated with chromosome repatterning, the taxonomic status of “scincus” and “albifasciatus” forms remains unsettled.

Comparative genomics and repetitive sequence divergence in the species of diploidNicotianasection Alatae

Combining phylogenetic reconstructions of species relationships with comparative genomic approaches is a powerful way to decipher evolutionary events associated with genome divergence. Here, we reconstruct the history of karyotype and tandem repeat evolution in species of diploid Nicotiana section Alatae. By analysis of plastid DNA, we resolved two clades with high bootstrap support, one containing N. alata, N. langsdorffii, N. forgetiana and N. bonariensis (called the n = 9 group) and another containing N. plumbaginifolia and N. longiflora (called the n = 10 group). Despite little plastid DNA sequence divergence, we observed, via fluorescent in situ hybridization, substantial chromosomal repatterning, including altered chromosome numbers, structure and distribution of repeats. Effort was focussed on 35S and 5S nuclear ribosomal DNA (rDNA) and the HRS60 satellite family of tandem repeats comprising the elements HRS60, NP3R and NP4R. We compared divergence of these repeats in diploids and polyploids of Nicotiana. There are dramatic shifts in the distribution of the satellite repeats and complete replacement of intergenic spacers (IGSs) of 35S rDNA associated with divergence of the species in section Alatae. We suggest that sequence homogenization has replaced HRS60 family repeats at sub-telomeric regions, but that this process may not occur, or occurs more slowly, when the repeats are found at intercalary locations. Sequence homogenization acts more rapidly (at least two orders of magnitude) on 35S rDNA than 5S rDNA and sub-telomeric satellite sequences. This rapid rate of divergence is analogous to that found in polyploid species, and is therefore, in plants, not only associated with polyploidy.

Extensive Chromosomal Repatterning and the Evolution of Sterility Barriers in Hybrid Sunflower Species

New species may arise via hybridization and without a change in ploidy. This process, termed homoploid hybrid speciation, is theoretically difficult because it requires the development of reproductive barriers in sympatry or parapatry. Theory suggests that isolation may arise through rapid karyotypic evolution and/or ecological divergence of hybrid neospecies. Here, we investigate the role of karyotypic change in homoploid hybrid speciation by generating detailed genetic linkage maps for three hybrid sunflower species, Helianthus anomalus, H. deserticola, and H. paradoxus, and comparing these maps to those previously generated for the parental species, H. annuus and H. petiolaris. We also conduct a quantitative trait locus (QTL) analysis of pollen fertility in a BC2 population between the parental species and assess levels of pollen and seed fertility in all cross-combinations of the hybrid and parental species. The three hybrid species are massively divergent from their parental species in karyotype; gene order differences were observed for between 9 and 11 linkage groups (of 17 total), depending on the comparison. About one-third of the karyoypic differences arose through the sorting of chromosomal rearrangements that differentiate the parental species, but the remainder appear to have arisen de novo (six breakages/six fusions in H. anomalus, four breakages/three fusions in H. deserticola, and five breakages/five fusions in H. paradoxus). QTL analyses indicate that the karyotypic differences contribute to reproductive isolation. Nine of 11 pollen viability QTL occur on rearranged chromosomes and all but one map close to a rearrangement breakpoint. Finally, pollen and seed fertility estimates for F1's between the hybrid and parental species fall below 11%, which is sufficient for evolutionary independence of the hybrid neospecies.

Quantum speciation in Aegilops: molecular cytogenetic evidence from rDNA cluster variability in natural populations.

Data are presented on quantum speciation in the Sitopsis section of the genus Aegilops (Poaceae, Monocotyledones). Two small, peripheral, isolated, wild populations of annual cross-pollinated Ae. speltoides and annual self-pollinated Ae. sharonensis are located 30 m apart on different soil types. Despite the close proximity of the two species and their close relatedness, no mixed groups are known. Comparative molecular cytogenetic analysis based on the intrapopulation variability of rRNA-encoding DNA (rDNA) chromosomal patterns of individual Ae. speltoides geno-types revealed an ongoing dynamic process of permanent chromosomal rearrangements. Chromosomal mutations can arise de novo and can be eliminated. Analysis of the progeny of the investigated genotypes testifies that inheritance of de novo rDNA sites happens frequently. Heterologous recombination and/or transposable elements-mediated rDNA transfer seem to be the mechanisms for observed chromosomal repatterning. Consequently, several modified genomic forms, intermediate between Ae. speltoides and Ae. sharonensis, permanently arise in the studied wild population of Ae. speltoides, which make it possible to recognize Ae. sharonensis as a derivative species of Ae. speltoides, as well as to propose rapidness and canalization of quantum speciation in Sitopsis species.

Viability of X-autosome translocations in mammals: an epigenomic hypothesis from a rodent case-study.

X-autosome translocations are highly deleterious chromosomal rearrangements due to meiotic disruption, the effects of X-inactivation on the autosome, and the necessity of maintaining different replication timing patterns between the two segments. In spite of this, X-autosome translocations are not uncommon. We here focus on the genus Taterillus (Rodentia, Gerbillinae) which provides two sister lineages differing by two autosome-gonosome translocations. Despite the recent and dramatic chromosomal repatterning characterising these lineages, the X-autosome translocated species all display intercalary heterochromatic blocks (IHBs) between the autosomal and the ancestral sexual segments. These blocks, composed of highly amplified telomeric repeats and rDNA clusters, are not observed on the chromosomes of the non-translocated species, nor the Y1 and Y2 of the translocated species. Such IHBs are found in all mammals documented for X-autosome translocation. We propose an epigenomic hypothesis which explains the viability of X-autosome translocations in mammals. This posits that constitutive heterochromatin is probably selected for in X-autosome translocations since it may (1) prevent facultative heterochromatinization of the inactivated X from spreading to the autosomal part, and (2) allow for the independent regulation of replication timing of the sex and autosomal segments.

Activity of the En/Spm-like transposons in meiosis as a base for chromosome repatterning in a small, isolated, peripheral population of Aegilops speltoides Tausch.

Chromosomal repatterning is considered to be one of the main mechanisms for plant genome evolution. Here, we report the first cytogenetic evidence for the involvement of En/Spm transposons in ongoing chromosomal repatterning leading to the rise of new fertile genomic forms in a small, isolated, peripheral plant population. Cytogenetical screening of original individual plants of Aegilops speltoides Tausch. with different phenotypes revealed a wide spectrum of chromosomal abnormalities including extra chromosomes, chromosomal rearrangements, and variability in chromosomal position/number of 45S and 5S rDNA sites. Cytogenetic analysis of the dynamics of En/Spm transposons in meiosis indicates that: (i) this type of transposon is active during male gametogenesis; (ii) separately or in conjunction with rDNA they form clusters in the hot spots of large chromosomal rearrangements; (iii) appearance of at least part of the mobile rDNA sites in genome of Ae. speltoides are connected with meiotic activity of En/Spm transposons. PCR screening for the site-selected transposon insertions confirm the presence of combined fragments that consist partly of the sequence of En/Spm transposon and partly of 5S rDNA sequence.

PATTERNS OF KARYOTYPIC MEGAEVOLUTION INREITHRODONTOMYS: EVIDENCE FROM A CYTOCHROME-bPHYLOGENETIC HYPOTHESIS

Phylogenetic relationships among 7 species of Reithrodontomys were examined using the complete mitochondrial cytochrome-b gene. DNA-sequence data were analyzed using parsimony (weighted and unweighted) and genetic distance methods. In all analyses, R. sumichrasti, R. megalotis, and R. zacatecae formed a monophyletic clade. Likewise, R. montanus and R. raviventris were depicted as sister taxa in all analyses. Our study supports recognition of R. zacatecae as a species distinct from R. megalotis and recognition of R. raviventris as distinct from R. montanus. Patterns of chromosomal evolution were examined using differential staining as resolved by 3 DNA probes and fluorescent in situ hybridization. Using the phylogeny based on DNA sequences, we concluded that there has been extensive chromosomal repatterning (in most cases karyotypic megaevolution) in the evolution of R. zacatecae, R. megalotis, R. sumichrasti, R. montanus, R. raviventris, and R. humulis.

Comparative cytogenetics of hamsters of the genus Calomyscus

Karyotypes of Calomyscus from different regions of Turkmenistan, Iran, and Azerbaijan were studied using chromosome banding (G- and C-banding) and analyses of meiosis in laboratory hybrids. Extensive variation in the diploid number and the number of autosomal arms (FNa) was revealed (2n = 30, FNa = 44; 2n = 32, FNa = 42; 2n = 44, FNa = 46; 2n = 44, FNa = 58; 2n = 37, FNa = 44; 2n = 50, FNa = 50; 2n = 52, FNa = 56). Centric and tandem fusions and heterochromatin changes were identified as the major modes of karyotype evolution in this group. Natural hybrids between individuals with different karyotypes were recorded, and regular chromosome pairing in meiosis was observed in laboratory hybrids. Fluorescence in situ hybridization with a 353-bp BspRI complex tandem repeat indicated that chromosomal repatterning occurred recently within the genus. There is no unequivocal evidence suggesting the role of chromosomal change in the speciation of the populations of Calomyscus examined.

Spontaneous Origin of Monosomics and Nullisomics in Clarkia unguiculata (Onagraceae) Hybrids

Artificial hybridizations between Clarkia unguiculata (Onagraceae) translocation homozygotes identified a fifth chromosome arrangement (E). Hybridizations between individuals homozygous for chromosome arrangement E and others homozygous for either arrangement C or D yielded only monosomic progeny. In most cells the longest potential chain (five or seven chromosomes) in these 17-chromosome translocation heterozygotes broke into smaller chains and univalents. The members of the chain disjoined alternately, as in naturally occurring Clarkia translocation heterozygotes. The monosomics were vigorous and morphologically normal, but only partly fertile. Self-pollination of a monosomic produced 16-, 17-, and 18-chromosome plants. The nullisomics were vigorous and morphologically normal, but highly sterile. In this regard they are similar to those of Clarkia amoena, which arise by chromosome repatterning, and are not comparable to the classical sort.