Chromosomal evolution in Rodentia

Investigating the evolutionary dynamics of diploid number variation in Ctenomys (Ctenomyidae, Rodentia)

Contrary to predictions from classical hybrid sterility models of chromosomal speciation, some organisms display high rates of karyotype variation. Ctenomys are the current mammals with the greatest interspecific and intraspecific chromosomal variation. A large number of species have been studied cytogenetically. The diploid numbers of chromosomes range from 2n = 10 to 2n = 70. Here, we analyzed karyotype evolution in Ctenomys using comparative phylogenetic methods. We found a strong phylogenetic signal with chromosome number. This refutes the chromosomal megaevolution model, which proposes the independent accumulation of multiple chromosomal rearrangements in each closely related species. We found that Brownian motion (BM) described the observed characteristic changes more thoroughly than the Ornstein-Uhlenbeck and Early-Burst models. This suggests that the evolution of chromosome numbers occurs by a random walk along phylogenetic clades. However, our data indicate that the BM model alone does not fully characterize the chromosomal evolution of Ctenomys.

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.

Integration of fluorescence in situ hybridization and chromosome-length genome assemblies revealed synteny map for guinea pig, naked mole-rat, and human

Descriptions of karyotypes of many animal species are currently available. In addition, there has been a significant increase in the number of sequenced genomes and an ever-improving quality of genome assembly. To close the gap between genomic and cytogenetic data we applied fluorescent in situ hybridization (FISH) and Hi-C technology to make the first full chromosome-level genome comparison of the guinea pig (Cavia porcellus), naked mole-rat (Heterocephalus glaber), and human. Comparative chromosome maps obtained by FISH with chromosome-specific probes link genomic scaffolds to individual chromosomes and orient them relative to centromeres and heterochromatic blocks. Hi-C assembly made it possible to close all gaps on the comparative maps and to reveal additional rearrangements that distinguish the karyotypes of the three species. As a result, we integrated the bioinformatic and cytogenetic data and adjusted the previous comparative maps and genome assemblies of the guinea pig, naked mole-rat, and human. Syntenic associations in the two hystricomorphs indicate features of their putative ancestral karyotype. We postulate that the two approaches applied in this study complement one another and provide complete information about the organization of these genomes at the chromosome level.

Cross-species chromosome painting and repetitive DNA mapping illuminate the karyotype evolution in true crocodiles (Crocodylidae)

Crocodilians have maintained very similar karyotype structures and diploid chromosome numbers for around 100 million years, with only minor variations in collinearity. Why this karyotype structure has largely stayed unaltered for so long is unclear. In this study, we analyzed the karyotypes of six species belonging to the genera Crocodylus and Osteolaemus (Crocodylidae, true crocodiles), among which the Congolian endemic O. osborni was included and investigated. We utilized various techniques (differential staining, fluorescence in situ hybridization with repetitive DNA and rDNA probes, whole chromosome painting, and comparative genomic hybridization) to better understand how crocodile chromosomes evolved. We studied representatives of three of the four main diploid chromosome numbers found in crocodiles (2n = 30/32/38). Our data provided new information about the species studied, including the identification of four major chromosomal rearrangements that occurred during the karyotype diversification process in crocodiles. These changes led to the current diploid chromosome numbers of 2n = 30 (fusion) and 2n = 38 (fissions), derived from the ancestral state of 2n = 32. The conserved cytogenetic tendency in crocodilians, where extant species keep near-ancestral state, contrasts with the more dynamic karyotype evolution seen in other major reptile groups.

Exploring objective feature sets in constructing the evolution relationship of animal genome sequences

BACKGROUND

Exploring evolution regularities of genome sequences and constructing more objective species evolution relationships at the genomic level are high-profile topics. Based on the evolution mechanism of genome sequences proposed in our previous research, we found that only the 8-mers containing CG or TA dinucleotides correlate directly with the evolution of genome sequences, and the relative frequency rather than the actual frequency of these 8-mers is more suitable to characterize the evolution of genome sequences.

RESULT

Therefore, two types of feature sets were obtained, they are the relative frequency sets of CG1 + CG2 8-mers and TA1 + TA2 8-mers. The evolution relationships of mammals and reptiles were constructed by the relative frequency set of CG1 + CG2 8-mers, and two types of evolution relationships of insects were constructed by the relative frequency sets of CG1 + CG2 8-mers and TA1 + TA2 8-mers respectively. Through comparison and analysis, we found that evolution relationships are consistent with the known conclusions. According to the evolution mechanism, we considered that the evolution relationship constructed by CG1 + CG2 8-mers reflects the evolution state of genome sequences in current time, and the evolution relationship constructed by TA1 + TA2 8-mers reflects the evolution state in the early stage.

CONCLUSION

Our study provides objective feature sets in constructing evolution relationships at the genomic level.

The genomic landscape, causes, and consequences of extensive phylogenomic discordance in Old World mice and rats

A species tree is a central concept in evolutionary biology whereby a single branching phylogeny reflects relationships among species. However, the phylogenies of different genomic regions often differ from the species tree. Although tree discordance is often widespread in phylogenomic studies, we still lack a clear understanding of how variation in phylogenetic patterns is shaped by genome biology or the extent to which discordance may compromise comparative studies. We characterized patterns of phylogenomic discordance across the murine rodents (Old World mice and rats) - a large and ecologically diverse group that gave rise to the mouse and rat model systems. Combining new linked-read genome assemblies for seven murine species with eleven published rodent genomes, we first used ultra-conserved elements (UCEs) to infer a robust species tree. We then used whole genomes to examine finer-scale patterns of discordance and found that phylogenies built from proximate chromosomal regions had similar phylogenies. However, there was no relationship between tree similarity and local recombination rates in house mice, suggesting that genetic linkage influences phylogenetic patterns over deeper timescales. This signal may be independent of contemporary recombination landscapes. We also detected a strong influence of linked selection whereby purifying selection at UCEs led to less discordance, while genes experiencing positive selection showed more discordant and variable phylogenetic signals. Finally, we show that assuming a single species tree can result in high error rates when testing for positive selection under different models. Collectively, our results highlight the complex relationship between phylogenetic inference and genome biology and underscore how failure to account for this complexity can mislead comparative genomic studies.

Paper mulberry leaves as a potential sterilant: evidence from Microtus fortis-a laboratory study

Introduction

The Yangtze vole (Microtus fortis) is a small herbivorous rodent that usually causes damage to crops and forests in China. Various measures were used to control their population including chemical rodenticides. However, rodenticides may cause secondary damage to the environment and the ecosystem. Therefore, the development of new rodent sterilants is urgent. Considering that some compounds of paper mulberry leaves have been verified that can inhibit the biosynthesis of sexual hormone, we aimed to explore the antifertility effect of paper mulberry leaves on M. fortis.

Methods

In this study, voles were divided into three groups including a male group, a female group, and a breeding group, and paper mulberry leaves were added into basal fodder of voles maintained in laboratory, of which the proportion of leaf weight was 50%. In each group, voles were fed with mixed fodder as treatment (BP) and voles were fed with basal fodder as contrast (CK).

Results and discussion

After feeding for more than 1 month, the results indicated that paper mulberry leaves attracted voles to feed, but inhibited their growth and reproduction. Since the second week, food intakes of BP have been significantly higher than CK (p< 0.05). However, weights of voles in male and female groups were 72.283 ± 7.394 g and 49.717 ± 2.278 g in the fifth week, and both were significantly reduced compared with their original weight (p< 0.05). Meanwhile, testicular volumes of male voles fed with BP were significantly smaller than CK (former: 318.000 ± 44.654 mm³, latter: 459.339 ± 108.755 mm³); the testosterone level, sperm number, and vitality of BP were obviously weaker than CK. Female uteruses and oophoron of BP grew slower, and the organ coefficients of uterus and oophoron fed BP were both significantly lower than CK (p< 0.05). The first reproduction of BP couple voles spent 45 days, while CK spent only 21 days. These results suggest that paper mulberry leaves could be the potential resource to produce sterilants to control rodent populations by delaying their sexual growth and reproduction. If it was practical, the apparent advantages of paper mulberry are that it is an abundant resource and the inhibitory effect could be effective in both male and female individuals. Our conclusion also supports the transformation of rodent management from lethal management to fertility control, which would be more ecologically friendly to agriculture and the ecosystem.

Mitochondrial DNA Diversity of Mesocricetus auratus and Other Cricetinae Species among Cricetidae Family

Unique anatomical and physiological features have made hamster species desirable research models. Comparative genomics and phylogenetic analysis of the hamster family members to clarify their evolution and genetic relationship, can provide a genetic basis for the comprehension of the variable research results obtained using different hamster models. The Syrian golden hamster (Mesocricetus auratus) is the most widely used species. In this study, we sequenced the complete mitochondrial genome (mitogenome) of M. auratus, compared it with the mitogenome of other Cricetinae subfamily species, and defined its phylogenetic position in the Cricetidae family. Our results show that the mitogenome organization, gene arrangement, base composition, and genetic analysis of the protein coding genes (PCGs) of M. auratus are similar to those observed in previous reports on Cricetinae species. Nonetheless, our analysis clarifies some striking differences of M. auratus relative to other subfamily members, namely distinct codon usage frequency of TAT (Tyr), AAT (Asn), and GAA (Glu) and the presence of the conserved sequence block 3 (CSB-3) in the control region of M. auratus mitogenome and other hamsters (not found in Arvicolinae). These results suggest the particularity of amino acid codon usage bias of M. auratus and special regulatory signals for the heavy strand replication in Cricetinae. Additionally, Bayesian inference/maximum likelihood (BI/ML) tree shows that Cricetinae and Arvicolinae are sister taxa sharing a common ancestor, and Neotominae split prior to the split between Cricetinae and Arvicolinae. Our results support taxonomy revisions in Cricetulus kamensis and Cricetulus migratorius, and further revision is needed within the other two subfamilies. Among the hamster research models, Cricetulus griseus is the species with highest sequence similarity and closer genetic relationship with M. auratus. Our results show mitochondrial DNA diversity of M. auratus and other Cricetinae species and provide genetic basis for judgement of different hamster models, promoting the development and usage of hamsters with regional characteristics.

Convergent and divergent neural circuit architectures that support acoustic communication

Vocal communication is used across extant vertebrates, is evolutionarily ancient, and been maintained, in many lineages. Here I review the neural circuit architectures that support intraspecific acoustic signaling in representative anuran, mammalian and avian species as well as two invertebrates, fruit flies and Hawaiian crickets. I focus on hindbrain motor control motifs and their ties to respiratory circuits, expression of receptors for gonadal steroids in motor, sensory, and limbic neurons as well as divergent modalities that evoke vocal responses. Hindbrain and limbic participants in acoustic communication are highly conserved, while forebrain participants have diverged between anurans and mammals, as well as songbirds and rodents. I discuss the roles of natural and sexual selection in driving speciation, as well as exaptation of circuit elements with ancestral roles in respiration, for producing sounds and driving rhythmic vocal features. Recent technical advances in whole brain fMRI across species will enable real time imaging of acoustic signaling partners, tying auditory perception to vocal production.

Chromosome-scale genome assembly of the African giant pouched rat (Cricetomys ansorgei) and evolutionary analysis reveals evidence of olfactory specialization

The Southern giant pouched rat, Cricetomys ansorgei, is a large rodent best known for its ability to detect landmines using its impressive sense of smell. Their powerful chemosensory abilities enable subtle discrimination of chemical social signals, and female pouched rats demonstrate a unique reproductive physiology hypothesized to be mediated by pheromonal mechanisms. Thus, C. ansorgei represents a novel mammalian model for chemosensory physiology, social behavior, and pheromonal control of reproductive physiology. We present the first chromosome-scale genomic sequence of the pouched rat encoding 22,671 protein coding genes, including 1571 olfactory receptors, and provide a glance into the evolutionary history of this species. Functional enrichment analysis reveals genetic expansions specific to the pouched rat are enriched for functions related to olfactory specialization. Overall, this assembly is of reference-quality, and will serve as a useful and informative genomic sequence on which we can confidently base future molecular research involving the pouched rat.

Centromere drive: model systems and experimental progress

Centromeres connect chromosomes and spindle microtubules to ensure faithful chromosome segregation. Paradoxically, despite this conserved function, centromeric DNA evolves rapidly and centromeric proteins show signatures of positive selection. The centromere drive hypothesis proposes that centromeric DNA can act like a selfish genetic element and drive non-Mendelian segregation during asymmetric female meiosis. Resulting fitness costs lead to genetic conflict with the rest of the genome and impose a selective pressure for centromeric proteins to adapt by suppressing the costs. Here, we describe experimental model systems for centromere drive in yellow monkeyflowers and mice, summarize key findings demonstrating centromere drive, and explain molecular mechanisms. We further discuss efforts to test if centromeric proteins are involved in suppressing drive-associated fitness costs, highlight a model for centromere drive and suppression in mice, and put forth outstanding questions for future research.

The emergence of a new sex-system (XX/XY1Y2) suggests a species complex in the "monotypic" rodent Oecomys auyantepui (Rodentia, Sigmodontinae)

X-autosome translocation (XY₁Y₂) has been reported in distinct groups of vertebrates suggesting that the rise of a multiple sex system within a species may act as a reproductive barrier and lead to speciation. The viability of this system has been linked with repetitive sequences located between sex and autosomal portions of the translocation. Herein, we investigate Oecomys auyantepui, using chromosome banding and Fluorescence In Situ Hybridization with telomeric and Hylaeamys megacephalus whole-chromosome probes, and phylogenetic reconstruction using mtDNA and nuDNA sequences. We describe an amended karyotype for O. auyantepui (2n = 64♀65♂/FNa = 84) and report for the first time a multiple sex system (XX/XY₁Y₂) in Oryzomyini rodents. Molecular data recovered O. auyantepui as a monophyletic taxon with high support and cytogenetic data indicate that O. auyantepui may exist in two lineages recognized by distinct sex systems. The Neo-X exhibits repetitive sequences located between sex and autosomal portions, which would act as a boundary between these two segments. The G-banding comparisons of the Neo-X chromosomes of other Sigmodontinae taxa revealed a similar banding pattern, suggesting that the autosomal segment in the Neo-X can be shared among the Sigmodontinae lineages with a XY₁Y₂ sex system.

3D chromatin remodelling in the germ line modulates genome evolutionary plasticity

Chromosome folding has profound impacts on gene regulation, whose evolutionary consequences are far from being understood. Here we explore the relationship between 3D chromatin remodelling in mouse germ cells and evolutionary changes in genome structure. Using a comprehensive integrative computational analysis, we (i) reconstruct seven ancestral rodent genomes analysing whole-genome sequences of 14 species representatives of the major phylogroups, (ii) detect lineage-specific chromosome rearrangements and (iii) identify the dynamics of the structural and epigenetic properties of evolutionary breakpoint regions (EBRs) throughout mouse spermatogenesis. Our results show that EBRs are devoid of programmed meiotic DNA double-strand breaks (DSBs) and meiotic cohesins in primary spermatocytes, but are associated in post-meiotic cells with sites of DNA damage and functional long-range interaction regions that recapitulate ancestral chromosomal configurations. Overall, we propose a model that integrates evolutionary genome reshuffling with DNA damage response mechanisms and the dynamic spatial genome organisation of germ cells.

The role of genome folding in the heritability and evolvability of structural variations is not well understood. Here the authors investigate the impact of the three-dimensional genome topology of germ cells in the formation and transmission of gross structural genomic changes detected from comparing whole-genome sequences of 14 rodent species.

Tandem Repeat Diversity in Two Closely Related Hamster Species—The Chinese Hamster (Cricetulus griseus) and Striped Hamster (Cricetulus barabensis)

The Chinese hamster (Cricetulus griseus) and striped hamster (Cricetulus barabensis) are very closely related species with similar karyotypes. The karyotypes differ from each other by one Robertsonian rearrangement and X-chromosome morphology. The level of the tandem repeat (TR) sequences’ evolutional variability is high. The aim of the current work was to trace the TR distribution on the chromosomes of two very closely related species. The striped hamster genome has not yet been sequenced. We classified the Chinese hamster TR in the assemblies available and then compared the mode of the TR distribution in closely related species. Chinese and striped hamsters are separate species due to the relative species specificity of Chinese hamster TR and prominent differences in the TR distribution in both species. The TR variation observed within homologous striped hamster chromosomes is caused by a lack of inbreeding in natural populations. The set of TR tested could be used to examine the CHO lines’ instability that has been observed in heterochromatic regions.

Chromosome Changes in Soma and Germ Line: Heritability and Evolutionary Outcome

The origin and inheritance of chromosome changes provide the essential foundation for natural selection and evolution. The evolutionary fate of chromosome changes depends on the place and time of their emergence and is controlled by checkpoints in mitosis and meiosis. Estimating whether the altered genome can be passed to subsequent generations should be central when we consider a particular genome rearrangement. Through comparative analysis of chromosome rearrangements in soma and germ line, the potential impact of macromutations such as chromothripsis or chromoplexy appears to be fascinating. What happens with chromosomes during the early development, and which alterations lead to mosaicism are other poorly studied but undoubtedly essential issues. The evolutionary impact can be gained most effectively through chromosome rearrangements arising in male meiosis I and in female meiosis II, which are the last divisions following fertilization. The diversity of genome organization has unique features in distinct animals; the chromosome changes, their internal relations, and some factors safeguarding genome maintenance in generations under natural selection were considered for mammals.

Comparative transcriptomics of the Djungarian hamster hypothalamus during short photoperiod acclimation and spontaneous torpor

The energy-saving strategy of Djungarian hamsters (Phodopus sungorus, Cricetidae) to overcome harsh environmental conditions comprises of behavioral, morphological, and physiological adjustments, including spontaneous daily torpor, a metabolic downstate. These acclimatizations are triggered by short photoperiod and orchestrated by the hypothalamus. Key mechanisms of long-term photoperiodic acclimatizations have partly been described, but specific mechanisms that acutely control torpor remain incomplete. Here, we performed comparative transcriptome analysis on hypothalamus of normometabolic hamsters in their summer- and winter-like state to enable us to identify changes in gene expression during photoperiodic acclimations. Comparing nontorpid and torpid hamsters may also be able to pin down mechanisms relevant for torpor control. A de novo assembled transcriptome of the hypothalamus was generated from hamsters acclimated to long photoperiod or to short photoperiod. The hamsters were sampled either during long photoperiod normothermia, short photoperiod normothermia, or short photoperiod-induced spontaneous torpor with a body temperature of 24.6 ± 1.0 °C, or. The mRNA-seq analysis revealed that 32 and 759 genes were differentially expressed during photoperiod or torpor, respectively. Biological processes were not enriched during photoperiodic acclimatization but were during torpor, where transcriptional and metabolic processes were reinforced. Most extremely regulated genes (those genes with |log2(FC)| > 2.0 and padj < 0.05 of a pairwise group comparison) underpinned the role of known key players in photoperiodic comparison, but these genes exhibit adaptive and protective adjustments during torpor. Targeted analyses of genes from potentially involved hypothalamic systems identified gene regulation of previously described torpor-relevant systems and a potential involvement of glucose transport.

Molecular mechanisms and topological consequences of drastic chromosomal rearrangements of muntjac deer

Muntjac deer have experienced drastic karyotype changes during their speciation, making it an ideal model for studying mechanisms and functional consequences of mammalian chromosome evolution. Here we generated chromosome-level genomes for Hydropotes inermis (2n = 70), Muntiacus reevesi (2n = 46), female and male M. crinifrons (2n = 8/9) and a contig-level genome for M. gongshanensis (2n = 8/9). These high-quality genomes combined with Hi-C data allowed us to reveal the evolution of 3D chromatin architectures during mammalian chromosome evolution. We find that the chromosome fusion events of muntjac species did not alter the A/B compartment structure and topologically associated domains near the fusion sites, but new chromatin interactions were gradually established across the fusion sites. The recently borne neo-Y chromosome of M. crinifrons, which underwent male-specific inversions, has dramatically restructured chromatin compartments, recapitulating the early evolution of canonical mammalian Y chromosomes. We also reveal that a complex structure containing unique centromeric satellite, truncated telomeric and palindrome repeats might have mediated muntjacs' recurrent chromosome fusions. These results provide insights into the recurrent chromosome tandem fusion in muntjacs, early evolution of mammalian sex chromosomes, and reveal how chromosome rearrangements can reshape the 3D chromatin regulatory conformations during species evolution.

Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

Therian mammals have among the oldest and most conserved sex-determining systems known to date. Any deviation from the standard XX/XY mammalian sex chromosome constitution usually leads to sterility or poor fertility, due to the high differentiation and specialization of the X and Y chromosomes. Nevertheless, a handful of rodents harbor so-called unusual sex-determining systems. While in some species, fertile XY females are found, some others have completely lost their Y chromosome. These atypical species have fascinated researchers for over 60 years, and constitute unique natural models for the study of fundamental processes involved in sex determination in mammals and vertebrates. In this article, we review current knowledge of these species, discuss their similarities and differences, and attempt to expose how the study of their exceptional sex-determining systems can further our understanding of general processes involved in sex chromosome and sex determination evolution.

Comparative genomic mapping reveals mechanisms of chromosome diversification in Rhipidomys species (Rodentia, Thomasomyini) and syntenic relationship between species of Sigmodontinae

Rhipidomys (Sigmodontinae, Thomasomyini) has 25 recognized species, with a wide distribution ranging from eastern Panama to northern Argentina. Cytogenetic data has been described for 13 species with 12 of them having 2n = 44 with a high level of autosomal fundamental number (FN) variation, ranging from 46 to 80, assigned to pericentric inversions. The species are grouped in groups with low FN (46-52) and high FN (72-80). In this work the karyotypes of Rhipidomys emiliae (2n = 44, FN = 50) and Rhipidomys mastacalis (2n = 44, FN = 74), were studied by classical cytogenetics and by fluorescence in situ hybridization using telomeric and whole chromosome probes (chromosome painting) of Hylaeamys megacephalus (HME). Chromosome painting revealed homology between 36 segments of REM and 37 of RMA. We tested the hypothesis that pericentric inversions are the predominant chromosomal rearrangements responsible for karyotypic divergence between these species, as proposed in literature. Our results show that the genomic diversification between the karyotypes of the two species resulted from translocations, centromeric repositioning and pericentric inversions. The chromosomal evolution in Rhipidomys was associated with karyotypical orthoselection. The HME probes revealed that seven syntenic probably ancestral blocks for Sigmodontinae are present in Rhipidomys. An additional syntenic block described here is suggested as part of the subfamily ancestral karyotype. We also define five synapomorphies that can be used as chromosomal signatures for Rhipidomys.

Convergent evolution of a genomic rearrangement may explain cancer resistance in hystrico- and sciuromorpha rodents

The rodents of hystricomorpha and sciuromorpha suborders exhibit remarkably lower incidence of cancer. The underlying genetic basis remains obscure. We report a convergent evolutionary split of human 3p21.31, a locus hosting a large number of tumour-suppressor genes (TSGs) and frequently deleted in several tumour types, in hystrico- and sciuromorphs. Analysis of 34 vertebrate genomes revealed that the synteny of 3p21.31 cluster is functionally and evolutionarily constrained in most placental mammals, but exhibit large genomic interruptions independently in hystricomorphs and sciuromorphs, owing to relaxation of underlying constraints. Hystrico- and sciuromorphs, therefore, escape from pro-tumorigenic co-deletion of several TSGs in cis. The split 3p21.31 sub-clusters gained proximity to proto-oncogene clusters from elsewhere, which might further nullify pro-tumorigenic impact of copy number variations due to co-deletion or co-amplification of genes with opposing effects. The split of 3p21.31 locus coincided with the accelerated rate of its gene expression and the body mass evolution of ancestral hystrico- and sciuromorphs. The genes near breakpoints were associated with the traits specific to hystrico- and sciuromorphs, implying adaptive significance. We conclude that the convergently evolved chromosomal interruptions of evolutionarily constrained 3p21.31 cluster might have impacted evolution of cancer resistance, body mass variation and ecological adaptations in hystrico- and sciuromorphs.

Beaver and Naked Mole Rat Genomes Reveal Common Paths to Longevity

Long-lived rodents have become an attractive model for the studies on aging. To understand evolutionary paths to long life, we prepare chromosome-level genome assemblies of the two longest-lived rodents, Canadian beaver (Castor canadensis) and naked mole rat (NMR, Heterocephalus glaber), which were scaffolded with in vitro proximity ligation and chromosome conformation capture data and complemented with long-read sequencing. Our comparative genomic analyses reveal that amino acid substitutions at "disease-causing" sites are widespread in the rodent genomes and that identical substitutions in long-lived rodents are associated with common adaptive phenotypes, e.g., enhanced resistance to DNA damage and cellular stress. By employing a newly developed substitution model and likelihood ratio test, we find that energy and fatty acid metabolism pathways are enriched for signals of positive selection in both long-lived rodents. Thus, the high-quality genome resource of long-lived rodents can assist in the discovery of genetic factors that control longevity and adaptive evolution.

Hamster PIWI proteins bind to piRNAs with stage-specific size variations during oocyte maturation

In animal gonads, transposable elements are actively repressed to preserve genome integrity through the PIWI-interacting RNA (piRNA) pathway. In mice, piRNAs are abundantly expressed in male germ cells, and form effector complexes with three distinct PIWIs. The depletion of individual Piwi genes causes male-specific sterility with no discernible phenotype in female mice. Unlike mice, most other mammals have four PIWI genes, some of which are expressed in the ovary. Here, purification of PIWI complexes from oocytes of the golden hamster revealed that the size of the PIWIL1-associated piRNAs changed during oocyte maturation. In contrast, PIWIL3, an ovary-specific PIWI in most mammals, associates with short piRNAs only in metaphase II oocytes, which coincides with intense phosphorylation of the protein. An improved high-quality genome assembly and annotation revealed that PIWIL1- and PIWIL3-associated piRNAs appear to share the 5'-ends of common piRNA precursors and are mostly derived from unannotated sequences with a diminished contribution from TE-derived sequences, most of which correspond to endogenous retroviruses. Our findings show the complex and dynamic nature of biogenesis of piRNAs in hamster oocytes, and together with the new genome sequence generated, serve as the foundation for developing useful models to study the piRNA pathway in mammalian oocytes.

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.

Chromosomal Signatures Corroborate the Phylogenetic Relationships within Akodontini (Rodentia, Sigmodontinae)

Comparative chromosome-painting analysis among highly rearranged karyotypes of Sigmodontinae rodents (Rodentia, Cricetidae) detects conserved syntenic blocks, which are proposed as chromosomal signatures and can be used as phylogenetic markers. In the Akodontini tribe, the molecular topology (Cytb and/or IRBP) shows five low-supported clades (divisions: “Akodon”, “Bibimys”, “Blarinomys”, “Oxymycterus”, and “Scapteromys”) within two high-supported major clades (clade A: “Akodon”, “Bibimys”, and “Oxymycterus”; clade B: “Blarinomys” and “Scapteromys”). Here, we examine the chromosomal signatures of the Akodontini tribe by using Hylaeamysmegacephalus (HME) probes to study the karyotypes of Oxymycterus amazonicus (2n = 54, FN = 64) and Blarinomys breviceps (2n = 28, FN = 50), and compare these data with those from other taxa investigated using the same set of probes. We strategically employ the chromosomal signatures to elucidate phylogenetic relationships among the Akodontini. When we follow the evolution of chromosomal signature states, we find that the cytogenetic data corroborate the current molecular relationships in clade A nodes. We discuss the distinct events that caused karyotypic variability in the Oxymycterus and Blarinomys genera. In addition, we propose that Blarinomys may constitute a species complex, and that the taxonomy should be revised to better delimit the geographical boundaries and their taxonomic status.

Accelerated Evolution of Limb-Related Gene Hoxd11 in the Common Ancestor of Cetaceans and Ruminants (Cetruminantia)

Reduced numbers of carpal and tarsal bones (wrist and ankle joints) are extensively observed in the clade of Cetacea and Ruminantia (Cetruminantia). Homebox D11 (Hoxd11) is one of the important genes required for limb development in mammals. Mutations in Hoxd11 can lead to defects in particular bones of limbs, including carpus and tarsus. To test whether evolutionary changes in Hoxd11 underlie the loss of these bones in Cetruminantia, we sequenced and analyzed Hoxd11 coding sequences and compared them with other 5' HoxA and HoxD genes in a taxonomic coverage of Cetacea, Ruminantia and other mammalian relatives. Statistical tests on the Hoxd11 sequences found an accelerated evolution in the common ancestor of cetaceans and ruminants, which coincided with the reduction of carpal and tarsal bones in this clade. Five amino acid substitutions (G222S, G227A, G229S, A240T and G261V) and one amino acid deletion (G254Del) occurred in this lineage. In contrast, other 5' HoxA and HoxD genes do not show this same evolutionary pattern, but instead display a highly conserved pattern of evolution in this lineage. Accelerated evolution of Hoxd11, but not other 5' HoxA and HoxD genes, is probably related to the reduction of the carpal and tarsal bones in Cetruminantia. Moreover, we found two amino acid substitutions (G110S and D223N) in Hoxd11 that are unique to the lineage of Cetacea, which coincided with hindlimb loss in the common ancestor of cetaceans. Our results give molecular evidence of Hoxd11 adaptive evolution in cetaceans and ruminants, which could be correlated with limb morphological adaptation.

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.

A centromere satellite concomitant with extensive karyotypic diversity across the Peromyscus genus defies predictions of molecular drive

A common feature of eukaryotic centromeres is the presence of large tracts of tandemly arranged repeats, known as satellite DNA. However, these centromeric repeats appear to experience rapid evolution under forces such as molecular drive and centromere drive, seemingly without consequence to the integrity of the centromere. Moreover, blocks of heterochromatin within the karyotype, including the centromere, are hotspots for chromosome rearrangements that may drive speciation events by contributing to reproductive isolation. However, the relationship between the evolution of heterochromatic sequences and the karyotypic dynamics of these regions remains largely unknown. Here, we show that a single conserved satellite DNA sequence in the order Rodentia of the genus Peromyscus localizes to recurrent sites of chromosome rearrangements and heterochromatic amplifications. Peromyscine species display several unique features of chromosome evolution compared to other Rodentia, including stable maintenance of a strict chromosome number of 48 among all known species in the absence of any detectable interchromosomal rearrangements. Rather, the diverse karyotypes of Peromyscine species are due to intrachromosomal variation in blocks of repeated DNA content. Despite wide variation in the copy number and location of repeat blocks among different species, we find that a single satellite monomer maintains a conserved sequence and homogenized tandem repeat structure, defying predictions of molecular drive. The conservation of this satellite monomer results in common, abundant, and large blocks of chromatin that are homologous among chromosomes within one species and among diverged species. Thus, such a conserved repeat may have facilitated the retention of polymorphic chromosome variants within individuals and intrachromosomal rearrangements between species-both factors that have previously been hypothesized to contribute towards the extremely wide range of ecological adaptations that this genus exhibits.

Chromosomal Localization of 18S-28S rDNA and (TTAGGG)n Sequences in Two South African Dormice of the Genus Graphiurus (Rodentia: Gliridae)

Classical cytogenetics and mapping of 18S-28S rDNA and (TTAGGG)n sequences by fluorescence in situ hybridization (FISH) was performed on Graphiurus platyops (GPL) and Graphiurus ocularis (GOC) metaphases with the aim to characterize the genomes. In both species, inverted DAPI karyotypes showed the same diploid number, 2n = 46, and hybridization of the (TTAGGG)n probe revealed interstitial telomeric sequences (ITSs) at the centromeres of almost all bi-armed chromosomes. FISH with the rDNA probe localized nucleolus organizer regions (NORs), at the terminal ends of the p arms of the subtelocentric pairs 16 and 17 in both species and detected additional signals on GPL8 and GOC18, 19, and 22. The species have similar karyotypes, but their chromosome pairs 18-22 differ in morphology; these are acrocentric in G. platyops, as also confirmed by C-banding, and subtelocentric in G. ocularis. These differences in pairs 18-22 were also highlighted by hybridization of the telomeric probe (TTAGGG)n, which showed the small p arms in G. ocularis enriched with ITSs. FISH of rDNA probes detected multiple NOR loci in G. ocularis, underlining the intense evolutionary dynamics related to these genes. Although the Graphiurus species analyzed have similar karyotypes, the results on the repetitive sequences indicate a complex pattern of genomic reorganization and evolution occurring in these phylogenetically close species.

Adaptive evolutionary expansion of the ribonuclease 6 in Rodentia

Ribonuclease 6 (RNase6 or RNase K6) is a protein that belongs to a superfamily thought to be the sole verte-brate-specific enzyme known for a wide range of physiological functions, including digestion, cytotoxicity, angiogenesis, male reproduction and host defense. In our study, 51 functional genes and 11 pseudogenes were identified from 27 Rodentia species. Intriguingly, in the 3 main lineages of rodents there were multiple RNase6s identified in all species of Ctenohystrica, whereas only a single RNase6 was observed in other Rodentia species examined except for 2 species in the mouse-related clade. The evolutionary scenario of "birth (gene duplication) and death (gene deactivation)" and gene sorting have been demonstrated in Ctenohystrica. In addition, bursts of positive selection, diversification of isoelectric point and positive net charge have been identified in Ctenohystrica, especially at two key sites that are involved in antimicrobial function. Site Trp30 has undergone positive selection and Ile45 has changed into other residues in Group B and Group C of the Ctenohystrica. Our results demonstrated a complex and intriguing evolutionary pattern of rodent RNase6, and indicated that functional modification may have occurred, which establishes an important theoretical foundation for future functional assays in rodent RNase6.

Multiple intrasyntenic rearrangements and rapid speciation in voles

Remarkably stable genomic chromosome elements (evolutionary conserved segments or syntenies) are the basis of large-scale chromosome architecture in vertebrate species. However, these syntenic elements harbour evolutionary important changes through intrachromosomal rearrangements such as inversions and centromere repositioning. Here, using FISH with a set of 20 region-specific probes on a wide array of 28 species, we analyzed evolution of three conserved syntenic regions of the Arvicolinae ancestral karyotype. Inside these syntenies we uncovered multiple, previously cryptic intrachromosomal rearrangements. Although in each of the three conserved blocks we found inversions and centromere repositions, the blocks experienced different types of rearrangements. In two syntenies centromere repositioning predominated, while in the third region, paracentric inversions were more frequent, whereas pericentric inversions were not detected. We found that some of the intrachromosomal rearrangements, mainly paracentric inversions, were synapomorphic for whole arvicoline genera or tribes: genera Alexandromys and Microtus, tribes Ellobini and Myodini. We hypothesize that intrachromosomal rearrangements within conserved syntenic blocks are a major evolutionary force modulating genome architecture in species-rich and rapidly-evolving rodent taxa. Inversions and centromere repositioning may impact speciation and provide a potential link between genome evolution, speciation, and biogeography.

Identification of differentially expressed placental transcripts during multiple gestations in the Eurasian beaver (Castor fiber L.)

The Eurasian beaver is one of the largest rodents that, despite its high impact on the environment, is a non-model species that lacks a reference genome. Characterising genes critical for pregnancy outcome can serve as a basis for identifying mechanisms underlying effective reproduction, which is required for the success of endangered species conservation programs. In the present study, high-throughput RNA sequencing (RNA-seq) was used to analyse global changes in the Castor fiber subplacenta transcriptome during multiple pregnancy. De novo reconstruction of the C. fiber subplacenta transcriptome was used to identify genes that were differentially expressed in placentas (n=5) from two females (in advanced twin and triple pregnancy). Analyses of the expression values revealed 124 contigs with significantly different expression; of these, 55 genes were identified using MegaBLAST. Within this group of differentially expressed genes (DEGs), 18 were upregulated and 37 were downregulated in twins. Most DEGs were associated with the following gene ontology terms: cellular process, single organism process, response to stimulus, metabolic process and biological regulation. Some genes were also assigned to the developmental process, the reproductive process or reproduction. Among this group, four genes (namely keratin 19 (Krt19) and wingless-type MMTV integration site family - member 2 (Wnt2), which were downregulated in twins, and Nik-related kinase (Nrk) and gap junction protein β2 (Gjb2), which were upregulated in twins) were assigned to placental development and nine (Krt19, Wnt2 and integrin α₇ (Itga7), downregulated in twins, and Nrk, gap junction protein β6 (Gjb6), GATA binding protein 6 (Gata6), apolipoprotein A-I (ApoA1), apolipoprotein B (ApoB) and haemoglobin subunit α₁ (HbA1), upregulated in twins) were assigned to embryo development. The results of the present study indicate that the number of fetuses affects the expression profile in the C. fiber subplacental transcriptome. Enhancement of transcriptomic resources for C. fiber will improve understanding of the pathways relevant to proper placental development and successful reproduction.

Mammalian Comparative Genomics Reveals Genetic and Epigenetic Features Associated with Genome Reshuffling in Rodentia

Understanding how mammalian genomes have been reshuffled through structural changes is fundamental to the dynamics of its composition, evolutionary relationships between species and, in the long run, speciation. In this work, we reveal the evolutionary genomic landscape in Rodentia, the most diverse and speciose mammalian order, by whole-genome comparisons of six rodent species and six representative outgroup mammalian species. The reconstruction of the evolutionary breakpoint regions across rodent phylogeny shows an increased rate of genome reshuffling that is approximately two orders of magnitude greater than in other mammalian species here considered. We identified novel lineage and clade-specific breakpoint regions within Rodentia and analyzed their gene content, recombination rates and their relationship with constitutive lamina genomic associated domains, DNase I hypersensitivity sites and chromatin modifications. We detected an accumulation of protein-coding genes in evolutionary breakpoint regions, especially genes implicated in reproduction and pheromone detection and mating. Moreover, we found an association of the evolutionary breakpoint regions with active chromatin state landscapes, most probably related to gene enrichment. Our results have two important implications for understanding the mechanisms that govern and constrain mammalian genome evolution. The first is that the presence of genes related to species-specific phenotypes in evolutionary breakpoint regions reinforces the adaptive value of genome reshuffling. Second, that chromatin conformation, an aspect that has been often overlooked in comparative genomic studies, might play a role in modeling the genomic distribution of evolutionary breakpoints.

Exceptional Chromosomal Evolution and Cryptic Speciation of Blind Mole Rats Nannospalax leucodon (Spalacinae, Rodentia) from South-Eastern Europe

Mole rats are exclusively subterranean and highly specialized rodents. Their long lifespans, remarkable anti-cancer mechanisms, and various distinctive adaptive features make them a useful research model. Moreover, opposing convergence of morphological traits, they have developed extremely high karyotype variability. Thus, 74 chromosomal forms have been described so far and new ones are being revealed continuously. These evolved during the process of rapid radiation and occur in different biogeographical regions. During research into their reproductive biology we have already provided substantial evidence for species-level separation of these taxa. Here, we review diverse chromosomal forms of the lesser blind mole rat, Mediterranean Nannospalax leucodon, distributed in South-eastern Europe, their karyotype records, biogeography, origin, and phylogeny from our extensive research. In the light of new data from molecular genetic studies, we question some former valuations and propose a cryptospecies rank for seven reproductively isolated chromosomal forms with sympatric and parapatric distribution and clear ecogeographical discrepances in their habitats, as well as new experimental and theoretical methods for understanding the courses of speciation of these unique fossorial mammals.

Nematofauna of Rodents of the Families Heteromyidae and Cricetidae from the Mexican Plateau

As a part of an ongoing project to inventory the helminth parasites of rodents in Mexico, 85 specimens of 2 families of rodents were collected from the Mexican Plateau: Cricetidae ( Neotoma sp., Neotoma leucodon , Onychomys arenicola , Peromyscus sp., Peromyscus eremicus , and Reithrodontomys sp.) and Heteromyidae ( Chaetodipus sp., Chaetodipus eremicus , Chaetodipus hispidus , Dipodomys merriami , Dipodomys ordii , Dipodomys ornatus, Dipodomys spectabilis , Liomys irroratus , Perognathus sp., and Perognathus flavus ). A total of 13 taxa of helminths were found: Heteromyoxyuris longejector, Heteromyoxyuris otomii, Heteromyoxyuris sp., Onchocercidae gen. sp. 1 and sp. 2, Physalopteridae gen. sp., Protospirura dipodomis, Pterygodermatites dipodomis, Subulura sp., Syphacia sp., Trichuris dipodomis, Vexillata liomyos, and Vexillata armande. The highest species richness was recorded in D. merriami (7 taxa). This study is the first report of nematodes from O. arenicola (Physalopteridae gen. sp.) and C. eremicus (H. longejector) and for V. liomyos from D. merriami . All reports of these species of nematodes represent new collection localities in Mexico.

Intense genomic reorganization in the genus Oecomys (Rodentia, Sigmodontinae): comparison between DNA barcoding and mapping of repetitive elements in three species of the Brazilian Amazon

Oecomys Thomas, 1906 is one of the most diverse and widely distributed genera within the tribe Oryzomyini. At least sixteen species in this genus have been described to date, but it is believed this genus contains undescribed species. Morphological, molecular and cytogenetic study has revealed an uncertain taxonomic status for several Oecomys species, suggesting the presence of a complex of species. The present work had the goal of contributing to the genetic characterization of the genus Oecomys in the Brazilian Amazon. Thirty specimens were collected from four locations in the Brazilian Amazon and three nominal species recognized: Oecomys auyantepui (Tate, 1939), Oecomys bicolor (Tomes, 1860) and Oecomys rutilus (Anthony, 1921). COI sequence analysis grouped Oecomys auyantepui, Oecomys bicolor and Oecomys rutilus specimens into one, three and two clades, respectively, which is consistent with their geographic distribution. Cytogenetic data for Oecomys auyantepui revealed the sympatric occurrence of two different diploid numbers, 2n=64/NFa=110 and 2n=66/NFa=114, suggesting polymorphism while Oecomys bicolor exhibited 2n=80/NFa=142 and Oecomys rutilus 2n=54/NFa=90. The distribution of constitutive heterochromatin followed a species-specific pattern. Interspecific variation was evident in the chromosomal location and number of 18S rDNA loci. However, not all loci showed signs of activity. All three species displayed a similar pattern for 5S rDNA, with only one pair carrying this locus. Interstitial telomeric sites were found only in Oecomys auyantepui. The data presented in this work reinforce intra- and interspecific variations observed in the diploid number of Oecomys species and indicate that chromosomal rearrangements have led to the appearance of different diploid numbers and karyotypic formulas.

Extensive Chromosomal Reorganization in the Evolution of New World Muroid Rodents (Cricetidae, Sigmodontinae): Searching for Ancestral Phylogenetic Traits

Sigmodontinae rodents show great diversity and complexity in morphology and ecology. This diversity is accompanied by extensive chromosome variation challenging attempts to reconstruct their ancestral genome. The species Hylaeamys megacephalus–HME (Oryzomyini, 2n = 54), Necromys lasiurus—NLA (Akodontini, 2n = 34) and Akodon sp.–ASP (Akodontini, 2n = 10) have extreme diploid numbers that make it difficult to understand the rearrangements that are responsible for such differences. In this study we analyzed these changes using whole chromosome probes of HME in cross-species painting of NLA and ASP to construct chromosome homology maps that reveal the rearrangements between species. We include data from the literature for other Sigmodontinae previously studied with probes from HME and Mus musculus (MMU) probes. We also use the HME probes on MMU chromosomes for the comparative analysis of NLA with other species already mapped by MMU probes. Our results show that NLA and ASP have highly rearranged karyotypes when compared to HME. Eleven HME syntenic blocks are shared among the species studied here. Four syntenies may be ancestral to Akodontini (HME2/18, 3/25, 18/25 and 4/11/16) and eight to Sigmodontinae (HME26, 1/12, 6/21, 7/9, 5/17, 11/16, 20/13 and 19/14/19). Using MMU data we identified six associations shared among rodents from seven subfamilies, where MMU3/18 and MMU8/13 are phylogenetic signatures of Sigmodontinae. We suggest that the associations MMU2entire, MMU6proximal/12entire, MMU3/18, MMU8/13, MMU1/17, MMU10/17, MMU12/17, MMU5/16, MMU5/6 and MMU7/19 are part of the ancestral Sigmodontinae genome.

Clues on Syntenic Relationship among Some Species of Oryzomyini and Akodontini Tribes (Rodentia: Sigmodontinae)

Sigmodontinae rodents represent one of the most diverse and complex components of the mammalian fauna of South America. Among them most species belongs to Oryzomyini and Akodontini tribes. The highly specific diversification observed in both tribes is characterized by diploid complements, which vary from 2n = 10 to 86. Given this diversity, a consistent hypothesis about the origin and evolution of chromosomes depends on the correct establishment of synteny analyzed in a suitable phylogenetic framework. The chromosome painting technique has been particularly useful for identifying chromosomal synteny. In order to extend our knowledge of the homeological relationships between Akodontini and Oryzomyini species, we analyzed the species Akodon montensis (2n = 24) and Thaptomys nigrita (2n = 52) both from the tribe Akodontini, with chromosome probes of Hylaeamys megacephalus (2n = 54) of the tribe Oryzomyini. The results indicate that at least 12 of the 26 autosomes of H. megacephalus show conserved synteny in A. montensis and 14 in T. nigrita. The karyotype of Akodon montensis, as well as some species of the Akodon cursor species group, results from many chromosomal fusions and therefore the syntenic associations observed probably represent synapomorphies. Our finding of a set of such associations revealed by H. megacephalus chromosome probes (6/21; 3/25; 11/16/17; and, 14/19) provides phylogenetic information for both tribes. An extension of these observations to other members of Akodontini and Oryzomyini tribes should improve our knowledge about chromosome evolution in both these groups.

Genome-wide comparative chromosome maps of Arvicola amphibius, Dicrostonyx torquatus, and Myodes rutilus

The subfamily Arvicolinae consists of a great number of species with highly diversified karyotypes. In spite of the wide use of arvicolines in biological and medicine studies, the data on their karyotype structures are limited. Here, we made a set of painting probes from flow-sorted chromosomes of a male Palearctic collared lemming (Dicrostonyx torquatus, DTO). Together with the sets of painting probes made previously from the field vole (Microtus agrestis, MAG) and golden hamster (Mesocricetus auratus, MAU), we carried out a reciprocal chromosome painting between these three species. The three sets of probes were further hybridized onto the chromosomes of the Eurasian water vole (Arvicola amphibius) and northern red-backed vole (Myodes rutilus). We defined the diploid chromosome number in D. torquatus karyotype as 2n = 45 + Bs and showed that the system of sex chromosomes is X1X2Y1. The probes developed here provide a genomic tool-kit, which will help to investigate the evolutionary biology of the Arvicolinae rodents. Our results show that the syntenic association MAG1/17 is present not only in Arvicolinae but also in some species of Cricetinae; and thus, should not be considered as a cytogenetic signature for Arvicolinae. Although cytogenetic signature markers for the genera have not yet been found, our data provides insight into the likely ancestral karyotype of Arvicolinae. We conclude that the karyotypes of modern voles could have evolved from a common ancestral arvicoline karyotype (AAK) with 2n = 56 mainly by centric fusions and fissions.

Biochemical adaptations to dive-derived hypoxia/reoxygenation in semiaquatic rodents

To meet the challenges presented by dive-derived hypoxia/reoxygenation transition, the aquatic mammals possess multi-level adaptations. However, the adjustments of the semiaquatic animals as modern analogs of evolutionary intermediates between ancestral terrestrial mammals and their fully aquatic descendants are still not fully elucidated. The aim of this study was to analyze the total lactate dehydrogenase (LDH) activity (in the lactate to pyruvate direction), the LDH patterns and the antioxidant defense in the tissues (heart, kidney, liver, lung, muscle, spleen) of semiaquatic rodents such as Eurasian beaver (Castor fiber), muskrat (Ondatra zibethicus) and nutria (Myocastor coypus). Samples from Wistar rat were used for comparison. Semiaquatic rodents had higher catalase activity compared to rats. The superoxide dismutase activity was higher and the catalase activity was lower in almost all tissues of muskrat than of both beaver and nutria. Comparing beaver and nutria, no significant differences in the antioxidant enzyme activities were found for the heart, kidney and liver. In beaver, most of the examined tissues (heart, kidney, lung and spleen) use lactate as preference to glucose as a substrate but in muskrat the heart, liver and skeletal muscle showed the increased LDH activity. Nutria had the unusual LDH properties that are needed to be further investigated. Our results suggest that beaver, nutria and muskrat have distinct mechanisms of adaptation to diving hypoxia/reoxygenation and support the hypothesis that semiaquatic mammals are the intermediate animals that help to define which potential selection factors and mechanical constraints may have directed the evolution of the aquatic forms.

A First Generation Comparative Chromosome Map between Guinea Pig (Cavia porcellus) and Humans

The domesticated guinea pig, Cavia porcellus (Hystricomorpha, Rodentia), is an important laboratory species and a model for a number of human diseases. Nevertheless, genomic tools for this species are lacking; even its karyotype is poorly characterized. The guinea pig belongs to Hystricomorpha, a widespread and important group of rodents; so far the chromosomes of guinea pigs have not been compared with that of other hystricomorph species or with any other mammals. We generated full sets of chromosome-specific painting probes for the guinea pig by flow sorting and microdissection, and for the first time, mapped the chromosomal homologies between guinea pig and human by reciprocal chromosome painting. Our data demonstrate that the guinea pig karyotype has undergone extensive rearrangements: 78 synteny-conserved human autosomal segments were delimited in the guinea pig genome. The high rate of genome evolution in the guinea pig may explain why the HSA7/16 and HSA16/19 associations presumed ancestral for eutherians and the three syntenic associations (HSA1/10, 3/19, and 9/11) considered ancestral for rodents were not found in C. porcellus. The comparative chromosome map presented here is a starting point for further development of physical and genetic maps of the guinea pig as well as an aid for genome assembly assignment to specific chromosomes. Furthermore, the comparative mapping will allow a transfer of gene map data from other species. The probes developed here provide a genomic toolkit, which will make the guinea pig a key species to unravel the evolutionary biology of the Hystricomorph rodents.

A High-Resolution Comparative Chromosome Map of Cricetus cricetus and Peromyscus eremicus Reveals the Involvement of Constitutive Heterochromatin in Breakpoint Regions

Compared to humans and other mammals, rodent genomes, specifically Muroidea species, underwent intense chromosome reshuffling in which many complex structural rearrangements occurred. This fact makes them preferential animal models for studying the process of karyotype evolution. Here, we present the first combined chromosome comparative maps between 2 Cricetidae species, Cricetus cricetus and Peromyscus eremicus, and the index species Mus musculus and Rattus norvegicus. Comparative chromosome painting was done using mouse and rat paint probes together with in silico analysis from the Ensembl genome browser database. Hereby, evolutionary events (inter- and intrachromosomal rearrangements) that occurred in C. cricetus and P. eremicus since the putative ancestral Muroidea genome could be inferred, and evolutionary breakpoint regions could be detected. A colocalization of constitutive heterochromatin and evolutionary breakpoint regions in each genome was observed. Our results suggest the involvement of constitutive heterochromatin in karyotype restructuring of these species, despite the different levels of conservation of the C. cricetus (derivative) and P. eremicus (conserved) genomes.

Comprehensive Analyses of White-Handed Gibbon Chromosomes Enables Access to 92 Evolutionary Conserved Breakpoints Compared to the Human Genome

Gibbon species (Hylobatidae) impress with an unusually high number of numerical and structural chromosomal changes within the family itself as well as compared to other Hominoidea including humans. In former studies applying molecular cytogenetic methods, 86 evolutionary conserved breakpoints (ECBs) were reported in the white-handed gibbon (Hylobates lar, HLA) with respect to the human genome. To analyze those ECBs in more detail and also to achieve a better understanding of the fast karyotype evolution in Hylobatidae, molecular data for these regions are indispensably necessary. In the present study, we obtained whole chromosome-specific probes by microdissection of all 21 HLA autosomes and prepared them for aCGH. Locus-specific DNA probes were also used for further molecular cytogenetic characterization of selected regions. Thus, we could map 6 yet unreported ECBs in HLA with respect to the human genome. Additionally, in 26 of the 86 previously reported ECBs, the present approach enabled a more precise breakpoint mapping. Interestingly, a preferred localization of ECBs within segmental duplications, copy number variant regions, and fragile sites was observed.