Integration of molecular cytogenetics, dated molecular phylogeny, and model-based predictions to understand the extreme chromosome reorganization in the Neotropical genus Tonatia (Chiroptera: Phyllostomidae)

Mechanisms of Rapid Karyotype Evolution in Mammals

Chromosome reshuffling events are often a foundational mechanism by which speciation can occur, giving rise to highly derivative karyotypes even amongst closely related species. Yet, the features that distinguish lineages prone to such rapid chromosome evolution from those that maintain stable karyotypes across evolutionary time are still to be defined. In this review, we summarize lineages prone to rapid karyotypic evolution in the context of Simpson's rates of evolution-tachytelic, horotelic, and bradytelic-and outline the mechanisms proposed to contribute to chromosome rearrangements, their fixation, and their potential impact on speciation events. Furthermore, we discuss relevant genomic features that underpin chromosome variation, including patterns of fusions/fissions, centromere positioning, and epigenetic marks such as DNA methylation. Finally, in the era of telomere-to-telomere genomics, we discuss the value of gapless genome resources to the future of research focused on the plasticity of highly rearranged karyotypes.

High rates of structural rearrangements have shaped the chromosome evolution in dysploid Phaseolus beans

KEY MESSAGE

Karyotypes evolve through numerical and structural chromosome rearrangements. We show that Phaseolus leptostachyus, a wild bean, underwent a rapid genome reshuffling associated with the reduction from 11 to 10 chromosome pairs, but without whole genome duplication, the highest chromosome evolution rate known for plants. Plant karyotypes evolve through structural rearrangements often associated with polyploidy or dysploidy. The genus Phaseolus comprises ~ 90 species, five of them domesticated due to their nutritional relevance. Most of the species have 2n = 22 karyotypes and are highly syntenic, except for three dysploid karyotypes of species from the Leptostachyus group (2n = 20) that have accumulated several rearrangements. Here, we investigated the degrees of structural rearrangements among Leptostachyus and other Phaseolus groups by estimating their chromosomal evolution rates (CER). For this, we combined our oligo-FISH barcode system for beans and chromosome-specific painting probes for chromosomes 2 and 3, with rDNA and a centromeric probe to establish chromosome orthologies and identify structural rearrangements across nine Phaseolus species. We also integrated the detected rearrangements with a phylogenomic approach to estimate the CERs for each Phaseolus lineage. Our data allowed us to identify translocations, inversions, duplications and deletions, mostly in species belonging to the Leptostachyus group. Phaseolus leptostachyus showed the highest CER (12.31 rearrangements/My), a tenfold increase in contrast to the 2n = 22 species analysed. This is the highest rate known yet for plants, making it a model species for investigating the mechanisms behind rapid genome reshuffling in early species diversification.

High-density linkage maps and chromosome level genome assemblies unveil direction and frequency of extensive structural rearrangements in wood white butterflies (Leptidea spp.)

Karyotypes are generally conserved between closely related species and large chromosome rearrangements typically have negative fitness consequences in heterozygotes, potentially driving speciation. In the order Lepidoptera, most investigated species have the ancestral karyotype and gene synteny is often conserved across deep divergence, although examples of extensive genome reshuffling have recently been demonstrated. The genus Leptidea has an unusual level of chromosome variation and rearranged sex chromosomes, but the extent of restructuring across the rest of the genome is so far unknown. To explore the genomes of the wood white (Leptidea) species complex, we generated eight genome assemblies using a combination of 10X linked reads and HiC data, and improved them using linkage maps for two populations of the common wood white (L. sinapis) with distinct karyotypes. Synteny analysis revealed an extensive amount of rearrangements, both compared to the ancestral karyotype and between the Leptidea species, where only one of the three Z chromosomes was conserved across all comparisons. Most restructuring was explained by fissions and fusions, while translocations appear relatively rare. We further detected several examples of segregating rearrangement polymorphisms supporting a highly dynamic genome evolution in this clade. Fusion breakpoints were enriched for LINEs and LTR elements, which suggests that ectopic recombination might be an important driver in the formation of new chromosomes. Our results show that chromosome count alone may conceal the extent of genome restructuring and we propose that the amount of genome evolution in Lepidoptera might still be underestimated due to lack of taxonomic sampling.

Chromosome painting and phylogenetic analysis suggest that the genus Lophostoma (Chiroptera, Phyllostomidae) is paraphyletic

The subfamily Phyllostominae (Chiroptera, Phyllostomidae) comprises 10 genera of Microchiroptera bats from the Neotropics. The taxonomy of this group is controversial due to incongruities in the phylogenetic relationships evident from different datasets. The genus Lophostoma currently includes eight species whose phylogenetic relationships have not been resolved. Integrative analyzes including morphological, molecular and chromosomal data are powerful tools to investigate the phylogenetics of organisms, particularly if obtained by chromosomal painting. In the present work we performed comparative genomic mapping of three species of Lophostoma (L. brasiliense 2n = 30, L. carrikeri 2n = 26 and L. schulzi 2n = 26), by chromosome painting using whole chromosome probes from Phyllostomus hastatus and Carollia brevicauda; this included mapping interstitial telomeric sites. The karyotype of L. schulzi (LSC) is a new cytotype. The species L. brasiliense and L. carrikeri showed interstitial telomeric sequences that probably resulted from expansions of repetitive sequences near pericentromeric regions. The addition of chromosomal painting data from other species of Phyllostominae allowed phylogeny construction by maximum parsimony, and the determination that the genera of this subfamily are monophyletic, and that the genus Lophostoma is paraphyletic. Additionally, a review of the taxonomic status of LSC is suggested to determine if this species should be reclassified as part of the genus Tonatia.

Positive Selection and Gene Expression Analyses from Salivary Glands Reveal Discrete Adaptations within the Ecologically Diverse Bat Family Phyllostomidae

The leaf-nosed bats (Phyllostomidae) are outliers among chiropterans with respect to the unusually high diversity of dietary strategies within the family. Salivary glands, owing to their functions and high ultrastructural variability among lineages, are proposed to have played an important role during the phyllostomid radiation. To identify genes underlying salivary gland functional diversification, we sequenced submandibular gland transcriptomes from phyllostomid species representative of divergent dietary strategies. From the assembled transcriptomes, we performed an array of selection tests and gene expression analyses to identify signatures of adaptation. Overall, we identified an enrichment of immunity-related gene ontology terms among 53 genes evolving under positive selection. Lineage-specific selection tests revealed several endomembrane system genes under selection in the vampire bat. Many genes that respond to insulin were under selection and differentially expressed genes pointed to modifications of amino acid synthesis pathways in plant-visitors. Results indicate salivary glands have diversified in various ways across a functional diverse clade of mammals in response to niche specializations.

Comparative cytogenetics of two species of Dermanura (Chiroptera, Phyllostomidae) in Midwestern Brazil

Dermanura Gervais, 1856 is represented by small frugivorous bats of the Stenodermatinae subfamily. The taxonomy of this group presents controversies and has been subject to changes, especially since the morphological characters evaluated have left gaps that are difficult to fill regarding good species characterization. Previous studies performed in Dermanura cinerea Gervais, 1856 found that the karyotype of this species has a diploid number of chromosomes equal to 30 and 56 autosomal arms. The objective of the present study was to describe, for the first time, the karyotypes of the species Dermanura anderseni (Osgood, 1916) and Dermanura gnoma (Handley, 1987) based on classical cytogenetic markers. For both species, the diploid number found was 2n = 30 and NFa = 56. Two pairs of chromosomes showed markings of the nucleolus organizing regions (AgNORs) in the species D. anderseni and only one pair in D. gnoma, differing from what has already been described for D. cinerea. The two species analyzed here also showed differences in the sex chromosome system, with D. gnoma showing a neo-XY type system while in D. anderseni the classic XY sexual system was observed. In both species, visualization of the constitutive heterochromatin occurred in the pericentromeric region of all chromosomes, as well as in the short arms of the subtelocentric chromosomes. The present work represents an important expansion of karyotypic information for the subfamily Stenodermatinae, bringing chromosomal features that are possible to use in the taxonomic implications of the group.

Repetitive DNA Profiles Reveal Evidence of Rapid Genome Evolution and Reflect Species Boundaries in Ground Beetles

Genome architecture is a complex, multidimensional property of an organism defined by the content and spatial organization of the genome's component parts. Comparative study of entire genome architecture in model organisms is shedding light on mechanisms underlying genome regulation, evolution, and diversification, but such studies require costly analytical approaches which make extensive comparative study impractical for most groups. However, lower-cost methods that measure a single architectural component (e.g., distribution of one class of repeats) have potential as a new data source for evolutionary studies insofar as that measure correlates with more complex biological phenomena, and for which it could serve as part of an explanatory framework. We investigated copy number variation (CNV) profiles in ribosomal DNA (rDNA) as a simple measure reflecting the distribution of rDNA subcomponents across the genome. We find that signatures present in rDNA CNV profiles strongly correlate with species boundaries in the breve species group of Bembidion, and vary across broader taxonomic sampling in Bembidion subgenus Plataphus. Profiles of several species show evidence of re-patterning of rDNA-like sequences throughout the genome, revealing evidence of rapid genome evolution (including among sister pairs) not evident from analysis of traditional data sources such as multigene data sets. Major re-patterning of rDNA-like sequences has occurred frequently within the evolutionary history of Plataphus. We confirm that CNV profiles represent an aspect of genomic architecture (i.e., the linear distribution of rDNA components across the genome) via fluorescence in-situ hybridization. In at least one species, novel rDNA-like elements are spread throughout all chromosomes. We discuss the potential of copy number profiles of rDNA, or other repeats, as a low-cost tool for incorporating signal of genomic architecture variation in studies of species delimitation and genome evolution. [Bembidion; Carabidae; copy number variation profiles; rapid genome evolution; ribosomal DNA; species delimitation.].

Extensive genomic reshuffling involved in the karyotype evolution of genus Cerradomys (Rodentia: Sigmodontinae: Oryzomyini)

Rodents of the genus Cerradomys belong to the tribe Oryzomyini and present high chromosome variability with diploid numbers ranging from 2n=46 to 60. Classical cytogenetics and fluorescence in situ hybridization (FISH) with telomeric and whole chromosome-specific probes of another Oryzomyini, Oligoryzomys moojeni (OMO), were used to assess the karyotype evolution of the genus. Results were integrated into a molecular phylogeny to infer the hypothetical direction of chromosome changes. The telomeric FISH showed signals in telomeres in species that diverged early in the phylogeny, plus interstitial telomeric signals (ITS) in some species from the most derived clades (C. langguthi,C. vivoi, C. goytaca, and C. subflavus). Chromosome painting revealed homology from 23 segments of C. maracajuensis and C. marinhus to 32 of C. vivoi. Extensive chromosome reorganization was responsible for karyotypic differences in closely related species. Major drivers for genomic reshuffling were in tandem and centric fusion, fission, paracentric and pericentric inversions or centromere repositioning. Chromosome evolution was associated with an increase and decrease in diploid number in different lineages and ITS indicate remnants of ancient telomeres. Cytogenetics results corroborates that C. goytaca is not a junior synonym of C. subflavus since the karyotypic differences found may lead to reproductive isolation.

Chromosomal dynamics in space and time: evolutionary history of Mycetophylax ants across past climatic changes in the Brazilian Atlantic coast

Fungus-farming ants of the genus Mycetophylax exhibit intra and interspecific chromosome variability, which makes them suitable for testing hypotheses about possible chromosomal rearrangements that endure lineage diversification. We combined cytogenetic and molecular data from Mycetophylax populations from coastal environments to trace the evolutionary history of the clade in light of chromosomal changes under a historical and geographic context. Our cytogenetic analyses revealed chromosomal differences within and among species. M. morschi exhibited three distinct karyotypes and considerable variability in the localization of 45S rDNA clusters. The molecular phylogeny was congruent with our cytogenetic findings. Biogeographical and divergence time dating analyses estimated that the most recent common ancestor of Mycetophylax would have originated at about 30 Ma in an area including the Amazon and Southern Grasslands, and several dispersion and vicariance events may have occurred before the colonization of the Brazilian Atlantic coast. Diversification of the psammophilous Mycetophylax first took place in the Middle Miocene (ca. 18-10 Ma) in the South Atlantic coast, while "M. morschi" lineages diversified during the Pliocene-Pleistocene transition (ca. 3-2 Ma) through founder-event dispersal for the Northern coastal regions. Psammophilous Mycetophylax diversification fits into the major global climatic events that have had a direct impact on the changes in sea level as well as deep ecological impact throughout South America. We assume therefore that putative chromosomal rearrangements correlated with increased ecological stress during the past climatic transitions could have intensified and/or accompanied the divergence of the psammophilous Mycetophylax. We further reiterate that "M. morschi" comprises a complex of at least three well-defined lineages, and we emphasize the role of this integrative approach for the identification and delimitation of evolutionary lineages.

Karyotype, evolution and phylogenetic reconstruction in Micronycterinae bats with implications for the ancestral karyotype of Phyllostomidae

BACKGROUND

The Micronycterinae form a subfamily of leaf-nosed bats (Phyllostomidae) that contains the genera Lampronycteris Sanborn, 1949, and Micronycteris Gray, 1866 (stricto sensu), and is characterized by marked karyotypic variability and discrepancies in the phylogenetic relationships suggested by the molecular versus morphological data. In the present study, we investigated the chromosomal evolution of the Micronycterinae using classical cytogenetics and multidirectional chromosome painting with whole-chromosomes probes of Phyllostomus hastatus and Carollia brevicauda. Our goal was to perform comparative chromosome mapping between the genera of this subfamily and explore the potential for using chromosomal rearrangements as phylogenetic markers.

RESULTS

The Micronycterinae exhibit great inter- and intraspecific karyotype diversity, with large blocks of telomere-like sequences inserted within or adjacent to constitutive heterochromatin regions. The phylogenetic results generated from our chromosomal data revealed that the Micronycterinae hold a basal position in the phylogenetic tree of the Phyllostomidae. Molecular cytogenetic data confirmed that there is a low degree of karyotype similarity between Lampronycteris and Micronycteris specimens analyzed, indicating an absence of synapomorphic associations in Micronycterinae.

CONCLUSIONS

We herein confirm that karyotypic variability is present in subfamily Micronycterinae. We further report intraspecific variation and describe a new cytotype in M. megalotis. The cytogenetic data show that this group typically has large blocks of interstitial telomeric sequences that do not appear to be correlated with chromosomal rearrangement events. Phylogenetic analysis using chromosome data recovered the basal position for Micronycterinae, but did not demonstrate that it is a monophyletic lineage, due to the absence of common chromosomal synapomorphy between the genera. These findings may be related to an increase in the rate of chromosomal evolution during the time period that separates Lampronycteris from Micronycteris.

Chromosomal Evolution in Chiroptera

Chiroptera is the second largest order among mammals, with over 1300 species in 21 extant families. The group is extremely diverse in several aspects of its natural history, including dietary strategies, ecology, behavior and morphology. Bat genomes show ample chromosome diversity (from 2n = 14 to 62). As with other mammalian orders, Chiroptera is characterized by clades with low, moderate and extreme chromosomal change. In this article, we will discuss trends of karyotypic evolution within distinct bat lineages (especially Phyllostomidae, Hipposideridae and Rhinolophidae), focusing on two perspectives: evolution of genome architecture, modes of chromosomal evolution, and the use of chromosome data to resolve taxonomic problems.

Centromeric enrichment of LINE-1 retrotransposons and its significance for the chromosome evolution of Phyllostomid bats

Despite their ubiquitous incidence, little is known about the chromosomal distribution of long interspersed elements (LINEs) in mammalian genomes. Phyllostomid bats, characterized by lineages with distinct trends of chromosomal evolution coupled with remarkable ecological and taxonomic diversity, represent good models to understand how these repetitive sequences contribute to the evolution of genome architecture and its link to lineage diversification. To test the hypothesis that LINE-1 sequences were important modifiers of bat genome architecture, we characterized the distribution of LINE-1-derived sequences on genomes of 13 phyllostomid species within a phylogenetic framework. We found massive accumulation of LINE-1 elements in the centromeres of most species: a rare phenomenon on mammalian genomes. We hypothesize that expansion of these elements has occurred early in the radiation of phyllostomids and recurred episodically. LINE-1 expansions on centromeric heterochromatin probably spurred chromosomal change before the radiation of phyllostomids into the extant 11 subfamilies and contributed to the high degree of karyotypic variation observed among different lineages. Understanding centromere architecture in a variety of taxa promises to explain how lineage-specific changes on centromere structure can contribute to karyotypic diversity while not disrupting functional constraints for proper cell division.

Heterochromatin variation and LINE-1 distribution in Artibeus (Chiroptera, Phyllostomidae) from Central Amazon, Brazil

Species in the subgenus Artibeus Leach, 1821 are widely distributed in Brazil. Conserved karyotypes characterize the group with identical diploid number and chromosome morphology. Recent studies suggested that the heterochromatin distribution and accumulation patterns can vary among species. In order to assess whether variation can also occur within species, we have analyzed the chromosomal distribution of constitutive heterochromatin in A. planirostris (Spix, 1823) and A. lituratus (Olfers, 1818) from Central Amazon (North Brazil) and contrasted our findings with those reported for other localities in Brazil. In addition, Ag-NOR staining and FISH with 18S rDNA, telomeric, and LINE-1 probes were performed to assess the potential role that these different repetitive markers had in shaping the current architecture of heterochromatic regions. Both species presented interindividual variation of constitutive heterochromatin. In addition, in A. planirostris the centromeres of most chromosomes are enriched with LINE-1, colocated with pericentromeric heterochromatin blocks. Overall, our data indicate that amplification and differential distribution of the investigated repetitive DNAs might have played a significant role in shaping the chromosome architecture of the subgenus Artibeus.