Evolutionary dynamics of autosomal-heterosomal rearrangements in a multiple-X chromosome system of tiger beetles (Cicindelidae)

High chromosomal mobility of rDNA clusters in holocentric chromosomes of Triatominae, vectors of Chagas disease (Hemiptera-Reduviidae)

The subfamily Triatominae (Hemiptera-Reduviidae) includes more than 150 blood-sucking species, potential vectors of the protozoan Trypanosoma cruzi, causative agent of Chagas disease. A distinctive cytogenetic characteristic of this group is the presence of extremely stable chromosome numbers. Unexpectedly, the analyses of the chromosomal location of ribosomal gene clusters and other repetitive sequences place Triatominae as a significantly diverse hemipteran subfamily. Here, we advance the understanding of Triatominae chromosomal evolution through the analysis of the 45S rDNA cluster chromosomal location in 92 Triatominae species. We found the 45S rDNA clusters in one to four loci per haploid genome with different chromosomal patterns: On one or two autosomes, on one, two or three sex chromosomes, on the X chromosome plus one to three autosomes. The movement of 45S rDNA clusters is discussed in an evolutionary context. Our results illustrate that rDNA mobility has been relatively common in the past and in recent evolutionary history of the group. The high frequency of rDNA patterns involving autosomes and sex chromosomes among closely related species could affect genetic recombination and the viability of hybrid populations, which suggests that the mobility of rDNA clusters could be a driver of species diversification.

Evolutionary pattern of karyotypes and meiosis in pholcid spiders (Araneae: Pholcidae): implications for reconstructing chromosome evolution of araneomorph spiders

BACKGROUND

Despite progress in genomic analysis of spiders, their chromosome evolution is not satisfactorily understood. Most information on spider chromosomes concerns the most diversified clade, entelegyne araneomorphs. Other clades are far less studied. Our study focused on haplogyne araneomorphs, which are remarkable for their unusual sex chromosome systems and for the co-evolution of sex chromosomes and nucleolus organizer regions (NORs); some haplogynes exhibit holokinetic chromosomes. To trace the karyotype evolution of haplogynes on the family level, we analysed the number and morphology of chromosomes, sex chromosomes, NORs, and meiosis in pholcids, which are among the most diverse haplogyne families. The evolution of spider NORs is largely unknown.

RESULTS

Our study is based on an extensive set of species representing all major pholcid clades. Pholcids exhibit a low 2n and predominance of biarmed chromosomes, which are typical haplogyne features. Sex chromosomes and NOR patterns of pholcids are diversified. We revealed six sex chromosome systems in pholcids (X0, XY, X1X20, X1X2X30, X1X2Y, and X1X2X3X4Y). The number of NOR loci ranges from one to nine. In some clades, NORs are also found on sex chromosomes.

CONCLUSIONS

The evolution of cytogenetic characters was largely derived from character mapping on a recently published molecular phylogeny of the family. Based on an extensive set of species and mapping of their characters, numerous conclusions regarding the karyotype evolution of pholcids and spiders can be drawn. Our results suggest frequent autosome-autosome and autosome-sex chromosome rearrangements during pholcid evolution. Such events have previously been attributed to the reproductive isolation of species. The peculiar X1X2Y system is probably ancestral for haplogynes. Chromosomes of the X1X2Y system differ considerably in their pattern of evolution. In some pholcid clades, the X1X2Y system has transformed into the X1X20 or XY systems, and subsequently into the X0 system. The X1X2X30 system of Smeringopus pallidus probably arose from the X1X20 system by an X chromosome fission. The X1X2X3X4Y system of Kambiwa probably evolved from the X1X2Y system by integration of a chromosome pair. Nucleolus organizer regions have frequently expanded on sex chromosomes, most probably by ectopic recombination. Our data suggest the involvement of sex chromosome-linked NORs in achiasmatic pairing.

Multidisciplinary approach detects speciation within the kissing bug Panstrongylus rufotuberculatus populations (Hemiptera, Heteroptera, Reduviidae)

BACKGROUND

Panstrongylus rufotuberculatus (Hemiptera-Reduviidae) is a triatomine species with a wide geographic distribution and a broad phenotypic variability. In some countries, this species is found infesting and colonising domiciliary ecotopes representing an epidemiological risk factor as a vector of Trypanosoma cruzi, etiological agent of Chagas disease. In spite of this, little is known about P. rufotuberculatus genetic diversity.

METHODS

Cytogenetic studies and DNA sequence analyses of one nuclear (ITS-2) and two mitochondrial DNA sequences (cyt b and coI) were carried out in P. rufotuberculatus individuals collected in Bolivia, Colombia, Ecuador and Mexico. Moreover, a geometric morphometrics study was applied to Bolivian, Colombian, Ecuadorian and French Guiana samples.

OBJECTIVES

To explore the genetic and phenetic diversity of P. rufotuberculatus from different countries, combining chromosomal studies, DNA sequence analyses and geometric morphometric comparisons.

FINDINGS

We found two chromosomal groups differentiated by the number of X chromosomes and the chromosomal position of the ribosomal DNA clusters. In concordance, two main morphometric profiles were detected, clearly separating the Bolivian sample from the other ones. Phylogenetic DNA analyses showed that both chromosomal groups were closely related to each other and clearly separated from the remaining Panstrongylus species. High nucleotide divergence of cyt b and coI fragments were observed among P. rufotuberculatus samples from Bolivia, Colombia, Ecuador and Mexico (Kimura 2-parameter distances higher than 9%).

MAIN CONCLUSIONS

Chromosomal and molecular analyses supported that the two chromosomal groups could represent different closely related species. We propose that Bolivian individuals constitute a new Panstrongylus species, being necessary a detailed morphological study for its formal description. The clear morphometric discrimination based on the wing venation pattern suggests such morphological description might be conclusive.

Deciphering the Origin and Evolution of the X1X2Y System in Two Closely-Related Oplegnathus Species (Oplegnathidae and Centrarchiformes)

Oplegnathus fasciatus and O. punctatus (Teleostei: Centrarchiformes: Oplegnathidae), are commercially important rocky reef fishes, endemic to East Asia. Both species present an X1X2Y sex chromosome system. Here, we investigated the evolutionary forces behind the origin and differentiation of these sex chromosomes, with the aim to elucidate whether they had a single or convergent origin. To achieve this, conventional and molecular cytogenetic protocols, involving the mapping of repetitive DNA markers, comparative genomic hybridization (CGH), and whole chromosome painting (WCP) were applied. Both species presented similar 2n, karyotype structure and hybridization patterns of repetitive DNA classes. 5S rDNA loci, besides being placed on the autosomal pair 22, resided in the terminal region of the long arms of both X1 chromosomes in females, and on the X1 and Y chromosomes in males. Furthermore, WCP experiments with a probe derived from the Y chromosome of O. fasciatus (OFAS-Y) entirely painted the X1 and X2 chromosomes in females and the X1, X2, and Y chromosomes in males of both species. CGH failed to reveal any sign of sequence differentiation on the Y chromosome in both species, thereby suggesting the shared early stage of neo-Y chromosome differentiation. Altogether, the present findings confirmed the origin of the X1X2Y sex chromosomes via Y-autosome centric fusion and strongly suggested their common origin.

Insights into the karyotype evolution and speciation of the beetle Euchroma gigantea (Coleoptera: Buprestidae)

Euchroma Dejean, 1833 (Buprestidae: Coleoptera) is a monotypic genus comprising the species Euchroma gigantea, with populations presenting a degree of karyotypic variation/polymorphism rarely found within a single taxonomic (specific) unit, as well as drastically incompatible meiotic configurations in populations from extremes of the species range. To better understand the complex karyotypic evolution of E. gigantea, the karyotypes of specimens from five populations in Brazil were investigated using molecular cytogenetics and phylogenetic approaches. Herein, we used FISH with histone genes as well as sequencing of the COI to determine differential distribution of markers and relationships among populations. The analyses revealed new karyotypes, with variability for chromosome number and morphology of multiple sex chromosome mechanisms, occurrence of B chromosome variants (punctiform and large ones), and high dispersion of histone genes in different karyotypes. These data indicate that chromosomal polymorphism in E. gigantea is greater than previously reported, and that the species can be a valuable model for cytogenetic studies. The COI phylogenetic and haplotype analyses highlighted the formation of three groups with chromosomally polymorphic individuals. Finally, we compared the different karyotypes and proposed a model for the chromosomal evolution of this species. The species E. gigantea includes at least three cytogenetically polymorphic lineages. Moreover, in each of these lineages, different chromosomal rearrangements have been fixed. Dispersion of repetitive sequences may have favored the high frequency of these rearrangements, which could be related to both adaptation of the species to different habitats and the speciation process.

Estimating tempo and mode of Y chromosome turnover: explaining Y chromosome loss with the fragile Y hypothesis

Chromosomal sex determination is phylogenetically widespread, having arisen independently in many lineages. Decades of theoretical work provide predictions about sex chromosome differentiation that are well supported by observations in both XY and ZW systems. However, the phylogenetic scope of previous work gives us a limited understanding of the pace of sex chromosome gain and loss and why Y or W chromosomes are more often lost in some lineages than others, creating XO or ZO systems. To gain phylogenetic breadth we therefore assembled a database of 4724 beetle species' karyotypes and found substantial variation in sex chromosome systems. We used the data to estimate rates of Y chromosome gain and loss across a phylogeny of 1126 taxa estimated from seven genes. Contrary to our initial expectations, we find that highly degenerated Y chromosomes of many members of the suborder Polyphaga are rarely lost, and that cases of Y chromosome loss are strongly associated with chiasmatic segregation during male meiosis. We propose the "fragile Y" hypothesis, that recurrent selection to reduce recombination between the X and Y chromosome leads to the evolution of a small pseudoautosomal region (PAR), which, in taxa that require XY chiasmata for proper segregation during meiosis, increases the probability of aneuploid gamete production, with Y chromosome loss. This hypothesis predicts that taxa that evolve achiasmatic segregation during male meiosis will rarely lose the Y chromosome. We discuss data from mammals, which are consistent with our prediction.

High-Resolution Physical Chromosome Mapping of Multigene Families in Lagria villosa (Tenebrionidae): Occurrence of Interspersed Ribosomal Genes in Coleoptera

The organization and mapping of multigene families can produce useful genetic markers, and its use may elucidate the mechanisms of karyotype variation and genomic organization in different groups of eukaryotes. To date, few species of Coleoptera have been analyzed using FISH for the location of multigene families. The purpose of this study was to use high-resolution chromosome mapping to establish the genomic organization of the 18S rDNA, 5S rDNA and histone H3 gene families in Lagria villosa. FISH was performed using 18S rDNA, 5S rDNA and histone H3 probes prepared via PCR labeling. Fiber-FISH for 18S and 5S rDNA indicated that both ribosomal elements are colocalized in the short arm of chromosome 4. Additionally, FISH, using the histone H3 probe, revealed that this sequence is found in only one autosomal pair and did not colocalize with rDNA. Fiber-FISH with 5S and 18S probes, used to improve the mapping resolution of these regions, showed that both genes are closely interspersed with varying amounts of both DNA classes.

Identification and characterisation of putative seminal fluid proteins from male reproductive tissue EST libraries in tiger beetles

BACKGROUND

The study of proteins transferred through semen can provide important information for biological questions such as adaptive evolution, the origin of new species and species richness. The objective of this study was to identify seminal fluid proteins (SFPs) that may contribute to the study of the reproductive system of tiger beetles (cicindelids), a group of more than 2,500 species distributed worldwide that occupy a great diversity of habitats.

RESULTS

Two cDNA libraries were constructed from the male gonads of Calomera littoralis and Cephalota litorea. Expressed sequence tags (ESTs) were analysed by bioinformatics approaches and 14 unigenes were selected as candidate SFPs, which were submitted to Reverse Transcription Polymerase Chain Reaction (RT-PCR) to identify patterns of tissue-specific expression. We have identified four novel putative SFPs of cicindelids, of which similarity searches did not show homologues with known function. However, two of the protein classes (immune response and hormone) predicted by Protfun are similar to SFPs reported in other insects. Searches for homology in other cicindelids showed one lineage specific SFPs (rapidly evolving proteins), only present in the closely related species C. littoralis and Lophyra flexuosa and two conserved SFP present in other tiger beetles species tested.

CONCLUSIONS

This work represents the first characterisation of putative SFPs in Adephagan species of the order Coleoptera. The results will serve as a foundation for further studies aimed to understand gene (and protein) functions and their evolutionary implications in this group of ecologically relevant beetles.

Chromosomal evolution in tortricid moths: conserved karyotypes with diverged features

Moths of the family Tortricidae constitute one of the major microlepidopteran groups in terms of species richness and economic importance. Yet, despite their overall significance, our knowledge of their genome organization is very limited. In order to understand karyotype evolution in the family Tortricidae, we performed detailed cytogenetic analysis of Grapholita molesta, G. funebrana, Lobesia botrana, and Eupoecilia ambiguella, representatives of two main tortricid subfamilies, Olethreutinae and Tortricinae. Besides standard cytogenetic methods, we used fluorescence in situ hybridization for mapping of major rRNA and histone gene clusters and comparative genomic hybridization to determine the level of molecular differentiation of the W and Z sex chromosomes. Our results in combination with available data in the codling moth, Cydia pomonella, and other tortricids allow us a comprehensive reconstruction of chromosomal evolution across the family Tortricidae. The emerging picture is that the karyotype of a common ancestor of Tortricinae and Olethreutinae differentiated from the ancestral lepidopteran chromosome print of n = 31 by a sex chromosome-autosome fusion. This rearrangement resulted in a large neo-sex chromosome pair and a karyotype with n = 30 conserved in most Tortricinae species, which was further reduced to n = 28 observed in Olethreutinae. Comparison of the tortricid neo-W chromosomes showed differences in their structure and composition presumably reflecting stochasticity of molecular degeneration of the autosomal part of the neo-W chromosome. Our analysis also revealed conservative pattern of the histone distribution, which is in contrast with high rDNA mobility. Despite the dynamic evolution of rDNA, we can infer a single NOR-chromosome pair as an ancestral state not only in tortricids but probably in all Lepidoptera. The results greatly expand our knowledge of the genome architecture in tortricids, but also contribute to the understanding of chromosomal evolution in Lepidoptera in general.

M. Serrano A, Serrano J, Galián J. Patterns of rDNA chromosomal localization in Palearctic Cephalota and Cylindera (Coleoptera: Carabidae: Cicindelini) with different numbers of X-chromosomes

The ribosomal clusters of six Paleartic taxa belonging to the tiger beetle genera Cephalota Dokhtourow, 1883and Cylindera Westwood, 1831, with multiple sex chromosomes (XXY, XXXY and XXXXY) have been localised on mitotic and meiotic cells by fluorescence in situ hybridization (FISH), using a PCR-amplified 18S rDNA fragment as a probe. Four patterns of rDNA localization in these tiger beetles were found: 1. Two clusters located in one autosomal pair; 2. Two clusters located in one autosomal pair and one in an X chromosome; 3. Three clusters located in three heterosomes (XXY); 4. Two clusters located in one autosomal pair and two in the heterosomes (one of the Xs and the Y). These results illustrate that ribosomal cistrons have changed their number and localization during the evolution of these genera, showing a dynamic rather than a conservative pattern. These changes in rDNA localization are uncoupled with changes in the number of autosomes and/or heterosomes. A mechanism that involves transposable elements that carry ribosomal cistrons appears to be the most plausible explanation for these dynamics that involve jumping from one location in the genome to another, in some cases leaving copies in the original location.