B chromosome in the beetle Coprophanaeus cyanescens (Scarabaeidae): emphasis in the organization of repetitive DNA sequences

Comprehensive mapping of transposable elements reveals distinct patterns of element accumulation on chromosomes of wild beetles

Over the past decades, transposable elements (TEs) have been shown to play important roles shaping genome architecture and as major promoters of genetic diversification and evolution of species. Likewise, TE accumulation is tightly linked to heterochromatinization and centromeric dynamics, which can ultimately contribute to speciation. Despite growing efforts to characterize the repeat landscape of species, few studies have focused on mapping the accumulation profiles of TEs on chromosomes. The few studies on repeat accumulation profiles in populations are biased towards model organisms and inbred lineages. Here, we present a cytomolecular analysis of six mobilome-extracted elements on multiple individuals from a population of a species of wild-captured beetle, Dichotomius schiffleri, aiming to investigate patterns of TE accumulation and uncover possible trends of their chromosomal distribution. Compiling TE distribution data from several individuals allowed us to make generalizations regarding variation of TEs at the gross chromosome level unlikely to have been achieved using a single individual, or even from a whole-genome assembly. We found that (1) transposable elements have differential accumulation profiles on D. schiffleri chromosomes and (2) specific chromosomes have their own TE accumulation landscape. The remarkable variability of their genomic distribution suggests that TEs are likely candidates to contribute to the evolution of heterochromatin architecture and promote high genetic variability in species that otherwise display conserved karyotypes. Therefore, this variation likely contributed to genome evolution and species diversification in Dichotomius.

Karyotype diversity and chromosomal organization of repetitive DNA in Tityus obscurus (Scorpiones, Buthidae)

Background

Holocentric chromosomes occur in approximately 750 species of eukaryotes. Among them, the genus Tityus (Scorpiones, Buthidae) has a labile karyotype that shows complex multivalent associations during male meiosis. Thus, taking advantage of the excellent model provided by the Buthidae scorpions, here we analyzed the chromosomal distribution of several repetitive DNA classes on the holocentric chromosomes of different populations of the species Tityus obscurus Gervais, 1843, highlighting their involvement in the karyotypic differences found among them.

Results

This species shows inter- and intrapopulational karyotype variation, with seven distinct cytotypes: A (2n = 16), B (2n = 14), C (2n = 13), D (2n = 13), E (2n = 12), F (2n = 12) and G (2n = 11). Furthermore, exhibits achiasmatic male meiosis and lacks heteromorphic sex chromosomes. Trivalent and quadrivalent meiotic associations were found in some cytotypes. In them, 45S rDNAs were found in the terminal portions of two pairs, while TTAGG repeats were found only at the end of the chromosomes. In the cytotype A (2n = 16), the U2 snRNA gene mapped to pair 1, while the H3 histone cluster and C₀t-1 DNA fraction was terminally distributed on all pairs. Mariner transposons were found throughout the chromosomes, with the exception of one individual of cytotype A (2n = 16), in which it was concentrated in heterochromatic regions.

Conclusions

Chromosomal variability found in T. obscurus are due to rearrangements of the type fusion/fission and reciprocal translocations in heterozygous. These karyotype differences follow a geographical pattern and may be contributing to reproductive isolation between populations analyzed. Our results also demonstrate high mobility of histone H3 genes. In contrast, other multigene families (45S rDNA and U2 snRNA) have conserved distribution among individuals. The accumulation of repetitive sequences in distal regions of T. obscurus chromosomes, suggests that end of chromosome are not covered by the kinetochore.

Possible origin of B chromosome in Dichotomius sericeus (Coleoptera)

B chromosomes have so far been described in about 80 species of Coleoptera, mainly using conventional staining analysis. In this study, 152 individuals of the dung beetle Dichotomius sericeus (Coleoptera), collected from three isolated geographical areas in the State of Pernambuco, Brazil, were analyzed to determine the frequency, prevalence, distribution, meiotic behavior, and possible B chromosome origin. The cytogenetic analysis consisted of conventional staining, C-banding, triple fluorochrome staining (CMA3/DA/DAPI), and fluorescent in situ hybridization using ribosomal DNAs (rDNAs) and H3 histone gene as probes, as well as microdissection and chromosome painting of the B chromosome. The B chromosomes were detected in all populations analyzed. Analysis revealed the heterochromatic nature and the presence of G+C-rich blocks and 18S rDNA on the B chromosome. FISH with DNA from microdissected B chromosome painted the entire extension of the B chromosome for all populations, besides the pericentromeric regions of all the autosomes, as well as the X chromosome. Finally, cross-hybridization in nine related species of Dichotomius using the microdissected B chromosome as probe did not reveal any hybridization signal. The results suggest an intraspecific and monophyletic origin for B chromosomes in D. sericeus, probably from the second or third autosomal pair.

Chromosomal organization and evolutionary history of Mariner transposable elements in Scarabaeinae coleopterans

Background

With the aim to increase the knowledge on the evolution of coleopteran genomes, we investigated through cytogenetics and nucleotide sequence analysis Mariner transposons in three Scarabaeinae species (Coprophanaeus cyanescens, C. ensifer and Diabroctis mimas).

Results

The cytogenetic mapping revealed an accumulation of Mariner transposon in the pericentromeric repetitive regions characterized as rich in heterochromatin and C₀t-1 DNA fraction (DNA enriched with high and moderately repeated sequences). Nucleotide sequence analysis of Mariner revealed the presence of two major groups of Mariner copies in the three investigated coleoptera species.

Conclusions

The Mariner is accumulated in the centromeric area of the coleopteran chromosomes probably as a consequence of the absence of recombination in the heterochromatic regions. Our analysis detected high diversification of Mariner sequences during the evolutionary history of the group. Furthermore, comparisons between the coleopterans sequences with other insects and mammals, suggest that the horizontal transfer (HT) could have acted in the spreading of the Mariner in diverse non-related animal groups.