Chromosome puzzle in the southernmost populations of the medically important scorpion Tityus bahiensis (Perty 1833) (Buthidae), a polymorphic species with striking structural rearrangements

Chromosome diversity in Buthidae and Chactidae scorpions from Brazilian fauna: Diploid number and distribution of repetitive DNA sequences

In this work, we analyzed cytogenetically eight Chactidae and Buthidae, including the localization of repetitive DNA sequences. The chactids possess monocentric chromosomes and the highest diploid numbers (2n=50 in Brotheas amazonicus, 2n=36 in Chactopsis amazonica, 2n=30 in Neochactas sp.) when compared with buthids (2n=10 in Tityus bahiensis, 2n=14 in Tityus apiacas and Tityus metuendus, 2n=18 in Tityus aba, 2n=26 in Ischnotelson peruassu). The localization of rDNA genes and (TTAGG)n sequences exhibited a conserved pattern of two terminal/subterminal ribosomal cistrons and terminal telomere signals. However, the comparison between the data of C-banding, DAPI after FISH and Cot-DNA fraction indicated a variable quantity and distribution of these regions, as follow: (i) positive heterochromatin and Cot-DNA signals (B. amazonicus and I. peruassu), (ii) small blocks of heterochromatin with large Cot-DNA signals (T. metuendus), (iii) positive heterochromatic regions and absence of Cot-DNA signals (T. aba and T. apiacas), and (iv) negative heterochromatin and Cot-DNA signals (T. bahiensis). Therefore, our results revealed that there still is not a clear relation between quantity of heterochromatin and presence of monocentric or holocentric chromosomes and occurrence of chromosomal rearrangements, indicating that repetitive regions in scorpions must be analyzed using different cytogenetic approaches.

High Chromosomal Reorganization and Presence of Microchromosomes in Chactidae Scorpions from the Brazilian Amazon

Scorpions are of particular interest in cytogenomic studies, as they can present a high incidence of chromosomal rearrangements heterozygous in natural populations. In this study, we cytogenetically analyzed four species of Chactidae. In Brotheas, 2n = 40 was observed in Brotheas silvestris, 2n = 48 in Brotheas paraensis, and 2n = 50 (cytotype A) or 2n = 52 (cytotype B) among populations of Brotheas amazonicus. Our results showed a bimodal karyotype in Neochactas parvulus, 2n = 54, with microchromosomes and a concentration of constitutive heterochromatin in macrochromosomes. The 45S rDNA is located in only one pair of the karyotype, with different heteromorphisms of clusters of this rDNA in the cytotype B of B. amazonicus, with NOR-bearing chromosomes involved in multi-chromosomal associations during meiosis I. The U2 snDNA was mapped in the interstitial region of distinct karyotype pairs of three Chactidae species. Our results indicate the possible formation of cryptic species in B. amazonicus; the different 45S rDNA configurations in the genome of this species may result from amplification and degeneration. We suggest that the bimodal karyotype in N. parvulus results from fusion/fission events and that the unequal distribution of repetitive DNAs between macro and microchromosomes contributes to the maintenance of its asymmetry.

Kinetic Activity of Chromosomes and Expression of Recombination Genes in Achiasmatic Meiosis of Tityus (Archaeotityus) Scorpions

Several species of Tityus (Scorpiones, Buthidae) present multi-chromosomal meiotic associations and failures in the synaptic process, originated from reciprocal translocations. Holocentric chromosomes and achiasmatic meiosis in males are present in all members of this genus. In the present study, we investigated synapse dynamics, transcriptional silencing by γH2AX, and meiotic microtubule association in bivalents and a quadrivalent of the scorpion Tityus maranhensis. Additionally, we performed RT-PCR to verify the expression of mismatch repair enzymes involved in crossing-over formation in Tityus silvestris gonads. The quadrivalent association in T. maranhensis showed delay in the synaptic process and long asynaptic regions during pachytene. In this species, γH2AX was recorded only at the chromosome ends during early stages of prophase I; in metaphase I, bivalents and quadrivalents of T. maranhensis exhibited binding to microtubules along their entire length, while in metaphase II/anaphase II transition, spindle fibers interacted only with telomeric regions. Regarding T. silvestris, genes involved in the recombination process were transcribed in ovaries, testes and embryos, without significant difference between these tissues. The expression of these genes during T. silvestris achiasmatic meiosis is discussed in the present study. The absence of meiotic inactivation by γH2AX and holo/telokinetic behavior of the chromosomes are important factors for the maintenance of the quadrivalent in T. maranhensis and the normal continuation of the meiotic cycle in this species.

Tracking the trends of karyotype differentiation in the phylogenetic context of Gint, a scorpion genus endemic to the Horn of Africa (Scorpiones: Buthidae)

To determine the mechanisms of karyotype differentiation in scorpions of the genus Gint, we employed an integrative approach, combining cytogenetic data and sequence-based phylogeny. We cytogenetically examined six species with emphasis on multivalent meiotic configurations, 18S rDNA and (TTAGG)n distribution and compared chromosomal data with genetic divergence based on analysis of 16S rRNA and COI gene markers. Our results show that Gint species exhibit substantial karyotype diversity (2n = 18–45) and a high incidence of chromosome heterozygosity. Meiotic chromosome chains formed by up to six elements were found in 85% of analysed individuals, causing intraspecific chromosome variation in three species. Fluorescence in situ hybridization revealed that the 18S rDNA distribution pattern differed in Gint species, including at the intrapopulation level, but the chromosomal localization of (TTAGG)n motif was stable across species. Conspicuous interspecific differences in chromosome counts broadly corresponded with genetic divergence among Gint species. Our findings indicate that Gint karyotypes have undergone dynamic reorganization through independent fusions, fissions and reciprocal translocations. Owing to present chromosomal polymorphism, such structural changes shaping the genome architecture appear to be still ongoing in the populations of some Gint species.

Insights into the Karyotype Evolution of Charinidae, the Early-Diverging Clade of Whip Spiders (Arachnida: Amblypygi)

Whip spiders (Amblypygi) represent an ancient order of tetrapulmonate arachnids with a low diversity. Their cytogenetic data are confined to only a few reports. Here, we analyzed the family Charinidae, a lineage almost at the base of the amblypygids, providing an insight into the ancestral traits and basic trajectories of amblypygid karyotype evolution. We performed Giemsa staining, selected banding techniques, and detected 18S ribosomal DNA and telomeric repeats by fluorescence in situ hybridization in four Charinus and five Sarax species. Both genera exhibit a wide range of diploid chromosome numbers (2n = 42-76 and 22-74 for Charinus and Sarax, respectively). The 2n reduction was accompanied by an increase of proportion of biarmed elements. We further revealed a single NOR site (probably an ancestral condition for charinids), the presence of a (TTAGG)n telomeric motif localized mostly at the chromosome ends, and an absence of heteromorphic sex chromosomes. Our data collectively suggest a high pace of karyotype repatterning in amblypygids, with probably a high ancestral 2n and its subsequent gradual reduction by fusions, and the action of pericentric inversions, similarly to what has been proposed for neoamblypygids. The possible contribution of fissions to charinid karyotype repatterning, however, cannot be fully ruled out.