Scorpion Chromosomes Reveal Record-Low Karyotypes Hidden in Brazil’s Caatinga

Deep in the semi-arid backlands of northeastern Brazil, two species of scorpions are quietly rewriting what scientists thought they knew about chromosome evolution. A new study published in the journal Comparative Cytogenetics has analyzed, for the first time, multiple wild populations of two scorpion species in the genus Bothriurus, uncovering diploid chromosome numbers so low that they set a new record for the entire family Bothriuridae. The findings, drawn from eleven populations across five Brazilian states, offer a rare population-level window into how genomes reorganize themselves over evolutionary time.

Scorpions occupy a special place in arachnid cytogenetics. They are the third most extensively studied order of arachnids from a chromosomal perspective, and their male meiosis is unusual: it is achiasmatic, meaning that the physical crossovers familiar from many other animals are absent. Correct chromosome segregation is instead guaranteed by the persistence of the synaptonemal complex, the protein scaffold that holds paired chromosomes together, until late stages of the first meiotic division. With the exception of the family Buthidae, whose members carry holocentric chromosomes with diffuse centromere activity, all other cytogenetically characterized scorpion families, including Bothriuridae, possess monocentric chromosomes with a single, clearly defined centromere.

Despite this foundation of knowledge, the data remain strikingly sparse. The family Bothriuridae comprises 168 described species, yet fewer than 6 percent have had their chromosomes examined, and only three genera have been studied at all. Within Brazil, the genus Bothriurus is the most diverse of the family, with 17 described species, but only five had been cytogenetically characterized before this work. The new study, led by researchers working with specimens deposited at the Federal University of Piauí, focused on two species from northeastern Brazil: Bothriurus asper and Bothriurus rochai, each the namesake of a species group still beset by taxonomic uncertainty.

The sampling effort was unusually broad for scorpion cytogenetics. Two populations of B. asper were analyzed, one from Teresina in Piauí and one from Igarassu in Pernambuco, the species’ type locality. Nine populations of B. rochai were sampled across Ceará, Paraíba, Pernambuco, Piauí, and Rio Grande do Norte, spanning both the Caatinga dry forest and, in the case of Igarassu, the Atlantic Forest. Gonadal tissue from adult specimens was prepared on slides and stained with Giemsa to reveal chromosome counts, meiotic configurations, and chromosome morphology, following established protocols for scorpion cytogenetic preparations.

The results for B. asper were remarkably consistent. Both geographically distant populations, separated by hundreds of kilometers and different biomes, showed a diploid number of 2n = 30, a strong indication that this is the fixed karyotype for the species. But the meiotic cells told a subtler story. In the Igarassu specimen, most post-pachytene nuclei displayed thirteen bivalents plus a chromosomal chain of four elements arranged in a cross-shaped configuration, while a minority showed fifteen simple side-by-side bivalents. In the Teresina specimens, every cell showed the four-element chain, but in an open configuration rather than a closed ring. Such multivalent associations are the classic signature of reciprocal chromosomal translocations, rearrangements in which segments are exchanged between non-homologous chromosomes, and the open configuration in the Teresina population suggests a second, non-reciprocal translocation that moved a fragment from the end of one chromosome into an interstitial position on another.

B. rochai proved far more volatile. Its populations split cleanly into two karyotypic forms: individuals from Brasileira, Floriano, São Raimundo Nonato, Icó, Quixadá, and Maturéia carried 2n = 18, while those from Apodi, Cajazeiras, and João Câmara carried 2n = 16. These are the lowest diploid numbers ever reported for the family Bothriuridae, a group whose previously characterized species range from 2n = 28 up to 2n = 50. The meiotic behavior varied accordingly. Some populations showed only simple bivalents, while others, notably João Câmara and Cajazeiras, consistently displayed five large bivalents plus a chain of six chromosomes, again pointing to reciprocal translocations layered on top of whatever fission or fusion events changed the chromosome count.

To probe the physical consequences of these rearrangements, the researchers measured the diploid set length, the combined length of all chromosomes in a cell, using image analysis software and compared the values statistically with a generalized linear model. The differences were significant. Cells from individuals with 2n = 16 and from 2n = 18 individuals carrying the six-chromosome chain had systematically smaller chromosome sets than cells from 2n = 18 individuals with nine bivalents or a four-element chain. Because rearrangements such as fusions and fissions do not by themselves change total chromosome length, this pattern suggests that deletions or duplications of genetic material have accompanied the karyotypic divergence, adding a layer of complexity beyond simple one-step rearrangements. Determining which diploid number is ancestral, the authors note, will require phylogeographic or phylogenetic analyses.

Two molecular markers added further texture to the picture. Silver ion impregnation, which stains active nucleolus organizer regions, the chromosomal sites of the ribosomal RNA genes, revealed a single terminal pair of NORs in every B. rochai population examined, regardless of whether the population carried 16 or 18 chromosomes. This stability is notable: while NOR number and position are conserved in some scorpion lineages, other genera show variation, and a single terminal NOR pair is considered the ancestral condition for arachnids. In contrast, C-banding, which reveals constitutive heterochromatin, painted a more variable landscape. B. asper carried larger heterochromatic blocks than B. rochai, and within B. rochai the amount of heterochromatin differed among populations, with some populations showing no detectable C-banding signals at all, likely because their heterochromatin blocks are too small to resolve.

Perhaps the most intriguing correlation emerged from comparing heterochromatin quantity with meiotic instability. In scorpions, the amount of heterochromatin appears to be inversely related to the frequency of chromosomal rearrangements. Consistent with this trend, the B. rochai population with the least heterochromatin, Brasileira in Piauí, also showed the greatest diversity of post-pachytene configurations, including cells with nine bivalents, cells with a six-chromosome chain, and cells with a four-element chain. At the species level, B. asper, with its larger heterochromatic blocks, showed only a single distinct meiotic configuration. Fluorochrome staining added a final detail: prominent AT-rich regions detected with DAPI in some populations, while GC-rich regions stained uniformly with chromomycin A3, without specific bright signals.

Taken together, the study marks the first time multiple populations of any Bothriurus species have been cytogenetically characterized, and it substantially expands the karyotypic record for scorpions with monocentric chromosomes. The contrast between the stable karyotype of B. asper and the variable one of B. rochai suggests distinct evolutionary trajectories within the genus, with translocations and fission-fusion events playing a central role in reshaping genomes even as some features, like the single NOR pair, remain conserved. For a group whose species boundaries are still being untangled, such chromosomal characters may prove valuable diagnostic tools, and the authors hope the data will feed into broader biodiversity research in one of the world’s most understudied arachnid faunas.

Subject of Research: Comparative cytogenetics of chromosome number, meiotic behavior, heterochromatin, and nucleolus organizer regions in populations of two Bothriurus scorpion species from northeastern Brazil

Article Title: Comparative cytogenetics among populations of two Bothriurus species ( Scorpiones , Bothriuridae )

Article References: Lima, J. F. D., Schneider, M. C., Carvalho, L. S., & Pinto-da-Rocha, R. (2025). Comparative cytogenetics among populations of two Bothriurus species (Scorpiones, Bothriuridae). Comparative Cytogenetics, 19, 189-208. https://doi.org/10.3897/compcytogen.19.165160

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.165160

Keywords: scorpions, Bothriurus, cytogenetics, karyotype, chromosomal rearrangements, heterochromatin, nucleolus organizer regions, meiosis, Bothriuridae, Brazil, Caatinga, chromosome evolution