In this study, we investigated the mitotic and meiotic chromosomes of 11 Buthidae scorpion species, belonging to three genera (Ananteris, Rhopalurus and Tityus), to obtain detailed knowledge regarding the mechanisms underlying the intraspecific and/or interspecific diversity of chromosome number and the origin of the complex chromosome associations observed during meiosis. The chromosomes of all species did not exhibit a localised centromere region and presented synaptic and achiasmatic behaviour during meiosis I. Spermatogonial and/or oogonial metaphase cells of these buthids showed diploid numbers range from 2n = 6 to 2n = 28. In most species, multivalent chromosome associations were observed in pachytene and postpachytene nuclei. Moreover, intraspecific variability associated with the presence or absence of chromosome chains and the number of chromosomes in the complex meiotic configurations was observed in some species of these three genera. Silver-impregnated cells revealed that the number and location of nucleolar organiser regions (NORs) remained unchanged despite extensive chromosome variation; notably, two NORs located on the terminal or subterminal chromosome regions were commonly observed for all species. C-banded and fluorochrome-stained cells showed that species with conspicuous blocks of heterochromatin exhibited the lowest rate of chromosomal rearrangement. Based on the investigation of mitotic and meiotic cells, we determined that the intraspecific variability occurred as a consequence of fission/fusion-type chromosomal rearrangements in Ananteris and Tityus species and reciprocal translocation in Rhopalurus species. Furthermore, we verified that individuals presenting the same diploid number differ in structural chromosome organisation, giving rise to intraspecific differences of chromosome association in meiotic cells (bivalent-like elements or chromosome chains).
The study of diversity has become increasingly sophisticated, including the use of measures of phylogenetic diversity.
We calculate the spatial variation in species richness, taxonomic beta diversity, and alpha and beta phylogenetic diversity (PDα and PDβ, respectively) of Atlantic Forest harvestman communities using a data set containing 556 species from 68 sites, distributed in 12 Brazilian states.
We compare the congruence of phylogenetic and taxonomic diversity patterns, and also compare PDα with null model expectations, to check for phylogenetic clustering or overdispersion in communities.
Species richness and PDα are correlated, peaking in southern and south‐eastern coastal sites and decreasing towards the interior and towards the north‐east. PDα in north‐eastern sites was higher than expected, while a clustered phylogenetic pattern characterised most other sites.
Communities in the southern and south‐eastern regions were dominated by species from the large family Gonyleptidae, presenting a high richness and a low PDα. As the dominance of Gonyleptidae decreased towards the north, where local communities have fewer species, but a higher PDα, they contain representatives of other families. The beta diversity was more sensitive to the compositional changes involving closely related Gonyleptidae species, while PDβ is more influenced by deeper phylogenetic compositional changes, between more distant lineages.
Phylogenetic diversity may be of special importance to assess the conservation value of distantly related lineages. These species‐poor groups are less likely to influence taxonomic‐based diversity analyses, but their importance for conservation arises from their phylogenetic distinctiveness, captured by PDα and PDβ measures.
Buthid scorpions exhibit a high variability in diploid number within genera and even within species. Cytogenetically, Buthidae differs from other families of Scorpiones based on its low diploid numbers, holocentric chromosomes, and complex chromosomal chains, which form during meiosis. In this study, we analyzed the distribution of the 45S ribosomal DNA (rDNA) genes in the mitotic and meiotic chromosomes of seven buthid species belonging to the genera Rhopalurus and Tityus with the ultimate goal of elucidating the chromosome organization in these scorpions. The chromosome number ranged from 2n=6 to 2n=28. Despite the high variance in diploid number, all species examined carried their 45S rDNA sites in the terminal region of exactly two chromosomes. Analyses of meiotic cells revealed 45S rDNA clusters in the chromosomal chains of Rhopalurus agamemnon, Tityus bahiensis, Tityus confluens, and Tityus martinpaechi, or in bivalent-like configuration in Rhopalurus rochai, Tityus bahiensis, Tityus confluens, Tityus fasciolatus, and Tityus paraguayensis. In the species examined, the 45S rDNA sites colocalized with constitutive heterochromatin regions. In light of the high chromosome variability and maintenance of number and terminal position of 45S rDNA sites in buthids, the heterochromatin may act to conserve the integrity of the ribosomal genes.
Testicular cells of 4 buthid scorpions, Rhopalurus agamemnon (2n = 28), R. rochai (2n = 28), Tityus bahiensis (2n = 6), and T. fasciolatus (2n = 14), which show different types of chromosomal configurations in meiosis I, were subjected to cellular microspreading in order to (1) obtain knowledge about the organization and behavior of the synaptonemal complex (SC), and (2) acquire data about the mechanisms responsible for inter- and intraindividual chromosomal variation within Buthidae. Ultrastructural analysis of microspread nuclei revealed SCs with a well-preserved structure until late substages of prophase I, but did not detect kinetochore plates and recombination nodules. Pachytene cells of R. agamemnon, R. rochai and T. bahiensis exhibited single and unsynapsed axes continuous with totally synapsed SCs, indicating the occurrence of heterozygous chromosomal rearrangements. Although chromosome chains were not observed in T. fasciolatus, the presence of gaps and interlocks points out that this species also carries heterozygous rearrangements, involving a small chromosome segment. Especially in R. rochai, the cellular microspreading analysis was useful to clarify the origin of inter- and intraindividual variation in the number of bivalent-like elements and in the number of chromosomes involved in multivalent associations. It was found that more chromosomes were involved in rearrangements than previously established through investigations using light microscopy alone.
Scorpions represent an intriguing group of animals characterized by a high incidence of heterozygous chromosomal rearrangements. In this work, we examined six species of Tityus (Archaeotityus) from Brazilian fauna with a particular focus on elucidating the rearrangements responsible for the intraspecific variability of diploid number and the presence of long chromosomal chains in meiosis. To access any interpopulation diversity, we also studied individuals from four species representing distinct localities. Most species demonstrated intraspecific polymorphism in diploid number (2n = 19 and 2n = 20 in T. clathratus, T. mattogrossensis, and T. pusillus, 2n = 16, 2n = 17 and 2n = 18 in T. paraguayensis, and 2n = 16 and 2n = 24 in T. silvestris) and multi-chromosomal associations during meiosis I, which differed even among individuals with the same chromosome number. In some species, the heterozygous rearrangements were not fixed, resulting such as in Tityus clathatrus, in 11 different chromosomal configurations recognized within a same population. Based on meiotic chromosome behaviour, we suggested that independent rearrangements (fusion/fission and reciprocal translocations), occurring in different combinations, originated the multi-chromosomal chains. To evaluate the effects of these chromosome chains on meiotic segregation, we applied the chi-square test in metaphase II cells. The non-significant occurrence of aneuploid nuclei indicated that non-disjunction was negligible in specimens bearing heterozygous rearrangements. Finally, based on our analysis of many chromosome characteristics, e.g., holocentricity, achiasmate meiosis, endopolyploidy, ability to segregate heterosynaptic or unsynapsed chromosomes, ()n sequence located in terminal regions of rearranged chromosomes, we suggest that the maintenance of multi-chromosomal associations may be evolutionarily advantageous for these species.
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