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

Abstract

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 C ₀ t-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.

Keywords: Holocentric chromosomes; Multivalent association; Repetitive DNA; Scorpiones; Tityus.

Fig. 1

Karyotypes of Tityus obscurus with giemsa stain (a, c, e, g, i, k) and C-banded (b, d, f, h, j, l, n): (a, b) cytotype A, 2n = 16. c, d cytotype B, 2n = 14. e, f cytotype C, 2n = 13. g, h cytotype D, 2n = 13. i, j cytotype E, 2n = 12. k, l cytotype F, 2n = 12. m, n cytotype G, 2n = 11. Barr = 10 μm

Fig. 2

Meiotic configuration on different specimens of T. obscurus. a Cytotype B, with seven bivalents. b Cytotype C and D, with one trivalent and five bivalents. c Cytotype E, with six bivalents. d Cytotype F with three bivalents and two trivalent. e Cytotype G with four bivalents and one trivalent. f Cytotype A with eight bivalents. g Cytotype A with six bivalents and one quadrivalent. h Cytotype A with quadrivalent at beginning of pachytene (arrows points the asynaptic region). i Synaptonemal complex analysis in an early pachytene of cytotype A with quadrivalent; the arrow points the asynaptic region of the quadrivalent. j An schematic interpretation of Fig. 2I. k Cytotype A with quadrivalent at end of pachytene; note the full pairing of the quadrivalente. l Metaphase II, with eight chromosomes. Arrows in (b) (d) and (e) point trivalent associations. Barr = 10 μm

Fig. 3

FISH with TTAGG (a-e) and 45S rDNA (f-j) probes in mitotic (a, c, e, h, i, j) and meiotic (b, d, f, g) chromosomes. a, f cytotype A. b, g Post-pachytene in cytotype A, a translocation bearer. c, d, h Cytotype F; the insert in (c) shows a nucleus with telomeres polarized to a single region; in (d) pachytene on the same sample in 3C. e Cytotype C (f) Cytotype E. The arrows in (f-j) point ribosomal sites. Barr = 10 μm

Fig. 4

Heterochromatin and multigenic family mapping in 2n = 16 specimens. a Cot-1 DNA. The insert in (a) shows a meiotic cell in pachytene. b DAPI staining. c CMA₃ staining. d FISH H3 histone probe showing terminal hybridization. e U2 snRNA gene maping in a 2n = 16 specimen, a translocation bearer. f U2 snRNA gene in a 2n =16 specimen with eight bivalents. Barr = 10 μm

Fig. 5

Mariner mapping in T. obscurus. a-f 2n = 16, a translocation bearer. a Mitotic metaphase. b Post-pachytene. c and d Post-pachytene after FISH and C-banding, with Mariner distribution in the constitutive heterochromatin of quadrivalent. e and f Interphase nucleus with packed distribution of Mariner. g Metaphase I of 2n = 16 specimen with eight bivalents. h 2n = 12, a translocation bearer. i 2n = 13, a translocation bearer. Barr = 10 μm

Fig. 6

Schematic interpretation of karyotype evolution hypothesis proposed to specimens of Tityus obscurus from Santarém, Brazil

Fig. 7

Collection places of individuals of T. obscurus and geographic distribution of the cytotypes: Santarém (white circle); Belém (grey triangle); Benevides (white triangle); Santa Bárbara (black triangle); Áfua (black square); Moju (white square); Acará (grey circle); Bragança (black circle); Rurópolis (grey square). The letters adjacent to the karyotypes represent their cytotypes. Note the greater diversity of karyotypes in Santarém (putative distribution center for the species). The arrows next Santarém indicate the dispersion of these cytotypes to marginal regions. This map is the work of Bruno Rafael Ribeiro de Almeida and used with permission