The satellite DNA AflaSAT-1 in the A and B chromosomes of the grasshopper Abracris flavolineata

Abstract

Background: Satellite DNAs (satDNAs) are organized in repetitions directly contiguous to one another, forming long arrays and composing a large portion of eukaryote genomes. These sequences evolve according to the concerted evolution model, and homogenization of repeats is observed at the intragenomic level. Satellite DNAs are the primary component of heterochromatin, located primarily in centromeres and telomeres. Moreover, satDNA enrichment in specific chromosomes has been observed, such as in B chromosomes, that can provide clues about composition, origin and evolution of this chromosome. In this study, we isolated and characterized a satDNA in A and B chromosomes of Abracris flavolineata by integrating cytogenetic, molecular and genomics approaches at intra- and inter-population levels, with the aim to understand the evolution of satDNA and composition of B chromosomes.

Results: AflaSAT-1 satDNA was shared with other species and in A. flavolineata, was associated with another satDNA, AflaSAT-2. Chromosomal mapping revealed centromeric blocks variable in size in almost all chromosomes (except pair 11) of A complement for both satDNAs, whereas for B chromosome, only a small centromeric signal occurred. In distinct populations, variable number of AflaSAT-1 chromosomal sites correlated with variability in copy number. Instead of such variability, low sequence diversity was observed in A complement, but monomers from B chromosome were more variable, presenting also exclusive mutations. AflaSAT-1 was transcribed in five tissues of adults in distinct life cycle phases.

Conclusions: The sharing of AflaSAT-1 with other species is consistent with the library hypothesis and indicates common origin in a common ancestor; however, AflaSAT-1 was highly amplified in the genome of A. flavolineata. At the population level, homogenization of repeats in distinct populations was documented, but dynamic expansion or elimination of repeats was also observed. Concerning the B chromosome, our data provided new information on the composition in A. flavolineata. Together with previous results, the sequences of heterochromatic nature were not likely highly amplified in the entire B chromosome. Finally, the constitutive transcriptional activity suggests a possible unknown functional role, which should be further investigated.

Keywords: B chromosome; Repetitive DNA; Tandem repeat; Transcription.

Fig. 1

Agarose gel electrophoresis (a) of integrated genomic DNA (1) and the same sample digested with HindII (2); (b) result of amplification of AflaSAT-1; (c) sequence of an entire monomeric unit of AflaSAT-1. In (a, b), L corresponds to 1 Kb plus marker, and asterisks indicate the monomer, dimer, trimer and tetramer. The arrows in (c) show the internal primers

Fig. 2

Haplotype network for the 11 haplotypes and their relationships from sequences of AflaSAT-1 obtained from different populations. Black dots correspond to substitutions, and the haplotype circle diameter corresponds to abundance. The different populations are represented in different colors

Fig. 3

FISH in mitotic embryo chromosomes (a-f) and in distended chromatin fiber (g) of AflaSAT-1 and AflaSAT-2 satDNAs in individuals without (a-d, g) and with one B chromosome (e, f). Arrows in (c, e, f) show the pair 11 and the arrowhead in (e, f) the B chromosome. In (e, f), the B chromosome is highlighted in an inset. Each probe is indicated directly in the panel. Bar = 5 μm

Fig. 4

Chromosomal location of AflaSAT-1 in three different populations of A. flavolineata. (a) Sta Bárbara do Pará/PA, (b) Cabo/PE and (c) Posadas/AR. The X chromosome is indicated, and the arrows point to the pair 11. In (b), the largest chromosome, chromosome 1, is also indicated

Fig. 5

Relative quantification of AflaSAT-1 from different populations of A. flavolineata in comparison with Rio Claro/SP 0B male individuals. (1) Rio Claro/SP male 0B, (2) Rio Claro/SP male 1B, (3) Rio Claro/SP female 0B, (4) Cabo/PE male 0B, (5) Sta Bárbara do Pará/PA male 0B, (6) Posadas/AR male 0B

Fig. 6

RT-PCR electrophoresis of AflaSAT-1 using as template cDNA obtained from different tissues, nymphs and embryos. he = head, gc = gastric caecum, tes = testis, ov = ovariole, leg = saltatory leg, nym = nymph, emb = embryo, C- = negative control