Abstract
Supernumerary chromosomes (B chromosomes) occur in approximately 15% of eukaryote species. Although these chromosomes have been extensively studied, knowledge concerning their specific molecular composition is lacking in most cases. The accumulation of repetitive DNAs is one remarkable characteristic of B chromosomes, and the occurrence of distinct types of multigene families, satellite DNAs and some transposable elements have been reported. Here, we describe the organization of repetitive DNAs in the A complement and B chromosome system in the grasshopper species Abracris flavolineata using classical cytogenetic techniques and FISH analysis using probes for five multigene families, telomeric repeats and repetitive C0t-1 DNA fractions. The 18S rRNA and H3 histone multigene families are highly variable and well distributed in A. flavolineata chromosomes, which contrasts with the conservation of U snRNA genes and less variable distribution of 5S rDNA sequences. The H3 histone gene was an extensively distributed with clusters occurring in all chromosomes. Repetitive DNAs were concentrated in C-positive regions, including the pericentromeric region and small chromosomal arms, with some occurrence in C-negative regions, but abundance was low in the B chromosome. Finally, the first demonstration of the U2 snRNA gene in B chromosomes in A. flavolineata may shed light on its possible origin. These results provide new information regarding chromosomal variability for repetitive DNAs in grasshoppers and the specific molecular composition of B chromosomes.
Highlights
Repetitive DNAs comprise a large portion of eukaryotic genomes, including tandem arrays and scattered repeats
The ribosomal DNAs, followed by histone genes and to a lesser extent U small nuclear RNA genes, have been mapped cytogenetically, revealing clusters located in one chromosomal loci or dispersed in some chromosomes
Flavolineata is similar to the previous description provided by Cella and Ferreira [41]. These results mirror the common pattern seen in grasshoppers [50], with the exception of the presence of metaor submetacentric chromosomes, which are less common in acridid karyotypes and may be related to pericentric inversions
Summary
Repetitive DNAs comprise a large portion of eukaryotic genomes, including tandem arrays and scattered repeats. The ribosomal DNAs (rDNAs), followed by histone genes and to a lesser extent U small nuclear RNA (snRNA) genes, have been mapped cytogenetically, revealing clusters located in one chromosomal loci or dispersed in some chromosomes (see for example [4,5,6,7,8,9,10,11]). The presence of multigene families in B chromosomes has been reported in some species (see references below)
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