Chromosomal Mapping of Repetitive DNAs in the Grasshopper Abracris flavolineata Reveal Possible Ancestry of the B Chromosome and H3 Histone Spreading

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.

Figure 1. Classical cytogenetic characterization ofAbracris flavolineata chromosomal complement at mitotic metaphase (a,d) and metaphase I (b,c,e–g) cells.

(a–c) conventional staining, (d,e) C-banding and (f,g) CMA₃ fluorochrome staining in cells with one or two B chromosomes. Insets show the B element obtained from another mitotic metaphase (a), and B chromosomes stained with DAPI (f,g). The B and X chromosomes are indicated in all cells, and (f) chromosomes with CMA₃-positive blocks are also indicated. The metaphases (e,g) are partial. Bar = 5 µm.

Figure 2. Cytogenetic mapping of repetitive DNAs in gastric caeca female mitotic cells bearing one B chromosome.

Each probe used is indicated directly in the images using colors. Insets in (f,h) show the B chromosome with a faint centromeric signal for the C₀t-1 DNA probe. Bar = 5 µm.

Figure 3. FISH with the U2 snRNA gene probe in individuals with one (a) and two (b) B chromosomes.

(a) gastric caeca female mitotic metaphase cell, (b) male metaphase I cell, (c,d) selected B chromosomes showing the symmetrical location of the U2 snDNA clusters in the two arms. Note the double signal in each arm of the B elements. (a,b) Bar = 5 µm, (c,d) Bar = 2.5 µm.