The B-chromosomes of two species ofCylindrotettix (Leptysminae, Acrididae)

Evolution of chromosome number in grasshoppers (Orthoptera: Caelifera: Acrididae)

Orthoptera have some of the largest genomes of all insects. At the same time, the architecture of their genomes remains poorly understood. Comparative cytological data across a wide range of taxa, even for basic parameters such as chromosome number, may provide important insights into the evolution of these genomes and help answer the question of why some species attained such large sizes. We collected and compiled more than 1,000 records of chromosome numbers of 339 genera (13.8% of 2,452 known genera) and 769 species (6.2% of 12,250 known species) of Caelifera, the suborder of Orthoptera that includes those taxa with short antennae. Within the family Acrididae, most of the records come from the subfamilies Oedipodinae (N = 325), Melanoplinae (N = 192) and Gomphocerinae (N = 254). Out of the 621 investigated species of Acrididae, 459 (73.9%) shared a chromosome number of 2n♂ = 23. Chromosome numbers of 2n♂ = 17 (12.2%) and 2n♂ = 21 (9.9%) were less common. The remaining 4.0% of species exhibited different chromosome numbers between 2n♂ = 8 (6 + XY) and 2n♂ = 27. Plotted on a phylogenetic tree, our results confirm that chromosome numbers, especially in the largest grasshopper family Acrididae, are highly conserved with a basic count of 2n♂ = 23 (22 + X0), sometimes reduced to, e.g., 2n♂ = 17 (16 + X0) in some genera of the slant-faced grasshopper subfamily Gomphocerinae. Species with divergent chromosome numbers occur in many of the groups we studied, but are not a systematic trait and have evolved multiple times independently. Our study supports the view that chromosome numbers are much more stable across the investigated Caelifera compared to Ensifera, the second suborder of Orthoptera that includes the long antennae bush crickets and crickets. Our results significantly extend our knowledge on the diversity of this character in Caelifera.

Organization of some repetitive DNAs and B chromosomes in the grasshopper Eumastusia koebelei koebelei (Rehn, 1909) (Orthoptera, Acrididae, Leptysminae)

B chromosomes occur in approximately 15% of eukaryotes and are usually heterochromatic and rich in repetitive DNAs. Here we describe characteristics of a B chromosome in the grasshopper Eumastusia koebelei koebelei (Rehn, 1909) through classical cytogenetic methods and mapping of some repetitive DNAs, including multigene families, telomeric repeats and a DNA fraction enriched with repetitive DNAs obtained from DOP-PCR. Eumastusia koebelei koebelei presented 2n=23, X0 and, in one individual, two copies of the same variant of a B chromosome were noticed, which are associated during meiosis. The C-positive blocks were located in the pericentromeric regions of the standard complement and along the entire length of the B chromosomes. Some G+C-rich heterochromatic blocks were noticed, including conspicuous blocks in the B chromosomes. The mapping of 18S rDNA and U2 snDNA revealed only autosomal clusters, and the telomeric probe hybridized in terminal regions. Finally, the DOP-PCR probe obtained from an individual without a B chromosome revealed signals in the heterochromatic regions, including the entire length of the B chromosome. The possible intraspecific origin of the B chromosomes, due to the shared pool of repetitive DNAs between the A and B chromosomes and the possible consequences of their association are discussed.

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

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.

The dicentric nature of the metacentric B chromosome of Metaleptea brevicornis adspersa (Acridinae, acrididae)

The meiotic behaviour of a large B isochromosome of Metaleptea brevicornis adspersa (Acridinae, Acrididae) was studied in both sexes using conventional preparations and in males by silver staining of surface-spread synaptonemal complexes and kinetochores. In males, both arms of the B chromosome synapse at zygotene-early pachytene suggesting its isochromosomal nature. Both arms separate precociously at late pachytene and remain joined only through the formation of a single distal chiasma in 92.5% and 91.7% of male and female meiocytes, respectively. In both sexes, the B chromosome tends to migrate precociously to one of the spindle poles, auto-orientating at the equator in 19.2% and 16.6% of metaphase I cells of males and females, respectively. In males, this produces a high frequency of anaphase I/telophase I lagging with the consequent inhibition of cytokinesis and the formation of second division restitution nuclei. For a better understanding of the synaptic and segregation behaviour of the B chromosome in males, pachytene synaptonemal complex (SC) formation was studied. In 0B males, pachytene nuclei invariably showed 11 fully paired SCs, each one with a densely stained pair of terminal kinetochores, and a single axis with a single kinetochore corresponding to the X univalent. In 1B males, both arms of the B chromosome appeared fully paired in 93. 8% of the nuclei, showing complete asynapsis in the rest. This frequency is almost exactly the same as distal chiasma formation in diplotene. The centric region of the SC of the B chromosome had two kinetochores connected by a single axial filament of variable length. The dicentric nature of the B was confirmed by silver staining of kinetochores at metaphase I and anaphase II stages. This is the first time that a dicentric B isochromosome has been described. Based on these results, two alternative models for the origin of isochromosomes are postulated.