Undertransmission of a supernumerary chromosome segment through heterozygous females possessing B chromosomes in the grasshopper Eyprepocnemis plorans

The transmission ratio (ks) for a supernumerary chromosome segment was studied in a total of 54 heterozygous females collected from two Spanish natural populations of the grasshopper Eyprepocnemis plorans. Our analysis clearly demonstrated that ks is negatively dependent on the number of B chromosomes in the female. The possible mechanisms by which B chromosomes may cause undertransmission of the supernumerary segment, and the implications of this phenomenon for the maintenance of this extra chromosome segment, are discussed.Key words: supernumerary segments, B chromosomes, Mendelian transmission, grasshoppers.

G banding in two species of grasshopper and its relationship to C, N, and fluorescence banding techniques

A G banding technique combining trypsin and hot saline treatments was used to analyze the chromosomes of two grasshopper species, Eyprepocnemis plorans and Locusta migratoria, both of which contain both standard and supernumerary heterochromatin. Although this technique does not produce G bands like those in mammalian chromosomes, it serves to characterize heterochromatic regions whose nature has been inferred from other banding techniques (C, N, CMA, and DAPI banding). The light regions revealed by G banding contain GC-rich DNA sequences, the more prominent of which coincide with nucleolus organizer regions (NORs). Furthermore, the proximal heterochromatin in E. plorans was heterogeneous, and the standard and supernumerary heterochromatin showed conspicuous differences in organization. Supernumerary heterochromatin is an exception to the regular patterns shown by the standard heterochromatin. The findings are related to the mechanism of action of these banding techniques.Key words: banding techniques, grasshoppers, Eyprepocnemis plorans, Locusta migratoria.

Distributive pairing in the grasshopper Chorthippus binotatus

Six males of the grasshopper Chorthippus binotatus were mosaic for the presence of extra (E) chromosomes in the germ line, but lacked them in the somatic cells of gastric caeca. E chromosomes were very similar to the X chromosome in size and meiotic properties (heteropycnosis and autopairing). X and E chromosomes associated frequently at diplotene, but the associations never persisted until metaphase I, which indicated that they were not chiasmate. When one E was present, X and E univalents segregated preferentially to opposite poles. In cells with two E, they formed a bivalent in almost all cells, and decreased the frequency of X–E associations by 20%. These cells showed a high frequency of nondisjunction between the two E chromosomes, such that they segregated independently despite the high persistence of their association at metaphase I. These results are interpreted and discussed in the light of the distributive pairing model.Key words: distributive pairing, X chromosome, E chromosome, Chorthippus, male meiosis.

Male germ line polysomy in the grasshopper Chorthippus binotatus: extra chromosomes are not transmitted

Some males of the grasshopper Chorthippus binotatus from a natural population in Sierra Nevada (Granada, Spain) were found to be polysomic mosaics for the presence of extra chromosomes (E) in a high proportion of testicular follicles. Transmission analysis of these chromosomes was performed in 21 controlled crosses, 2 of which involved a polysomic parent. While most spermatozoa produced by polysomic males carried E chromosomes, these chromosomes were not transmitted to the progeny, since all 22 embryos descended from a polysomic male parent lacked them.Key words: transmission, polysomy, grasshopper, Chorthippus binotatus.

Relationship between mitotic instability and accumulation of B chromosomes in males and females of Locusta migratoria

Mitotic instability of B chromosomes has been studied in males and females from two Spanish populations of Locusta migratoria. This instability is a common feature in embryos, adult somatic tissues such as gastric caeca, and ovariole wall cells and male and female germ lines. While B chromosomes accumulate in the male germ line independently of the existence or absence of intraindividual variation in the number of B chromosomes in gastric caeca cells, they do not accumulate in the female germ line despite meiotic instability. The relationship between the mitotic instability of the B chromosomes in germ lines and their mode of transmission is discussed.Key words: Locusta migratoria, B chromosomes, mitotic instability, accumulation.

Effects of B chromosomes on the activity of nucleolar organizer regions in the grasshopper Eyprepocnemis plorans: activation of a latent nucleolar organizer region on a B chromosome fused to an autosome

Four nucleolar organizer regions (NORs) are active in standard males of the grasshopper Eyprepocnemis plorans. They are located near the centromeric regions of the S9, S10, S11, and X chromosomes. Changes in the pattern of NOR activity have been observed in the presence of a B2 type supernumerary chromosome. Males with one B show a higher mean number of active NORs per cell than do zero B males owing to significant increases in the activity of the NORs on the S11 and the X. Zero B and one B embryos, however, showed similar patterns of activity. In a male carrying a centric fusion between a B and one of the L1 chromosomes, the activation of a latent NOR, present at the telomere of the long arm of the B, parallelled a significant decrease of NOR activity on the S9 and S10 bivalents stemming from a competition between different NORs in the presence of the B. Thus, while in zero B males the activity of the S10 NOR influences that of the NORs on the X and S9 in a negative way, in one B males it does not do so, although such an influence is observed in the B–L1 fusion male where the activity of the S10 NOR again decreases significantly. On the other hand, the activities of the NORs on the S9 and S11 show a significant positive interdependence in both zero B and one B males where S11 NOR activity is increased but do not do so in the B–L1 fusion male, which shows a significant decrease in the S9 NOR activity.

Key words: Eyprepocnemis plorans, B chromosome, nucleolus.

A test for equilocality of supernumerary heterochromatin distribution in grasshoppers

The presence of extra segments in four or more nonhomologous chromosome pairs led us to test the applicability of the principle of the principle of equilocality of heterochromatin distribution to supernumerary heterochromatin. In Chorthippus brunneus, which carries extra segments on all chromosomes except the L3, the 13 heterochromatic variants occupy all possible locations with 3 proximal, 5 interstitial, and 5 distal. This indicates that equilocality does not seem to apply to the supernumerary heterochromatin in this species. On the other hand, the four extra segments found in Chorthippus parallelus and five of the seven extra heterochromatic segments found in Chorthippus binotatus are all located distally on the M5–S8 chromosomes. Thus for these groups of chromosomes the extra heterochromatic segments do appear to show a tendency for an equilocal distribution.

Key words: Chorthippus, heterochromatin.

Meiotic behaviour of the holocentric chromosomes of Nezara viridula (Insecta, Heteroptera) analysed by C-banding and silver impregnation

While silver impregnation reveals the presence of kinetochores in monocentric chromosomes, it does not do so in the holocentric system of Nezara viridula. Here, C-banding and silver impregnation techniques reveal that C-heterochromatin is present in the greater part of the Y chromosome and at the nucleolus organizer region (NOR) of the largest autosome (A 1) and in the extra NOR located in the X chromosome of a single exceptional male. Furthermore, one telomere of each autosome appeared lightly C-banded. The largest, A1, bivalent shows chiasmata almost always located at the chromosome ends. This bivalent may orient axially or equatorially in metaphase I cells depending on whether it carries a single chiasma or two chiasmata, respectively. From our cytological analysis we deduce that centromeric activity is preferentially located at the two telomeric ends and that the presence of chiasmata at an end excludes such activity.Key words: diffuse centromere, C-banding, holocentric, insect chromosomes.