J. Cabrero

Location and expression of ribosomal RNA genes in grasshoppers: Abundance of silent and cryptic loci

We investigate regularities and restrictions in chromosome location of ribosomal RNA genes, analysed by fluorescent in situ hybridization (FISH), and their phenotypic expression assessed by nucleolus formation at first meiotic prophase cells, analysed by silver impregnation, in 49 grasshopper species. High variation was found for rDNA location between species within most genera analysed. The mean haploid number of rDNA loci detected by FISH was 2.47, but some species had up to 10 loci. Chromosome distribution of rDNA loci differed between the Gomphocerinae and Oedipodinae subfamilies, most loci being proximal to the centromere in the former and distal to it in the latter. Chromosomes 2, 3 and X frequently carried rDNA in Gomphocerinae species with 2n♂=17 chromosomes, whereas chromosomes 6 and 9 were the most frequent rDNA locations in the Oedipodinae. About 13% of the 126 rDNA loci detected by FISH were silent, although this figure might be even higher. The comparison of FISH and silver-impregnation results also suggested the existence of cryptic NORs, i.e. those forming small nucleoli with no apparent presence of rDNA revealed by FISH. This was especially clear after the same cells in two species were sequentially treated with both silver impregnation and FISH. The abundance of silent and cryptic loci might thus suggest that rDNA spreads through grasshopper genomes by the Dubcovsky and Dvorak mechanism—that is, the transposition of a few rRNA genes to new chromosome locations, their amplification giving rise to new NORs, and the elimination of the old NORs. The cryptic NORs might correspond to nascent NORs, i.e. a few rRNA gene copies moved to new locations, whereas the inactive rDNA loci might correspond to those being in the process of elimination.

Population Dynamics of A Selfish B Chromosome Neutralized by the Standard Genome in the Grasshopper Eyprepocnemis Plorans

Effects of the B chromosome polymorphism of the grasshopper Eyprepocnemis plorans were analyzed in two natural populations. Postmating sexual selection, female fertility, and survival were studied. The B chromosome lacks drive and has no detectable effects on fitness. A neutral B cannot invade a population and establish a polymorphism, but the confidence limits on our estimates cannot exclude the possibility that the polymorphism is maintained by a balance between weak drive and weak selection against individuals with two and three Bs. However, other lines of evidence favor the following model of the dynamics of the B in E. plorans. In a newly invaded population, the B has substantial drive, but the evolution of drive suppressor genes in the A chromosomes neutralizes the B drive so that it becomes near‐neutral and begins a random walk toward extinction by stochastic loss. Because the B is common by the time drive disappears, the random walk is likely to continue for a long time. If in the course of the random walk a variant B with greater drive appears, then it will displace the original variant, and a new cycle of drive suppression and drift to extinction occurs. A simulation model of this process suggested that the mean time to extinction is proportional to the two‐thirds power of the population size; it is much less affected by subpopulation size or the number of populations in a subdivided population.

POLYMORPHISM REGENERATION FOR A NEUTRALIZED SELFISH B CHROMOSOME

Long‐run evolution of B chromosomes is mainly made up by an evolutionary arms race between these selfish genetic elements and the standard genome. The suppression of B drive is one of the clearest expressions of genome defense against B chromosomes. After drive neutralization, the B is condemned to extinction unless a new variant showing drive can emerge and replace it. This paper reports the first empirical evidence for the substitution of a neutralized B variant by a new selfish B variant. Such a polymorphism regeneration has recently taken place in a natural population of the grasshopper Eyprepocnemis plorans.

Chromosome mapping of H3 and H4 histone gene clusters in 35 species of acridid grasshoppers

We analyse chromosome location of H3 and H4 histone gene clusters by fluorescence in-situ hybridization (FISH) in 35 species of Acrididae grasshoppers belonging to seven subfamilies. As in other organisms, H3 and H4 co-localized in the same chromosome region in the 11 species where double FISH was performed with the H3 and H4 DNA probes. Chromosome location of H3-H4 histone gene clusters showed high regularity in the species analysed, with all of them carrying a single H3-H4 cluster in an autosome which, in most cases, was located interstitially in the proximal chromosome third. In 17 out of the 21 species with 2n♂ = 23 acrocentric chromosomes, the H3-H4-carrying autosome was about eighth in order of decreasing size. Two of the four exceptions changed H3-H4 localization to proximal (Pezotettix giornae) or distal (Tropidopola graeca) in the eighth-sized autosome, but the remainder (the two Eyprepocnemis species) showed the H3-H4 cluster distally located in the second-sized autosome. All 14 species with 2n♂ = 17 chromosomes (including three long metacentric autosome pairs, five acrocentric autosome pairs and an acrocentric X chromosome) carried an interstitial H3-H4 cluster in the short arm of the smallest of the three long metacentric pairs. These results suggest that chromosome location of H3-H4 histone gene clusters seem to be highly conservative in Acrididae grasshoppers. The change in H3-H4 location from the acrocentric medium-sized autosome in the 2n♂ = 23 karyotype to the long metacentric autosome in the 2n♂ = 17 karyotype is most parsimoniously explained by common ancestry, i.e. by the involvement of the H3-H4-carrying acrocentric in the centric fusion that gave rise to the smallest of the three long metacentric autosomes of 2n♂ = 17 species.

A WIDESPREAD B CHROMOSOME POLYMORPHISM MAINTAINED WITHOUT APPARENT DRIVE

The transmission rates of the three main types of B chromosome present in the grasshopper Eyprepocnemis plorans were studied by two different methods: (i) cytological observation of their meiotic behavior in males and females and (ii) karyological analysis of the embryo offspring obtained either from gravid females collected in the field or from controlled crosses. We conclude that Bs are inherited in a random manner with no tendency toward accumulation or loss from either sex. The possible factors that might be responsible for the maintenance of this widespread polymorphism are discussed.

C-Heterochromatin content of supernumerary chromosome segments of grasshoppers: Detection of an euchromatic extra segment

SummaryThe response of 11 supernumerary segments to C-banding has been compared in six species of grasshoppers. A unique euchromatic supernumerary segment was present in the five populations of Omocestus bolivari analysed. It appears negatively heteropyenotic during the first prophase of meiosis and does not C-band. This euchromatic segment is distally located in the M6 chromosome and in every case was present in a heterozygous condition. It does not associate with the telomere of its unsegmented homologue and, consequently, the unequal M6 bivalents always segregate equationally for the extra segment in the first meiotic division. This euchromatic segment does not influence mean cell chiasma frequency but does influence chiasma position in the M6 bivalents carrying it. Four additional types of heterochromatic supernumerary chromosome segments may be distinguished in grasshoppers by C-banding: a) those darkly C-banded, b) those partly darkly C-banded, c) those lightly C-banded and d) those which do not C-band. All five types of segment appear to affect chiasma distribution in heterozygous monochiasmate bivalents but only those heterochromatic segments that do not C-band influence mean cell chiasma frequency.

Generating high variability of B chromosomes in Eyprepocnemis plorans (grasshopper)

Twenty-eight progeny analyses (PAs) performed on specimens of E. plorans collected from four natural Iberian populations have been informative about the transmission of rare B chromosome types or the de novo origin of some of them. At least 11 rare B-types have been found in addition to the predominant ones: B1 in Daimuz, B2 in Jete and Salobreña, and B5 in Fuengirola. The presence in two controlled crosses of one embryo carrying a B-type which was absent in the parents suggests that these B variants (B2iso and B1f1) have originated de novo. Eleven other PAs suggest that new B derivatives are recurrently arising in these populations. The most frequent B chromosome mutation was centromere misdivision that originated four different B-types (B2m1, B1iso, B2iso and Bmini). Other rearrangements were pericentric inversions (B2i1, B2i2 and B2i3), inverse tandem fusion (B2it1), centric fusion (B1f1) and deletions (B2d1 and B2d2). The four B derivatives produced by centromeric misdivision are significantly eliminated during sexual transmission, most probably owing to deficiencies in the control of chromosome movement by their hemicentromeres. Those derived from translocations showed Mendelian transmission but deletion B variants showed a tendency to elimination. Our results suggest that B chromosome substitution of B1 by B2 in the Salobreña and Jete populations could be achieved by differences in relative transmission efficiency, as in one controlled cross, where the female carried 1 B1 plus 1 B2, B2 was significantly overtransmitted and B1, eliminated.

Evolutionary dynamics of 5S rDNA location in acridid grasshoppers and its relationship with H3 histone gene and 45S rDNA location

We analyze the chromosomal location of 5S rDNA clusters in 29 species of grasshoppers belonging to the family Acrididae. There was extensive variation among species for the number and location of 5S rDNA sites. Out of 148 sites detected, 75% were proximally located, 21.6% were interstitial, and only 3.4% were distal. The number of 5S rDNA sites per species varied from a single chromosome pair (in six species) to all chromosome pairs (in five species), with a range of intermediate situations. Thirteen chromosomes from eight species carried two 5S rDNA clusters. At intraspecific level, differences among populations were detected in Eyprepocnemis plorans, and some heteromorphisms have also been observed in some species. Double FISH for 5S rDNA and H3 histone gene DNA, performed on 17 of these 29 species, revealed that both markers are sometimes placed in a same chromosome but at different location, whereas they appeared to co-localize in five species (Calliptamus barbarus, Heteracris adpersa, Aiolopus strepens, Oedipoda charpentieri and O. coerulescens). Double fiber-FISH in A. strepens and O. coerulescens showed that the two DNAs are closely interspersed with variable relative amounts of both classes of DNA. Finally, no correlation was observed between the number of 5S and 45S rDNA clusters in 23 species where this information was available. These results are discussed in the light of possible mechanisms of spread that led to the extensive variation in the number of clusters observed for both rDNA types in acridid grasshoppers.

Microdissection and chromosome painting of X and B chromosomes in Locusta migratoria.

Acquisition of knowledge of the nature and DNA content of B chromosomes has been triggered by a collection of molecular techniques, one of which, microdissection, has provided interesting results in a number of B chromosome systems. Here we provide the first data on the molecular composition of B chromosomes in Locusta migratoria, after microdissection of the B and X chromosomes, DNA amplification by one (B) or two (X) different methods, and chromosome painting. The results showed that B chromosomes share at least two types of repetitive DNA sequences with the A chromosomes, suggesting that Bs in this species most likely arose intraspecifically. One of these repetitive DNAs is located on the heterochromatic distal half of the B chromosome and in the pericentromeric regions of about half of the A chromosomes, including the X. The other type of repetitive DNA is located interspersedly over the non-centromeric euchromatic regions of all A chromosomes and in an interstitial part of the proximal euchromatic half of the B chromosome. Chromosome painting, however, did not provide results sufficiently reliable to determine, in this species, which A chromosome gave rise to the B; this might be done by detailed analysis of the microdissected DNA sequences

Host recombination is dependent on the degree of parasitism.

Parasites and hosts are involved in a continuous coevolutionary process leading to genetic changes in both counterparts. To understand this process, it is necessary to track host responses, one of which could be an increase in sex and recombination, such as is proposed by the Red Queen hypothesis. In this theoretical framework, the inducible recombination hypothesis states that B–chromosomes (genome parasites that prosper in natural populations of many living beings) elicit an increase in host chiasma frequency that is favoured by natural selection because it increases the proportion of recombinant progeny, some of which could be resistant to both B–chromosome effects and B–accumulation in the germline. We have found a clear parallelism between host recombination and the evolutionary status of the B–chromosome polymorphism, which provides explicit evidence for inducible recombination and strong support for the Red Queen hypothesis.