E Viseras

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.

Paternity displacement in the grasshopper Eyprepocnemis plorans

Three types of double crosses were carried out to investigate sperm competition in the grasshopper Eyprepocnemis plorans. Maximum likelihood estimation of paternity probability showed a high degree of second male sperm precedence (P2 = 90%). The results also showed that: (i) a single copulation may be enough for an efficient paternity displacement; (ii) males do not use sperm plugs, but they mate for a prolonged period of time to resist further copulation; and (iii) successive paternity displacements may be seen in the same female. The mechanisms controlling sperm competition in E. plorans, and the evolutionary implications of this phenomenon are discussed

Mitotic instability of B chromosomes during embryo development in Locusta migratoria

Mitotic instability of B chromosomes during embryo development has been studied in the locust Locusta migratoria. Direct cytological observation of B chromosome nondisjunction in embryos has shown that it occurs in 2.7 per cent of anaphase and telophase cells, and that this frequency is not significantly different among embryos of 5–9 days of development. We have defined three indices which have been shown to be very useful to quantifying mitotic instability: R, the ratio of embryos showing B chromosome instability, M, the median of the distribution of B chromosome numbers in a sample of embryo cells, which has been shown to be a good estimator of the original number of B chromosomes present in the zygote, and MI, the sum of all deviations (in absolute value) of B numbers with respect to M in the same embryo. Mitotic instability of B chromosomes is already apparent in 3-day-old embryos and reaches its maximum value on the fifth day of development. The intensity of mitotic instability, as measured by MI, varies significantly during the developmental period analysed but no definite trend was observed.

The B-chromosomes of Locusta migratoria I. Detection of negative correlation between mean chiasma frequency and the rate of accumulation of the B's; a reanalysis of the available data about the transmission of these B-chromosomes

B-chromosomes were studied in two Spanish populations of Locusta migratoria. Both exhibit a high frequency of B-carrying individuals (75% and 94%). In both the B-chromosomes are mitotically unstable and they accumulate in the male germ line. The mean rate of accumulation is 28.5% in Baños de la Encina and 31.9% in Carboneras. There are no significant differences in mean cell chiasma frequency and between-cell variance of chiasmata between follicles with different number of Bs within individual males. Likewise, there are no significant differences in mean cell chiasma frequency and between-cell variance between males with and without supernumeraries. However, a significant negative correlation exists between mean chiasma frequency and the rate of accumulation.

C-heterochromatin variation in the genus Eumigus (Orthoptera: Pamphagoidea)

Eumigus punctatus, E. monticulus and E. cucullatus all have 2n♂=19 and similar chromosome morphology, all the elements being telocentric. In E. cucullatus there are C-bands in all chromosomes, but not in E. monticulus. The possible origins of these differences and their cytotaxonomic significance are discussed.

Effects and maintenance of a pericentric inversion polymorphism in the grasshopper Aiolopus strepens

Seven natural populations of the grasshopper Aiolopus strepens were analysed to investigate the effects of a pericentric inversion in the smallest chromosome (S11) on chiasma formation and nucleolus organizer regions (NORs) activity. This polymorphism was stable in one population after comparing the frequencies of standard and inverted S11 chromosomes in two samples taken 5 years apart. Furthermore, mean cell chiasma frequency in this population tended to decrease over these 5 years. Males homozygous for the S11 inversion showed a higher frequency of inactivity of one of the NORs located on the L3 chromosome than that found in standard homozygous and heterozygous males. The results suggest that this stable polymorphism is maintained by heterosis, and that inversion homozygotes are less fit than standard homozygotes, which explains the low frequency of inverted chromosomes in all populations analysed. However, the possibility of recurrence of the inversion cannot be ruled out.