Chiasma redistribution in the presence of different sized supernumerary segments in a grasshopper: dependence on nonhomologous synapsis

Eight different sized supernumerary segments located at distal ends of the long arms of chromosomes M4, M5, M6, and S8 of the grasshopper Stenobothrus festivus were studied in males with regard to the synaptic process and chiasma distribution in the bivalents that carry them. The M4, M5, and M6 bivalents heterozygous for extra segments were always monochiasmate, in contrast to their bichiasmate condition observed in basic homozygotes. Furthermore, the presence of any of these extra segments led to chiasma redistribution in the carrier bivalents, so that such chiasmata were formed preferentially further away from the extra segment. The intensity of this effect is dependent on the size of the segment. Not all heteromorphic bivalents exhibited synaptonemal complexes with equalized axes at pachytene, but there was always a variable proportion of heterosynapsis around the distal ends of the long arms that was dependent on both the size of the segment and the size of the carrier chromosome. It is proposed that the absence of chiasmata in nonhomologous synapsed regions is responsible for the results obtained. Length measurements of the different extra segments and their carrier chromosomes between pachytene and diplotene indicated that synaptonemal complex is underrepresented in supernumerary heterochromatin.Key words: chiasma distribution, grasshopper, heterosynapsis, supernumerary segment, synaptonemal complex.

Temporal frequency stability and absence of effects on mating behaviour for an autosomal supernumerary segment in two natural populations of the grasshopper Eyprepocnemis plorans

Interannual evolution of a polymorphism for a supernumerary segment in the smallest autosome of the grasshopper Eyprepocnemis plorans has been analysed in two natural populations. The polymorphism seemed to be stable in both populations, despite its undertransmission through heterozygous females carrying B chromosomes. Analyses of the effects of the extra segment on mating behaviour failed to show differential mating success in any sex or consistent effects on mating pattern. These results are discussed in relation to the maintenance of this polymorphism in natural populations.Key words: supernumerary segments, heterochromatin, repetitive DNA.

A supernumerary chromosome segment in Locusta migratoria

A single female of Locusta migratoria was found to be heterozygous for a supernumerary heterochromatic segment distally located on the M6 autosome close to its nucleolus organiser region (NOR). Reactions to several chromosome banding techniques revealed its heterochromatic nature and its composition of GC-rich DNA sequences and likewise the NORs in this species. This suggests an origin for the extra segment by amplification of GC-rich DNA sequences contained in the distal NOR of the M6 chromosome, which is reinforced by the observation that the NOR of segmented M6 chromosomes produced the larger nucleolus in embryo prophase cells, such as would be expected from the presence of rRNA genes in the extra segment. No accumulation mechanism was detected in this female after analyzing the 213 embryo offspring produced, but an increase in the number of nucleoli per interphase nucleus was noted in heterozygous embryos in respect to standard homozygous ones.Key words: Locusta migratoria, supernumerary segments, nucleolar organizing regions, heterochromatin.

Effect of sperm genotype on chromatid segregation in female mice heterozygous for aberrant chromosome 1

SummaryAn aberrant chromosome 1 with two large homogeneously staining insertions was isolated from wild populations of Mus musculus musculus. The specific features of the aberrant chromosome have been described elsewhere (Agulnik et al. 1990). These include its preferential entry into the oocyte of heterozygous females, increased mortality of homozygotes and decreased fertility of homozygous females. Here we present data indicating that chromatid segregation in heterozygous females depends upon which sperm enters the oocyte before the second meiotic division: meioticdrive is powerful when it is sperm bearing normal chromosome 1, and segregation normalizesduring Mil when it is sperm bearing chromosome 1 with the extra segment. Experimental data are adduced and explanations offered for the observed phenomenon.

Transcriptive and replicative activity of the X chromosome in an autosomal segmental hyperploid in Drosophila and its significance

In the present investigation the transcription and replication patterns have been examined in different segments of the X chromosome and in certain specific segments (88B-92A) of an autosomal segmental hyperploid in which an extra segment 88B-92A (3R) is translocated to the X chromosome in addition to the normal two doses. Transcriptive activity monitored by [3H]uridine-labelling of these autosomal hyperploids reveals an enhanced hyperactivity of the male X chromosome while the female X chromosomes show no change in their activity. [3H]thymidine autoradiograms reveal that while the labelling frequencies of most replicating sites are distinctly lowered in the autosomal hyperploid males, no change within sexes is resolvable with regard to labelling-intensity profile. Furthermore, the X-autosome labelling frequency relation shows a distinct deviation from linearity, suggesting multiple events that lead to a higher template form of the X chromosome. These findings lead us to suggest that the signals emanating from autosome(s) do not interfere with the primary modulation inherent in the X chromosome, but act on a modulated organization of the same at a second step evoking higher activity in the male X chromosome. The results further reveal that the gene activity of the X chromosome remains unaffected by the pattern of pairing of the autosomal segments.

Chromosomal polymorphism for a heterochromatic supernumerary segment in a natural population of Tulipa australis Link (Liliaceae)

Five hundred bulbs of Tulipa australis Link, and where possible the associated young flowers, have been analysed cytogenetically in four natural populations collected from the Sierra Nevada of Spain. A majority of these were diploid (2n = 24), but a few triploids (3n = 36) were present in the population from the highest altitude. All the diploid plants from three of these populations were basic homozygotes with homomorphic bivalents. In one population, however, three types of plants were identified: homozygous standard plants (80%), plants heterozygous for a large supernumerary chromosome segment (19%), and plants homozygous for its same segment (1%). The segment was located terminally on the short arm of a submetacentric chromosome. It was heteropycnotic at meiotic prophase, showed dark C-banding at metaphase I, and was visible as a prominent heterochromatic chromocentre at interphase. The segment divided reductionally in a majority of anaphase I cells (88%), which means that chiasmata tend to be excluded from the arm carrying the segment. Seeds obtained from plants homozygous for the extra segment always carried one such a segment. One-half of the seedlings of heterozygous plants are themselves heterozygotes for the extra segment, while the other half lack the segment. Finally, 11% of the seeds from bulbs without a segment turned out to be segment heterozygotes. From these results, it is deduced that the extra segment is sexually transmitted in a Mendelian fashion and that it has little or no effect on male fertility and fecundity.Key words: polymorphism, chromosomal supernumerary segment, Tulipa australis.

EVIDENCE OF TANDEM DUPLICATION OF GENES IN A MERODIPLOID REGION OF PNEUMOCOCCAL MUTANTS RESISTANT TO SULFONAMIDE

ABSTRACT A Pneumococcal mutant, sulr-c, resistant to sulfonamides, and three transformants bearing associated d or d + resistance markers have earlier been reported to be unstable and show distinct patterns and frequencies of segregating stable progeny lacking the c marker. Each of the four strains showed a characteristic dosage of the genes involved in the merodiploidy. Complementary strands of DNA's from these stable and unstable strains were resolved and homoduplex and heteroduplex hybrids made from the separated DNA strands were used as donors in genetic transformations. Activities of a normal marker (streptomycin resistance) and those involved in the heterozygosity (c, d and d +) were quantitatively measured. From those heterodyplexes made up of opposite strands derived from a heterozygote and a stable strain, the normal marker is transferred efficiently, but the heterozygous markers are not. On the other hand, if both strands of a heteroduplex are derived from different heterozygotic strains, all markers can be transferred with usual efficiency to a stable recipient strain. The lowered efficiency in the former type of heteroduplex is attributed to an inhomology resulting from a tandem duplication in the merodiploid strains, and a postulated DNA repair process stimulated by it while in the form of the donor duplex. The inhomology probably includes (a) a microheterogeneity between the c site and the wild type locus, and (b) a more extensive incompatibility attributable to an extra segment of genome in a tandem duplication covering the c and d sites. The first of these inhomologies produces a lowered efficiency of transfer from all configurations of the particular d allele associated with the mutant c marker, and therefore accounts for the characteristic transfer patterns even from the native merodiploid DNA's.