Supernumerary Chromosome Segments Research Articles

Identification of Highly Variable Supernumerary Chromosome Segments in an Asexual Pathogen.

Supernumerary chromosome segments are known to harbor different transposons from their essential counterparts. The aim of this study was to investigate the role of transposons in the origin and evolution of supernumerary segments in the asexual fungal pathogen Fusarium virguliforme. We compared the genomes of 11 isolates comprising six Fusarium species that cause soybean sudden death syndrome (SDS) or bean root rot (BRR), and identified significant levels of genetic variation in A+T-rich repeat blocks of the essential chromosomes and in A+T-neutral regions of the supernumerary segments. The A+T-rich repeat blocks in the essential chromosomes were highly variable between F. virguliforme and non-F. virguliforme isolates, but were scarcely variable between F. virguliforme isolates. The A+T-neutral regions in the supernumerary segments, however, were highly variable between F. virguliforme isolates, with a statistically significant number (21 standard deviations above the mean) of single nucleotide polymorphisms (SNPs). And supernumerary sequence types and rearrangement patterns of some F. virguliforme isolates were present in an isolate of F. cuneirostrum but not in the other F. virguliforme isolates. The most variable and highly expressed region in the supernumerary segments contained an active DNA transposon that was a most conserved match between F. virguliforme and the unrelated fungus Tolypocladium inflatum. This transposon was absent from two of the F. virguliforme isolates. Furthermore, transposons in the supernumerary segments of some F. virguliforme isolates were present in non-F. virguliforme isolates, but were absent from the other F. virguliforme isolates. Two supernumerary P450 enzymes were 43% and 57% identical to their essential counterparts. This study has raised the possibility that transposons generate genetic variation in supernumerary chromosome segments by frequent horizontal transfer within and between closely related species.

Supernumerary Heterochromatin does not Affect Several Morphological and Physiological Traits in the Grasshopper Eyprepocnemis Plorans

Supernumerary heterochromatin in the form of autonomous B chromosomes or A-linked supernumerary chromosome segments (SCSs) constitutes the most frequent genomic polymorphism in grasshoppers (HEWITT 1979). Most B chromosomes are considered selfish genetic elements because they show drive mechanisms that enhance their maintenance in natural populations despite the negative effects they may produce on carriers (JONES 1985; BEUKEBOOM 1994). Even though SCSs have received less attention in the literature, it is now c1ear that many show drive (LOPEZ-LEONet al. 1992a) and should also be considered selfish elements integrated in the standard genome. The maintenance of these polymorphisms depends mainly on a balance of positive (drive) and negative (harmful effects) factors that influence their frequency in natural populations. In arder to reveal their evolutionary role it is necessary to investigate these two properties (drive and effects). The grasshopper Eyprepocnemis plorans has both polymorphisms in natural populations from the Iberian Peninsula. The three most widespread B types (Bi, B2 and Bs) lack drive (LOPEZ-LEONet al. 1992b) and fitness effects (LOPEZ-LEONet al. 1992c; CAMACHO et al. 1997a,b), so that they are expected to go extinct unless they are either replaced by a new selfish B variant (CAMACHOet al. 1997a,b) or they have unknown beneficial effects on fitness. The SCS 10cated proximally on the smallest autosome shows a transmission ratio significantly lower iban the Mendelian one through females possessing B chromosomes (LOPEZ-LEONet al. 1991, 1994). Therefore, it is also expected to become extinct unless it is beneficial for carriers. Surprisingly, ibis polymorphism seems to show frequency stability ayer time in natural populations (LOPEZ-LEONet al. 1995), suggesting the existence of some unknown factors influencing SCS frequency. Therefore, it is important to analyse the effects of both polymorphisms on as many different traits as possible. Here, we report the

Supernumerary chromosome segments and intrabivalent chiasma redistribution in Pyrgomorpha conica (Orthoptera)

Two kinds of supernumerary chromosome segments have been found on the megameric chromosome of the grasshopper Pyrgomorpha conica. One of them is euchromatic and distally located while the other one is heterochromatic and proximally located. Their effects on chiasma distribution in heterozygous, homozygous and doubly heterozygous bivalents for these segments have been analyzed. Both segments distort chiasma distribution so that the single chiasma is preferentially formed far from the location of each supernumerary segment. This effect is more pronounced in bivalents that are homozygous for the presence of the proximal heterochromatic segment. In doubly heterozygous bivalents that possess both segments on the same chromosome the chiasma distribution is significantly different from that observed in heterozygous bivalents for each segment and a significant frequency of univalents was scored.

Supernumerary chromosome segments in different genera of formicidae

Supernumerary chromosome segments have been detected in natural populations ofTapinoma nigerrimum, Aphaenogaster senilis, Aphaenogaster iberica, Plagiolepis schmitzii, andPlagiolepis pygmaea. Chromosomal location, frequencies, response to C-banding techniques, and other aspects of extra segments are studied. Previous hypotheses about the chromosome differentiation processes between related species of the genusTapinoma are examined using this new cytogenetic information. The possible significance of the supernumerary chromosome segments in the process of chromosomal evolution in ants is discussed.

Male and female segregation distortion for heterochromatic supernumerary segments on the S8 chromosome of the grasshopper Chorthippus jacobsi.

The mode of inheritance of supernumerary segments located on three different chromosome pairs was investigated in controlled crosses with specimens of the grasshopper Chorthippus jacobsi. While extra segments located on chromosomes M5 and M6 showed Mendelian inheritance, that on S8 did not. Thus, the two supernumerary heterochromatic chromosome segments located distally on the S8 chromosome accumulated through non-Mendelian transmission through both sexes. The observed transmission patterns may be explained by gametic selection for spermatozoa carrying segmented S8 chromosomes, in addition to meiotic drive for segmented S8 chromosomes in heterozygous females. The significance of these findings for the maintenance of these polymorphisms in natural populations is discussed.

Variation in the heterochromatin and nucleolar organizing regions of Allium subvillosum L. (Liliaceae)

Two kinds of polymorphisms have been found in a wild population of Allium subvillosum L. (Liliaceae): (i) extra C-bands and supernumerary chromosome segments on three chromosome pairs and (ii) the absence of the nucleolar organizing region (NOR) in one member of the third chromosome pair (there are two other NOR-bearing chromosome pairs in the karyotype of this species). The fluorescence pattern (chromomycin A3 (CMA3) positive and 4,6-diamidino-2-phenylindole (DAPI) negative) of the extra heterochromatin is similar to that of the standard. The absence of the NOR on the third chromosome pair is related to a variation in the NOR-associated heterochromatin (the chromosomes with no NOR do not have the distal C-band from the satellite, but the proximal C-band may be present). The characteristics of the karyotype of this species, its heterochromatin distribution, and the possible mechanisms by which the polymorphisms may originate are discussed.Key words: Allium, supernumerary segments, heterochromatin, polymorphisms, nucleolar organizing regions.

New hypotheses about the origin of supernumerary chromosome segments in grasshoppers

The existence of euchromatic supernumerary chromosome segments which do not seem to represent repeated material cannot be explained by the current hypotheses (duplication or translocation) concerning the origin of extra segments. An hypothesis is proposed to explain the origin of the euchromatic extra segments distally located on the megameric M6 chromosome of the species Omocestus bolivari and Chorthippus binotatus, namely that these segments represent a relic of an euchromatic region which formed part of the ancestral M6 chromosome of the gomphocerine grasshoppers. On the other hand, the absence of paracentromeric C-bands in some chromosomes of Chorthippus vagans and Ch. brunneus could have been achieved by means of transformation of heterochromatin into euchromatin

Chiasma redistribution in presence of supernumerary chromosome segments in grasshoppers: dependence on the size of the extra segment

The presence of two different supernumerary chromosome segments in each of four bivalents of the grasshopper Chorthippus binotatus permitted us to test confidently the effects of such extra material on chiasma distribution in the bivalents carrying them, since the same bivalent could carry any of two extra segments differing in nature, size, chromosomal location and C-banding response. All extra segments analysed modified significantly the chiasma distribution pattern, so that chiasmata formed preferentially in the furthest chromosomal regions from the extra segment. The significant difference in the intensity of this effect observed for the two extra segments on the M5 bivalent, however, is presumably due to their very different size, which demonstrates a direct dependence of the intensity of the chiasma redistribution on the size of the extra segment.

Effects of supernumerary chromosome segments on the activity of nucleolar organiser regions in the grasshopper Chorthippus binotatus

Eight different supernumerary chromosome segments have been detected in natural populations of the grasshopper Chorthippus binotatus. These differ in chromosomal location, size and their response to C-banding. Two different segments occur in each of the M5, M6, M7 and S8 chromosome pairs. Standard males which lack all eight extra segments show regular primary active nucleolar organiser regions (NORs) located interstitially in the L2 and L3 chromosomes plus occasional secondary active NORs located in paracentromeric regions of the M4 and M5 chromosomes which give rise to small nucleoli. However, males carrying extra distal segments on the M5 chromosome show a significant increase in the activity of the secondary NORs in either or both of the M4 and M5 chromosomes. In contrast, males carrying extra segments in chromosomes other than the M5 give a pattern of NOR activity similar to that of standard males. This increase in the activity of the secondary NORs is presumed to result from the presence of extra heterochromatin in the distal region of the M5. We propose that this region contains genes which regulate NOR expression and that the presence of extra segments in their vicinity affects their normal functioning, resulting in an increased transcription of the ribosomal DNA contained in the secondary NORs.

Chiasma redistribution in bivalents carrying supernumerary chromosome segments in grasshoppers

Chiasma redistribution in bivalents carrying supernumerary chromosome segments in grasshoppers

Cytological evidence for either polyspermy or polar-body activation in mosaic embryos of Chorthippus brunneus (Orthoptera, Acrididae)

Two embryos of Chorthippus brunneus from the same pod were haplo-diplo-pentaploid and diplo-triplopentaploid mosaics, respectively. The presence of supernumerary chromosome segments gave indications as to the possible origin of these two embryos. The C-banding patterns of both mosaic embryos show absence of homozygosity, which excludes accidental thelytoky as possible origin. The embryos must have arisen from fertilised eggs by means of processes like polyspermy or polar-body activation.

Chiasma frequency and distribution in the presence and absence of supernumerary chromosome segments in the grasshopper, euchorthippus pulvinatus gallicus

Chiasma frequency and distribution in the presence and absence of supernumerary chromosome segments in the grasshopper, euchorthippus pulvinatus gallicus

The effect of supernumerary heterochromatic chromosome segments on non-homologous chromosome associations in the grasshopper Camula pellucida.

The effect of supernumerary heterochromatic chromosome segments on non-homologous chromosome associations in the grasshopper Camula pellucida