The supernumerary segment system of Stethophyma

Three species of the genus Stethophyma have been cytologically examined and all three show variation both for supernumerary heterochromatic segments and for the distribution of standard heterochromatin among the autosomes. The European species, S. grossum, for example, shows considerable interpopulation variation for standard heterochromatin while two of the populations, from Spain and Austria, show supernumerary segment polymorphism. The segments are located interstitially on the S₁₁ chromosome but occupy different positions in the different populations. — In all species, the presence of the extra heterochromatic segments increases the mean chiasma frequency. Moreover, the influence of the segments upon mean chiasma frequency is different in different populations and in different species. In the Spanish population, the increase is both intra- and interchromosomal whereas in Austria the influence of the segment is completely interchromosomal. — In the American species, S. gracile and S. lineatum, where supernumerary heterochromatic segments are carried on both S₁₀ and S₁₁ chromosomes, the effect on chiasma frequency shows a dosage relationship, an increase in the number of segments per individual being correlated with an increase in mean chiasma frequency. It is suggested that the interstitial segments found in all species have originated by direct duplication of chromosome material. By contrast the terminal segments in S. lineatum and S. gracile may be derived by translocation from a B-chromosome since such a chromosome has been found in one individual of the former species. — The variation in segment structure and the distribution of standard heterochromatin, among the European species of S. grossum suggests that these systems have evolved independently in different populations.On educational leave from the Forest Research Laboratory, Fredericton, N. B. Canada.     

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.by S.A. Gerbi     

Extra heterochromatin in natural populations of Gomphocerus sibiricus (Orthoptera: Acrididae)

Two types of extra heterochromatin, in the form of either supernumerary segments or B-chromosomes, have been observed in natural populations of Gomphocerus sibiricus. The presence of a heterochromatic supernumerary segment affecting the M₇ pair in a population from the Valle de Arán disturbs the normal separation of this bivalent at anaphase I of meiosis and non-disjunction products, respectively deficient or duplicate for the M₇, are present at metaphase II.A stable B_m chromosome has been detected in a second population from Formigal. In a single individual where a B_m is present together with a small unstable B_s supernumerary chromosome both the B_m and the M₅ are unstable in the germ line and in the case of the M₅ this unstability leads to a state of germ line polysomy.     

Parallel polymorphism for supernumerary segments inChorthippus parallelus (zetterstedt)

A complex and parallel pattern of polymorphism for heterochromatic supernumerary segments in the M₇ and S₈ chromosomes has been found in 14 populations of the meadow grasshopperChorthippus parallelus. Nine distinct karyotype classes for these two chromosome pairs occur though they are not equally represented in different populations. Populations differ also with respect to the frequency of supernumerary segments they contain. In all populations the presence of supernumerary segments leads to a significant elevation of mean cell chiasma frequency compared to individuals from the same population lacking such segments. The extent of the effect appears to differ in different populations. The observed frequencies of S₈ karyotypes conform to the expectations of a Hardy-Weinberg distribution. Those of the M₇, however, do not, and in all but one of the 14 populations there is a significant excess of homokaryotypes. In the Ashurst population 26% of the individuals sampled were characterised by germ-line polysomy for the M₄ chromosome, either in the form of entire tetrasomics or more usually as mosaics ranging from tri- to hepta-somics. In all these polysomics the M₄ chromosomes in excess of two were regularly heteropycnotic at first meiotic prophase from zygotene to diakinesis. As a consequence of this multivalents are rare. Extra M₄ chromosomes do not modify the chiasma characteristics of the other chromosomes in the complement. Nor do they modify the action of the supernumerary segments in any way.     

Meiotic drive against an autosomal supernumerary segment promoted by the presence of a B chromosome in females of the grasshopper Eyprepocnemis plorans

Twenty-seven out of 50 progeny analyses performed with specimens of the grasshopper Eyprepocnemis plorans were informative about the transmission of a supernumerary heterochromatic chromosome segment. The simultaneous presence of a B chromosome in some of the parents involved in the crosses permitted us to test the relationship between both types of supernumerary heterochromatin with respect to their transmission. The results demonstrated that the supernumerary segment is partly eliminated through females possessing B chromosomes. The implications of this in relation to the occurrence of the extra segment in natural populations are discussed.by S.A. Gerbi     

Effects of different supernumerary segments on chiasma distribution of the polymorphic species Chorthippus jucundus (Orthoptera: Acrididae)

The analysis of 18 wild populations of the polymorphic species Chorthippus jucundus has revealed the presence of five distal extra segments. They differ in location, size and staining properties. Two of them (L₁H and L₁h) are located in the largest element and do not C-band, while the remainder (S₈H, S₈h and S₈h₁) appear in the shortest chromosome and show positive C-banding. Those located in the same chromosome differ basically in their size. However, any of these blocks induce a similar change in the pattern of chiasma distribution within the affected bivalent. Additionally four patterns of C-banding as regards the presence of distal bands in some autosomes have been detected in the different populations.     

The cytogenetic systems of grasshoppers and locusts

The australian plague locust (2n=23 male, 24 female) is distinctive in possessing three pairs of two-armed, short autosomes (S₉, S₁₀ and S₁₁). In two of these pairs (S₉, S₁₀) these arms are a constant feature but in the shortest (S₉) pair most individuals are either heterozygous for them or else are homozygous telocentric. Coupled with this five of the heterozygous individuals give evidence of occasional short-arm detachment.—In all the S-pairs the shorter of the two arms is invariably heterochromatic in character and in the S₉ and S₁₁ shows a bi- or tri-partite sub-structure which suggests they may have originated by tandem duplication. — Three of the other autosomes (L₂, M₃ and M₆) also have small heterochromatin(het)-blocks associated with them. At first meiotic prophase these frequently associate with the univalent X chromosome which itself displays an unconventional pattern of allocycly, its centric end appearing negatively heteropycnotic from leptotene through diplotene.—At metaphase I the het-blocks on the telocentric autosomes sometimes transform into swollen, negatively heteropycnotic, segments equivalent in appearance to that shown by the entire X at this stage. It is suggested that these puff-like structures represent an inter-chromosomal position effect conditional upon prior X/A het-association at first prophase.     

Morphologically differentiated sex chromosomes in neotropical freshwater fish

A general survey of the occurrence of morphologically differentiated sex chromosomes in the neotropical freshwater fishes is presented. The total number of 32 occurrences involving simple XX-XY and ZZ-ZW, and multiple X₁X₂Y, XY₁Y₂ and ZW₁W₂ sex chromosome systems is described, with comments on the aspects of sex chromosome evolution in this fish fauna. The occurrence of different sex chromosome systems in related species of the same genus, or in different populations of the same nominal species, involving male and sometimes female heterogamety, and differences in the molecular composition of sex-linked heterochromatin, are considered as indicative of the early stage of sex chromosomes evolution in fish.     

Non-random segregation during anaphase II in an individual of Arcyptera tornosi (Bol.) (Orthoptera) heterozygous for three supernumerary heterochromatic segments

An individual of Arcyptera tornosi heterozygous for distal heterochromatic segments affecting M₆, S₁₀ and S₁₁ chromosomes has been analyzed during all the meiotic stages in order to establish the pattern of meiotic segregation in anaphase I and II. S-bivalents invariably show an equational separation during anaphase I and the anaphase II separation is non-random, both chromatids with heterochromatic segments often segregating to the same pole. Differences are significant if compared with the expected segregation. Some aspects of this particular chromosome behaviour are briefly discussed.     

Asynchrony of DNA replication and mitotic spiralization along heterochromatic portions of Chinese hamster chromosomes

Sequence of DNA synthesis and mitotic chromosome spiralization along heterochromatic portions of the sex (X₁X₂) and of some marker chromosomes in cultured Chinese hamster cells were studied, employing two methods: study of segmentation pattern caused in chromosomes with colcemid, and autoradiography with tritiated thymidine. The heterochromatic portions of all chromosomes studied were characterized by striking internal asynchrony of DNA replication. In particular, they had segments that replicated relatively early. The short arm of the X₂ chromosome, heterochromatic in female somatic cells, had at least three such segments. Replication patterns of the long arms of the X₁ and X₂ chromosomes were different. In X₁ this arm contains several segments showing relatively early replication. The long arm of X₂ had no similar segments. The possible significance of the data obtained is discussed with regard to the problem of genetic inertness of heterochromatin. At the terminal stage of the S period, H³-thymidine seems to be incorporated into condensed chromatin of interphase nuclei. On the basis of the data obtained, it is proposed that during replication of heterochromatin consecutive despiralization of parts of it takes place.     

Spontaneous chromosome mutations in Truxaline grasshoppers

Three distinct mutant conditions are described in single male individuals from three species of short horn grasshopper. Of these, one is an entire germ line mutant of Myrmeleotettix maculatus, heterozygous for a centric fusion between single M₄ and M₅ telocentric chromosomes. In contrast, the remaining two mutants are present in mosaic form. One is heterozygous for an L₁-M₄ interchange in Omocestus viridulus, the other tetrasomic for the M₄ chromosome in Chorthippus parallelus which in addition is characterised by the inclusion of a supernumerary heterochromatic segment on one S₈ homologue. Centric fusion in Myrmehotettix maculatus has neither disturbed the chiasma potential of the elements constituting the fusion multiple nor, has it apparently influenced the production of balanced gametes. The pattern of chiasma formation in the L₁-M₄ interchange multiple lends support to the contention that the process of chiasma formation originates near the distal end of chromosome arms in Omocestus viridulus. There is no interaction between the two mutant conditions of tetrasomy and the presence of supernumerary segments in Chorthippus parallelus. Moreover, because of the precocious nature of two of the four M₄ homologues there is little tendency to form multivalents. The two M₄ bivalents share a similar mean chiasma frequency.     

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.     

The Genetic Identification of a Heterochromatic Segment on the X Chromosome of DROSOPHILA MELANOGASTER

An autosomal euchromatic maternal-effect mutant, abo (= abnormal oocyte), interacts with, or regulates the activity of, the heterochromatin of the sex chromosomes of Drosophila melanogaster. It is shown that this interaction or regulation with the X chromosome involves a specific heterochromatic locus or small region that maps to the distal penultimate one-eighth of the basal X-chromosome heterochromatic segment.     

A novel simple satellite DNA is colocalized with the Stalker retrotransposon in Drosophila melanogaster heterochromatin

In the T(1;2)dor ^var7 multibreak rearrangement the distal 1A-2B segment of the X chromosome of Drosophila melanogaster is juxtaposed to an inverted portion of the heterochromatin of chromosome 2. Analysis of mitotic chromosomes by a series of banding techniques has permitted us precisely to locate the heterochromatic breakpoint of this translocation in the h42 region of 2R. Cloning and sequencing of the eu-heterochromatic junction revealed that the translocated 1A-2B fragment is joined to (AACAC)_n repeats, which represent a previously undescribed satellite DNA in D. melanogaster. These repeated sequences have been estimated to account for about 1 Mb of the D. melanogaster genome. The repeats are located mainly in the Y chromosome and in the heterochromatin of the right arm of chromosome 2 (2Rh), where they are colocalized with the Stalker retrotransposon. Received: 3 October 1998 / Accepted: 3 December 1998     

Alpha and beta heterochromatin in polytene chromosome 2 ofDrosophila melanogaster

The formation of alpha and beta heterochromatin in chromosomes of Drosophila melanogaster was studied in salivary glands (SGs) and pseudonurse cells (PNCs). In SGs of X0, XY, XYY, XX and XXY individuals the amounts of alpha heterochromatin were similar, suggesting that the Y chromosome does not substantially contribute to alpha heterochromatin formation. Pericentric heterochromatin developed a linear sequence of blocks in PNCs, showing morphology of both alpha and beta heterochromatin. In situ hybridization with Rsp sequences (H _o clone) revealed that the most proximal heterochromatic segment of the mitotic map (region h39) formed a polytenized block in PNCs. Dot analysis showed that the clone had a hybridization rate with PNC-DNA very close to that with DNA from mainly diploid head cells, whereas the homologous SG-DNA was dramatically underrepresented. A similar increase of DNA representation in PNC was found for AAGAC satellite DNA. The mitotic region h44 was found not to polytenize in the SG chromosome, whereas in PNC chromosome 2 this region was partly polytenized and presented as an array of several blocks of alpha and beta heterochromatin. The mapping of deficiencies with proximal breakpoints in the most distal heterochromatin segments h35 in arm 2L and h46 in 2R showed that the mitotic eu-heterochromatin transitions were located in SG chromosomes distally to the polytene 40E and 41C regions, respectively. Thus, the transition zones between mitotic hetero- and euchromatin are located in banded polytene euchromatin. A scheme for dynamic organization of pericentric heterochromatin in nuclei with polytene chromosomes is proposed. Received: 17 November 1995; in revised form: 10 April 1996 / Accepted: 18 September 1996     

Population differences in the expression of nucleolus organizer regions in the grasshopperEyprepocnemis plorans

Summary Fluorescence in situ hybridization revealed the presence of ribosomal RNA genes in paracentromeric regions of all A chromosomes and in the distal half of B chromosomes in embryonic cells from Moroccan specimens of the grasshopperEyprepocnemis plorans. The expression of these genes was monitored by the presence of nucleoli attached to each chromosome bivalent in diplotene cells from males collected from two different Moroccan populations and was compared to previous data of Spanish populations. Whereas only the nucleolus organizer regions (NORs) on S₉–S₁₁ and X chromosomes were active in the Spanish specimens. Moroccan individuals showed NOR activity in all chromosomes. The rRNA genes on the B chromosome were inactive in both populations. The S₉ and S₁₀ NORs were less active in Moroccan specimens than in Spanish specimen, which might be partly explained by the negative interdependence for expression of the S₁₀ NOR with respect to those on L₂ and X chromosomes. On the other hand, the X NOR was more active in Moroccan specimens than in Spanish specimens, and this might be partly due to the positive effect that the presence of B chromosomes has on the expression of this NOR. The implications of these observations on current models of NOR activity regulation are discussed.Abbreviation NOR nucleolus organizer region     

Polymorphism involving heterochromatic segments in Metrioptera brachyptera

A complex pattern of polymorphism involving terminal heterochromatic segments on L₃ and L₄ chromosomes has been uncovered in eight populations of Metrioptera brachyptera. There are individuals in every population which carry reduced segments on one or both L₄'s. In six populations, enlarged heterochromatic segments have been encountered on the L₃ chromosomes in some individuals. The L₄ system is almost certainly stable although the frequency of L₄ karyotypes does not conform to a Hardy-Weinberg distribution in all populations. Stability of the L₃ polymorphism could not be ascertained. A reduction of L₄ heterochromatin leads to a significant rise both in mean cell chiasma frequency and between cell variance. The effect on chiasma frequency is transchromosomal. The normal pattern of strict chiasma localisation tends to be disrupted in germ lines which include modified L₄ chromosomes. There is a reduction in the number of proximal and distal chiasmata and an increase in the frequency of interstitial ones. It is proposed that the standard L₄ heterochromatin may function in conserving heterozygosity and promoting uniformity between parent and offspring. Partial removal may lead to an effective increase in recombination and produce a greater diversity of genotypes for selection to act upon.     

Cytogenetic studies on Chorthippus jucundus (Fisch.) (Orthoptera) III. The meiotic consequences of a spontaneous centric fusion

A spontaneous structural heterozygote originated from a centric fusion involving two acrocentric autosomes (M₅ and S₈) in a male Chorthippus jucundus (2n=16+X) was tested for the effects on chiasmata characteristics and the role that these could play in metaphase-I orientation of the trivalent. Results show non-existence of inter- or intra-chromosomal effects on chiasma frequency when compared with standard individuals, and displacement of the single chiasma on the S₈ to a distal position while no significant change is appreciated in the M₅ chiasma distribution. The presence of two chiasmata on the M₅ and one on the S₈ always induces a linear orientation of the trivalent. However, when two chiasmata are present (one on each chromosome) and independently of their location on each arm (proximal, interstitial of distal) any of the possible orientations (linear or convergent) can be attained. The frequency of linear orientation is higher than that expected at random in the resulting trivalent during metaphase-I and the rate of reorientation is very low.The meitotic behaviour of such a trivalent is discussed taking into account the already mentioned aspects together with the size of the fused element.     

The modification of crossing over in maize by extraneous chromosomal elements

Summary Five regions of the maize genome were tested for their response to endogenous factors influencing recombination. These included heterochromatic B chromosomes and abnormal chromosome 10 as well as the sex in which recombination occurred.The frequency of recombination in the proximal A ₂-Bt and Bt-Pr segments of chromosome 5 was increased in the presence of B chromosomes, with the male meiocytes showing a greater response than the female meiocytes. In addition, experiments involving 0, 1, 2 and 4 B's revealed a dosage effect of B chromosomes on crossing over in chromosome 5. Recombination in the proximal Wx-Gl ₁₅ interval of chromosome 9 was found to be slightly higher than normal in male flowers when two B chromosomes were present. This increase was accompanied by a decrease in the adjacent Sh-Wx segment. Crossing over in the distal C-Sh segment and in the C-Sh-Wx-Gl ₁₅ regions of female flowers was unaffected by B's.Comparisons of plants heterozygous for abnormal chromosome 10 (K10 k10) and homozygous for the standard chromosome 10 (k10 k10) showed that abnormal 10 greatly enhances crossing over in the A ₂-Bt and Bt-Pr segments of chromosome 5. In contrast to the finding with B's, the effect is greater in female than in male sporocytes. K10 showed no significant effect on recombination in the C-Sh-Wx-Gl ₁₅ region of chromosome 9 except in male sporocytes, where there was a slight increase in the Sh-Wx region of 0 B K10 k10 plants and a possible interaction with B chromosomes to raise the level of recombination between Wx and Gl ₁₅. The fact that the regions adjacent to the centromere of chromosome 9 show little or no response to the presence of K10 indicates that the proximal heterochromatin of this chromosome differs qualitatively from that of other maize chromosomes. This conclusion is supported by a comparison of the effects of B chromosomes, K10 and sex on crossing over in chromosomes 5 and 9.Dedicated to Dr. M. M. Rhoades on the occasion of his seventieth birthday.