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.     

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.     

Persistent heterochromatic association inMetrioptera brachyptera

The non-homologous and achiasmate association of L₃ and L₄ chromosomes leading to the formation of persistent pseudo-multiples has been detected in each of the natural populations sampled. Populations exhibit different mean frequencies and within a given sample a considerable range may be found. The mean chiasma frequencies of populations vary significantly but there is no correlation between these and the level of multiple formation. It is proposed that the mechanism controlling chiasma production and that determining the frequency of multiples act independently of one another. Individuals have been discovered where either one or both members of the L₃ or L₄ pair have been modified structurally. An analysis of these entire germ line conditions indicates that not only is heterochromatin primarily concerned in the formation and maintenance of multiples, but also, that their inception is correlated with the distal position of these segments on L₃ and L₄ chromosomes.     

Pseudomultiple production in Ageneotettix deorum deorum

Meiosis in Ageneotettix deorum deorum is characterised by extensive pseudomultiple formation during prophase. The association of non-homologous chromosomes takes place prior to pairing and chiasma formation and occurs to a varying degree in all prophase cells. These pseudomultiples originate during interphase as a consequence of the association of heterochromatin. All autosomes carry procentric heterochromatic segments of variable size and the L₁, L₃ and M₅ chromosomes also possess terminal heterochromatic regions. The association of non-homologous chromosomes during zygotene and pachytene does not appear to impede pairing or the frequency and distribution of chiasmata. — A majority of the pseudomultiples dissociate after diakinesis, during orientation and congression on the spindle. However in 4% of the cells examined, associations, mainly quadrivalents, persist through metaphase. — Heterochromatic associations of non-homologous chromosomes are again evident during second prophase, though here they involve only the centric heterochromatic regions; 9% of these associations persist through second metaphase. — The nature and behaviour of the pseudomultiples in Ageneotettix are pertinent to the interpretation of terminal associations in other Orthoptera and provide evidence that persistent associations can arise following a non-chiasmate association.On educational leave from the Forest Research Laboratory, Fredericton, N.B. Canada.     

Pseudo-multiple formation as a consequence of prolonged non-homologous chromosome association in Metrioptera brachyptera

Multiple configurations involving the L ₃ and L ₄ homologues have been observed in all individuals from a population sample of Metrioptera brachyptera. These associations which persist up to and including metaphase of the first meiotic division are non-homologous and achiasmate in character. They are conditioned by the persistent association of large distally located heterochromatic blocks on the L ₃ and L ₄ chromosomes and are not, as White has proposed, the result of crossing over in translocated terminal duplications. When the L ₃ and L ₄ chromosomes form bivalents the distal heterochromatin restricts crossing over in this region and chiasmata are localised proximally. In other members of the complement chiasmata are localised at either or both the centric and telomeric ends. A relationship is demonstrated between the pattern of chiasma localisation and that of chromosome pairing.     

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.     

The supernumerary segment system of Stethophyma

Three species of the genus Stethophyma carry supernumerary heterochromatic segments. The European species, S. grossa, has segments located close to the centromere on the S₁₁ chromosome pair, while the North American species, S. lineatum and S. gracile, have both interstitial and terminal segments on the S₁₀ and S₁₁ chromosomes. The latter species show a high degree of segment variation between individuals and the interstitial segments also show variation in size. The presence of segments on the S₁₀ and S₁₁ chromosomes, whether homozygous or heterozygous, modifies the pattern of chiasma distribution in these chromosomes when compared with that found in the basic homozygotes. When interstitial, they also lead to a high frequency of ring bivalents.Two points suggest that interstitially located supernumerary segments may prove to be more common than has previously been accepted. Firstly, combined equational and reductional segregation in unequal bivalents is only otherwise explicable in terms of chiasma formation in a short arm. Secondly the rod chromosomes of many Acridids may well be telocentric as in the case under study.It is proposed that these segments have arisen through a process of duplication with no evidence of interchromosomal movement.On educational leave from the Forest Research Laboratory, Canadian Forestry Service, Fredericton, New Brunswick, Canada.     

Intraspecific polymorphism of sex chromosome heterochromatin in two species of the Anopheles gambiae complex

The Hoechst 33258 banding pattern of the mitotic chromosomes of several laboratory and natural populations of the sibling species A. gambiae and A. arabiensis has been analyzed. A clear intraspecific polymorphism of sex chromosome heterochromatin has been observed. Nevertheless in each species heterochromatic variations fall within a characteristic species-specific pattern. Moreover, while laboratory populations tend to be monomorphic for a given heterochromatic variant, natural populations exhibit a high degree of intrapopulation polymorphism. The possible role of sex chromosome heterochromatin in controlling fertility and mating behaviour of Anopheles mosquitoes is discussed.     

Karyotype differences in the crenaticeps-group of Atractomorpha (orthoptera,acridoidea, pyrgomorphidae)

The four known species of the crenaticeps-group of the genus Atractomorpha have 2n ()=18+X0. All members of the complement are rod-chromosomes and the smallest autosome (no. 9) is megameric. The four species have similar amounts of euchromatin but differ markedly in the amount of heterochromatin present in their genomes. In A. similis, A. crenaticeps and the unnamed species, Species-1, there are distinct proximal segments of heterochromatin in the eight large autosomes. In A. similis these chromosomes also have prominent distal segments of heterochromatin. The fourth species, A. australis, has no visible heterochromatin in its eight large autosomes except for a small segment at the proximal end of autosome 4. In all four species, the heterochromatic segments influence chiasma frequency and chiasma position. Moreover the overall chiasma frequency is lowest in A. similis with most heterochromatin and highest in A. australis with least heterochromatin.     

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.     

Meiotic effects of supernumerary heterochromatin in Heteropternis obscurella

The endemic Australian grasshopper Heteropternis obscurella shows considerable variation in respect of both chromosome structure and chromosome behaviour. The structural differences depend upon different patterns of heterochromatin distribution as revealed by C-banding. These involve differences between populations in respect of polytypic variation in the size of paracentromeric C-bands and differences within populations in respect of polymorphisms both for terminal blocks of heterochromatin in autosomes 3 to 8 and a large proximal block of heterochromatin in autosome 7. The behavioural differences stem in part from genotypically determined variation in the chiasma distribution pattern which is markedly localised in a majority of populations but more randomly distributed in populations from the south of Western Australia. Behavioural differences also arise as secondary consequences of the presence of those heterochromatic blocks which occur as polymorphisms. The distal blocks on autosomes 5, 6, 7 and 8 lead to a redistribution of chiasmata to more proximal sites while the proximal block on 7 leads to the virtual abolition of chiasma formation in that bivalent and its replacement by a non-chiasmate mechanism of segregation. This depends upon a persistent proximal heterochromatic association between the pairing partners. The presence of distal C-blocks on bivalents 3 to 8 gives rise to persistent pseudomultiples, formed as a result of heterochromatic associations between these blocks. Such pseudomultiples involve any two or three of these six bivalents, provided they carry distal blocks, and their frequency rises dramatically in the presence of the proximal heterochromatic block on chromosome 7.     

An interchange which raises chiasma frequency

In the grasshopper Cibolacris parviceps a chromosomal interchange is described which considerably raises the chiasma frequency in all the chromosomes, including the interchange itself. Both of the chromosomes involved in the interchange have pronounced heterochromatic regions and the possible role of heterochromatin in altering recombination is discussed.     

Unequal crossing over and heterochromatin exchange in the X-Y bivalents of the deer mouse,Peromyscus beatae

Differences in length of the heterochromatic short arms of the X and Y chromosomes in individuals ofPeromyscus beatae are hypothesized to result from unequal crossing over. To test this hypothesis, we examined patterns of synapsis, chiasma formation, and segregation for maleP. beatae which were either heterozygous or homozygous for the amount of short-arm sex heterochromatin. Synaptonemal complex analysis demonstrated that mitotic differences in heterochromatic shortarm lengths between the X and Y chromosomes were reflected in early pachynema as corresponding differences in axial element lengths within the pairing region of the sex bivalent. These length differences were subsequently eliminated by synaptic adjustment such that by late pachynema, the synaptonemal complex configurations of the XY bivalent of heterozygotes were not differentiable from those of homozygotes. Crossing over between the heterochromatic short arms of the XY bivalent was documented by the routine appearance of a single chiasma in this region during diakinesis/metaphase I. Sex heterochromatin heterozygotes were characterized by the presence of asymmetrical chiasma between the X and Y short arms at diakinesis/metaphase I and sex chromosomes with unequal chromatid lengths at metaphase II. These data corroborate our hypothesis on the role of unequal crossing over in the production and propagation of X and Y heterochromatin variation and suggest that, in some cases, crossing over can occur during the process of synaptic adjustment.     

Parallel polymorphism for supernumerary segments in Chorthippus parallelus (Zetterstedt)

French populations of Chorthippus parallelus like those from Britain are polymorphic with respect to supernumerary segments on the M₇ and S₈ chromosomes. The pattern of polymorphism parallels that of British material on both morphological and behavioural grounds. It agrees also in its effect on mean cell chiasma frequency and the absence of any effect on between cell variance. Finally the agreement extends also to the distribution of karyotypes within populations, those of the S₈ showing a Hardy-Weinberg distribution whereas those of the M₇ show a deficiency of structurally heterozygous types. All these parallels imply a common origin for the British and French material, an origin which, so we must suppose, antedated the physical separation of Britain and France some 8,000 years ago.     

Heterochromatin content and chiasma distribution in the megameric chromosomes of Stethophyma gracile and S. lineatum (Orthoptera: Acrididae)

Single populations of Stethophyma gracile and S. lineatum from Eastern Canada show substantial differences in the relative amount of heterochromatic and euchromatic material in the extensively heterochromatic (megameric) M₉ autosome. No differences in chromosome phenotype between these two species have been previously reported. A new polymorphism with respect to an interstitial, supernumerary H-block was also found in the S. lineatum population, and it appears to have some interchromosomal influence upon chiasma condition. These findings suggest that the North American Stethophyma forms may parallel the extensive and deme-specific heterochromatin variation found in the European populations of the Palaearctic S. grossum. Special attention was focused upon the structural organization of the proximal and distal H-segments on the M₉ during the early prophase of the first meiotic division in the male sex. These conspicuous, hetregions, prior to diplotene, are clearly interrupted and contain short, euchromatic sequences. This condition can readily account for the absence of chiasma localization and the distribution of chiasmata along the H-blocks at diplotene-diakinesis in the megameric element. This is the only member of the complement showing no evidence of pronounced proximal or proximal/distal chiasma localization at diplotene. Possible explanations for this interesting phenomenon are suggested.     

C-bands and chiasma distribution inScilla amoena,S. ingridae,andS. mischtschenkoana (Hyacinthaceae)

An effect of C-band pattern and polymorphism on chiasma distribution in pollen meiosis was recently demonstrated inScilla siberica. A further meiotic banding study has been performed in the alliesS. amoena, S. ingridae, andS. mischtschenkoana in order to analyze the effect, if any, of their specific C-band patterns and cytochemically different heterochromatin types on recombination. No clear evidence for a preferential formation of chiasmata adjacent to homozygous intercalary heterochromatin and no consistent reduction of chiasma frequency near strongly heterozygous intercalary heterochromatin blocks, as observed inS. siberica, could be found. Terminal C-band heteromorphism is suspected to cause distal chiasma defaults. The results suggest once more that there is no uniform effect of heterochromatin on crossover distribution.     

Cytogenetic studies of the Australian rodent,Uromys caudimaculatus,a species showing extensive heterochromatin variation

The Australian rodent, Uromys caudimaculatus, consists of two chromosome races, a) The Southern Race is characterized by the possession of two to twelve B-chromosomes. These vary considerably in size, morphology, and C- and G-banding characteristics, they behave as univalent at meiosis and are inherited in a random manner, b) The Northern Race lacks Bchromosomes, but possesses large blocks of distal C-positive heterochromatin on between 18 and 28 of the 46 chromosomes. The C-blocks may be entirely G-positive, entirely G-negative, or G-banded, suggesting heterogeneity within the C-blocks. There is extensive variation both between and within populations of the northern race in the number and size of the distal heterochromatic blocks. There is no apparent difference between the races in chiasma frequency. The northern race does have a much higher proportion of interstitial versus distal chiasmata, although it is probable that this is merely a reflection of lack of crossing over in the heterochromatic blocks rather than an actual shift of chiasma localisation within the euchromatin. Despite the extensive differences between the races in the amount and organization of constitutive heterochromatin, hybrids show no abnormalities at meiosis and they are fully fertile.     

The effect of supernumerary chromosomes on meiosis in Puschkinia libanotica (Liliaceae)

The presence of the partly heterochromatic supernumerary chromosomes in pollen mother cells of Puschkinia libanotica raises the chiasma frequency in the A chromosome bivalents. The pattern of chiasma distribution along each of the five A bivalents was related to the DNA labelling pattern of mitotic chromosomes. Regions that showed heavy labelling at the end of the DNA synthetic phase had fewer chiasmata than lightly labelled regions. As this relation is the opposite to that found by Rees and Evans in another species we regard any correlation between labelling pattern and chiasma distribution as fortuitous.     

Characterisation of a very complex constitutive heterochromatin in two Gerbillus species (Rodentia)

A large amount of heterochromatin is observed in two species of the genus Gerbillus, G. nigeriae and G. hesperinus. The C-band material represents about one-half of the total karyotype length in the former species, and about one-third in the latter. Several banding techniques and various 5-bromodeoxyuridine (BrdU) treatments were used to characterise these heterochromatic segments. After applying the R-banding technique, three different staining responses of the heterochromatin can be distinguished. In G. nigeriae, strongly stained segments (R-band positive) appear in most chromosomes and, in particular, constitute the short arms of all the larger chromosomes. Palely staining heterochromatic segments (R-band negative) are less abundant in G. nigeriae but predominate in G. hesperinus. In addition, in both species an intermediate staining of heterochromatin is observed near the centromere or in the heterochromatic short arms of some acrocentric and small submetacentric chromosomes. Very short BrdU treatment during the end of the last cell cycle results in asymmetrical staining of chromatids in heterochromatic segments after applying the acridine orange or FPG (fluorescence plus Giemsa) technique. The alternating location of strongly staining segments in one or the other chromatid simulates sister chromatid exchanges (pseudo-SCE). This pattern persists after longer BrdU treatment during different stages of the last cell cycle and is independent of the R-staining properties of the heterochromatin. The lateral asymmetric appearance of the large heterochromatic segments in Gerbillus is interpreted as reflecting an uneven distribution of adenine and thymidine between the two strands of DNA.