Studies on chiasma distribution and chiasma movement in Zoniopoda tarsata (Orthoptera: Romaleidae)

A sample of 27 males of Zoniopoda tarsata from Argentina was studied cytologically. The three largest autosomal pairs and the X were characterized by the presence of interstitial C-bands. Chiasma position relative to the bands was analyzed at diplotene and diakinesis. The frequency of interstitial, terminal and total chiasmata per cell was studied for the whole autosomal bivalent set, analysing the variations between stages and among individuals. The comparison of interstitial chiasma frequencies between stages and among individuals and the study of chiasma position relative to the bands in pairs 1, 2 and 3 indicated that chiasma distribution varied from diplotene to diakinesis. Therefore, terminalization does exist in this species and the movement may occur towards the centromere. The frequency of terminal associations at diplotene showed a high negative correlation (r=-0.89; p<10^-5) with the number of interstitial chiasmata. This correlation would not be expected if the two kinds of association were produced by different (independent) mechanisms. Consequently, terminal associations were considered genuine chiasmata. The correlation between interstitial and total chiasmata was very much lower then the former (r=0.39; p=0.04). This fact, besides the relatively low variation for chiasma number, observed among individuals suggests that in this species the number of interestitial chiasmata, which are the most important in controlling the genetical recombination, is mainly regulated by changes in chiasma distribution, while variations in total chiasma frequency are of much lower magnitude.Member of Carrera del Investigador del Consejo Nacional de Investigaciones Cientificas y Técnicas (CONICET)     

Parallel polymorphisms for supernumerary heterochromatin in Dichroplus elongatus (Orthoptera):

Natural populations of Dichroplus elongatus often exhibit diverse forms of supernumerary heterochromatin. In Tafi Viejo, Tucumán (Argentina), parallel polymorphisms for extra segments on pairs M₆, S₉ (proximal) and S₁₀ (distal) and a mitotically unstable B chromosome were detected. The segments produce intrachromosome effects excluding chiasmata from their neigbourhood and displacing them to regions distal to the extra heterochromatin. The individuals bearing simultaneously the B and the segment on M₆ show a decrease in the frequency of interstitial chismata (X_i) while those carrying only segments on M₆ or S₁₀ have an increase of this variable when compared with standard males. The between cell variance of X_i is also higher in M₆ segment carriers. B individuals present an increase of abnormal spermatid frequency which may be explained by the simultaneous occurrence of two mechanisms: 1) lagging of supernumeraries and 2) physiological effects of B's affecting the meiosis even in cells lacking them. The intra and interchromosome effects may be important for the plasticity of populations allowing recombination to explore new chromosome regions. The B polymorphism seems to be stable which points to an advantage of B carriers and/or the occurrence of accumulation mechanisms counterbalancing their negative effects on fertility.     

Supernumerary segments in five species of grasshoppers (Orthoptera: Acridoidea)

Supernumerary segments have been observed in five species of grasshoppers: Calliptamus barbarus, Oedipoda fuscocincta, Acrotylus insubricus, Omocestus raymondi and Chorthippus ariasi. In four cases they are located in the S-chromosomes, but in A. insubricus they are carried by the megameric pair (M₉). In C. barbarus and O. fuscocincta we have observed non-homologous association during diplotene between the extra segments and the X-chromosome. The supernumerary segments of these two species are distally located. However, in anaphase-I the unequal bivalents divide reductionally in 30% and 20% of the cells, respectively. Finally, the supernumerary segments of C. barbarus do not produce any effect on mean chiasma frequency, but they decrease significantly the between-cell variance of chiasmata in males carrying them.     

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.     

Chiasma distribution in asynaptic Locusta migratoria

The formation of chiasmata in six full sib male partially asynaptic individuals of Locusta migratoria has been studied. The mean chiasma frequency per cell was 2.3 both at diplotene and metaphase I. Chiasmata tended to be distributed evenly among the bivalents. The frequency and distribution of the chiasmata in each type of bivalent (L, M, or S) depended on the level of asynapsis and on interference between the bivalents. Short bivalents were the most affected by interference, while M bivalents associated independently of L and S bivalent behaviour.     

Meiosis VII: “Detorsive bending” as a basis for geometric shapes of late prophase bivalents

A new model depending on mechanical properties of chromosomes is adduced as a basis for diplotene opening-out and for curvature occurring in grasshopper bivalents, during and subsequent to diplotene. Conditions underlying the model are: (1) rigid physical binding exists between sister chromatids, (2) each chromatid remains free of torsional strain if its pairing face is straight, i.e. the chromatid is bilaterally symmetrical, (3) reciprocal exchange together with stiff binding between sisters produces twist in each chromated before diplotene begins, (4) stiffening of the bivalent during late meiotic prophase removes the twist resulting from reciprocal exchange, (5) since sister binding prevents untwisting of chromatids about their long axes, untwisting would be achieved only in conjunction withbending of each chromated. It is shown that this bending, called detorsive bending, automatically produces opening out, not only in bivalents with one chiasma but also in those with more than one, especially if the chiasmata are interstitial.In bivalonts with two chiasmata, classes of curvature resulting when both chiasmata are interstitial (II), when one is interstitial and one terminal (IT) and when both are terminal (TT) are attributed to differences in strength of opening out at interstitial and at terminal chiasmata respectively. It is postulated that mechanisms responsible for opening out at terminal chiasmata are basically different from those at interstitial chiasmata.A theoretical basis of a method for cytological detection of chromatid interference is outlined and arguments are presented against the electrostatic hypothesis.     

18 μm replication units of chromosomal DNA fibers of differentiated cells of Pea (Pisum sativum)

A technique is presented for investigating possible early terminalisation of chiasmata, based on the analysis of labelling patterns in autoradiographs in X₂-labelled diplotene bivalents. Terminalisation is expected to produce unlabelled gaps in otherwise labelled bivalents which, given even a moderate amount of movement, is capable of resolution by this technique. The method has been evaluated using diplotene spermatocytes of Schistocerca gregaria as a test system. Very few unlabelled gaps were actually observed in X₂-labelled bivalents, in fact no more than in X₁-labelled bivalents, where chiasma terminalisation is not expected to produce gaps. Consequently it is concluded that the gaps observed are due to technical causes and that early terminalisation is an unimportant factor determining chiasma distribution in this system.     

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.     

Chromosome polymorphism in populations of the grasshopper Trimerotropis pallidipennis from southern Argentina

Three populations of the grasshopper Trimerotropis pallidipennis from southern Argentina have been studied cytologically. A very characteristic B-chromosome was found in all three. They also showed geographical variability in respect of the presence of pericentric inversions, and the inversion system was found to influence chiasma frequency. The Laguna Blanca population, which is on the hypothetical pathway the species is believed to have followed during its migration from northern to southern latitudes, has the same karyotype composition as the N. American form, with fixed inversions in the 3 largest autosomes and the X-chromosome. Its members have a high total chiasma frequency and a great number of interstitial chiasmata. The Sierra de la Ventana population, situated at the absolute eastern border of the species distribution is highly polymorphic with respect to the presence of inversions in the medium chromosomes. Its members have the lowest total chiasma frequency and a greatly reduced number of interstitial chiasmata. Situated geographically between the other two, the Choele-Choel population has the highest frequency of inversions and many of them are homozygous. Its members have a higher total chiasma frequency than that observed in specimens from Sierra de la Ventana, and a greatly reduced number of interstitial chiasmata, similar to that observed in individuals from the latter population.     

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 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.     

Recombination within extra segments: evidence from the grasshopper Chorthippus jucundus

A detailed analysis, using a modified silver staining technique which permits the visualization of the chromatid cores in bivalents at metaphase I, shows that some so-called chromosome associations are indeed true chiasmata. This study employs a supernumerary segment on the longest of the autosome pairs in the grasshopper Chorthippus jucundus that produces clear asymmetric bivalents in heterozygotes. Clear evidence of crossing over within the supernumerary segment explains part of the polymorphism of this segment, since unequal crossing over can produce different-sized segments. The origin of this segment and its mode of inheritance are also considered in the light of these results.     

Meiotic behavior of an unequal bivalent in the grasshopper Calliptamus palaestinensis Bdhr.

Summary Unequal bivalents were found in two of four populations of the short horned grasshopper, Calliptamus palaestinensis bdhr. sampled in Israel.The inequality of the homologues was due to an extra segment which was heterochromatic and apparently terminal. Pairing configurations at pachytene and position of chiasmata at later stages revealed however that the extra segment was interstitial, and the long member terminated in a minute segment homologous to the terminal part of its normal partner. The percentage of reductional divisions at anaphase I corresponded well with the percentage of terminal chiasmata (i.e. distal to the extra segment) observed at diakinesis.The assumption of an interstitial position of the extra segment would explain the previously puzzling examples of unequal bivalents in Orthoptera whose reductional divisions have not been readily accounted for otherwise.     

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.     

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.     

Comparative chiasma analysis using a computerised optical digitiser

A new computerised technique has been devised for measuring the distribution of chiasmata along diplotene bivalents. The method involves the introduction into the field of view of the microscope, of a fine light spot which can be accurately manipulated along the chromosomes of each bivalent. The data recorded include (a) the positions of the chiasmata along the bivalent in terms of their relative distances from the centromere and (b) the individual bivalent and cellular chiasma frequencies. — The method has been applied to the analysis of chiasma distribution patterns in the two known species of the genus Caledia, C. species nova 1 and C. captiva and in two chromosomal races of the latter. Statistical tests indicate that within bivalents at least 40% of the comparative distribution patterns of chiasmata between races and species are significantly different. Similar comparisons between populations within races reveal only 18% significant differences. — The observed distribution patterns of chiasmata in this genus suggest that chiasma formation is sequential from centromere to telomere. — The variation in the frequency and distribution of chiasmata between races and species suggests that the interference distances between successive chiasmata are, at least partially, independent of chiasma frequency and position. — The interracial and interspecific differences in chromosome structure are correlated with changes in chiasma pattern.     

Chromosomal similarities and differences among four Neotropical Elateridae (Conoderini and Pyrophorini) and other related species, with comments on the NOR patterns in Coleoptera

This work deals with the comparative cytogenetic analysis of four Neotropical Elateridae species and reviews the nucleolar organizer region (NOR) patterns on Coleoptera chromosomes, for the first time. The cytogenetic characterization of Conoderus malleatus (Conoderini), Pyrearinus candelarius, Pyrophorus divergens and Pyrophorus punctatissimus (Pyrophorini) was accomplished through the study of mitotic and meiotic cells submitted to standard (Giemsa) and differential staining [silver impregnation and GC‐specific chromomycin A₃ (CMA₃) plus AT‐specific 4′‐6‐diamidino‐2‐phenylindole (DAPI) fluorochromes]. The analysis of spermatogonial cells revealed the diploid numbers: 2n = 17 in C. malleatus and 2n = 15 in P. candelarius, P. divergens and P. punctatissimus. In these species, the X0 type sex‐determination system and the acrocentric morphology of almost all chromosomes were observed. The study of meiotic cells of the four species revealed the occurrence of total synapsis between the autosomes, the presence of one terminal or interstitial chiasma in the majority of the bivalents, and the reductional behaviour and regular segregation of all chromosomes. Although the three Pyrophorini species demonstrated many similar karyotypical characteristics, there was one discrepancy, which was noted in the diplotene cells and concerns the number of bivalents with two chiasmata; P. candelarius only presented one bivalent, P. divergens showed two bivalents and P. punctatissimus exhibited up to four bivalents with two chiasmata. Testicular cells impregnated with silver nitrate demonstrated two terminal NORs located on the fourth autosomal pair of the Conoderini species and on the second autosomal pair of the three Pyrophorini representatives. Use of CMA₃/distamycin A (DA)/DAPI staining on the P. candelarius and P. punctatissimus chromosomes revealed that the CMA₃ labelled regions were coincident with the NORs. The main strategies of karyotypical differentiation that have occurred among the four Elateridae species and other related species, and the general trends of the NOR shifts during Coleoptera chromosomal evolution are discussed in this work.     

Cyclic variation of chiasma frequency and distribution in Podarcis sicula (Reptilia: Lacertidae)

The seasonal variations of chiasma frequency and distribution have been studied in the lizard: Podareis sicula. In this species, as in Phyllodactylus (King & Hayman, Chromosoma 69: 131–154, 1978), chiasma frequencies vary following a definite annual cycle, and clearly different trends are shown by interstitial and terminal chiasmata.A comparison between these seasonal chiasma frequency variations and those of environmental temperature shows the existence of a clear correlation between these two parameters. However, this correlation is different in the two types of chiasmata, and may be different within the same type of chiasma depending on the period of the year.A more significant correlation is observed between chiasma cycles and annual variations of the haematic levels of sexual steroid hormones. In particular we observe a highly significant correlation between interstitial chiasma frequencies and testosterone concentration. A less precise correlation between terminal chiasma frequencies and estradiol concentration is also observed.In Podarcis, as in Phyllodactylus, the sperm that will be used for fertilization derive from the spermatocytes showing the highest rate of interstitial chiasmata. This supports the hypothesis that the cyclic variations in interstitial chiasma frequencies represent a mechanism to ensure an adequate level of variability in a given population. The above mentioned correlation between chiasma frequencies and steroia hormone concentrations suggests that the seasonal chiasma cycles are controlled by the same environmental and hormonal factors regulating the spermatogenetic cycle.     

Cytological studies of the two-spotted spider mite Tetranychus urticae Koch (tetranychidae,trombidiformes) II. Meiosis in growing oocytes

The morphology and behaviour of the holokinetic chromosomes of Tetranychus urticae Koch were studied in maturing oocytes from diplotene up to and including metaphase. Each of the three diplotene bivalents contains 2–3 chiasmata.During congression the dyads of the axially orientated bivalents make turning movements in opposite directions till they have reached an equatorial orientation which leads to an equational first meiotic division.It was argued that chiasmata terminalise towards both ends of the dyads and that each dyad contains two sister chromatids.It has been supposed that the inverted meiosis of organisms with holokinetic chromosomes can be brought about by the way in which chiasmata terminalise and the kinetochores are assembled subsequently on each chromatid.During congression vesicular strands form a Feulgen-negative zone round the bright plasm in which the bivalents are embedded. The question was raised whether these strands originate from the dense plasm surrounding each of the diplotene bivalents and bear some relation to the elimination chromatin present during the meiotic telophases.This investigation was supported by research grants from the Ministry of Public Health and Environment and from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).     

Genetic differentiation in populations of the yellow-necked mouse,Apodemus flavicollis,harbouring B chromosomes in different frequencies

Two alternative models are used to explain maintenance of polymorphism of B chromosomes (Bs) in populations of a great number of species. The parasitic model suggests deleterious effects of Bs on fitness of carriers, while the heterotic model assumes that, in the absence of drive, equilibrium is produced by beneficial effects of Bs at low numbers. In order to determine the potential contribution of Bs to genetic differentiation and diversity, four populations of Apodemus flavicollis, differing in frequency of Bs (from 0.23 to 0.38) and settled in ecologically different habitats, were analyzed by 471 AFLP markers. Although numerous loci were demonstrated to be population specific, none of them was associated with individuals with Bs. AMOVA showed that the presence of Bs does not affect population differentiation, pointing to greater genetic similarity of Bs to A chromosomes. The greatest genetic diversity (0.241) was found in the population settled in optimal conditions for this species featured by the lowest frequency of animals with Bs (0.23). We found that the majority of loci marked as loci under directional selection, are characteristic of either a population with lower or one with a higher frequency of Bs. Several loci detected as outliers were associated with environmental variables that could directly and/or indirectly influence population dynamics of A. flavicollis. Thus, we suggest that the different frequency of Bs carriers in populations is related to adaptive differentiation to diverse habitats, which is in accordance with the heterotic model of Bs maintenance.