C. García de la Vega

A hybrid zone between two subspecies of Chorthippus parallelus. X-chromosome variation through a contact zone

Two grasshopper subspecies, Chorthippus parallelus parallelus (Cpp) and Chorthippus parallelus erythropus (Cpe), meet along the Pyrenees where they hybridize and produce a hybrid zone. A contact zone located in the Col de Portalet has been analyzed for the distribution of chromosome markers on the sex (X) chromosome in pure and hybrid populations. C‐banding allowed us to distinguish both pure subspecific forms and recombinant forms, and to demonstrate their different frequencies through the contact zone. Interestingly, the distal C‐band (P) on the X chromosome that characterizes Cpp individuals occurs at very high frequency through the zone and then drops sharply, close to where pure Cpe populations are found. A novel interstitial C‐band (H) has been found, probably derived from that characterizing Cpe individuals (E). This marker band is only present in hybrid populations. These data are discussed in the light of the dynamics of the hybrid zone.

Heterochromatin differentiation between two species of the genus Dociostaurus (Orthoptera: Acrididae)

Dociostaurus jagoi and Dociostaurus genei are two acridoid grasshoppers which show large differences in heterochromatin content. The use of several banding techniques, including fluorochrome staining and restriction endonuclease digestion, provides further information on the characteristics of the constitutive heterochromatin regions revealed with the C-banding method. Thus both species show GC-enriched bands accompanying active nucleolar organizers which differ in location. Because of its bright response to DAPI and CMA3, the centromeric heterochromatin seems to have a bipartite nature in terms of DNA composition in D. genei, while it shows dull staining with both fluorochromes in D. jagoi. However, two endonucleases, Mbol and Sau3A, extensively digest these regions in both species. The supernumerary heterochromatic segments present only in D. genei seem to be AT-rich and are extensively digested with Alul. These results reveal heterogeneity in the distinct C-banded regions which, in turn, are equilocally distributed in both chromosome complements.

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.

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.

Equilocality and heterogeneity of constitutive heterochromatin : in situ localization of two families of highly repetitive DNA in Dociostaurus genei (Orthoptera)

The chromosome complement of the grasshopper Dociostaurus genei is characterized by the presence of constitutive heterochromatin (C-bands) located in the centromeric regions of all the chromosomes and in the distal regions of some autosomes in the form of supernumerary segments. A sequence analysis was carried out to obtain information about the molecular characteristics of both heterochromatic regions. Two families of tandemly repetitive DNA (DgT2 and DgA3) from D. genei were cloned and characterized. Data obtained from in situ hybridization indicate that these families are located solely in the regions of constitutive heterochromatin. The DgT2 clone is representative of a family of sequences which mainly forms the centromeric C-bands in each chromosome of the complement. The DgA3 family is the major component of the distal C-bands (supernumerary segments) present in most of the autosomal pairs. These results show the existence in D. genei of two different families of repetitive DNA restricted to different chromosomal domains. We discuss these results in the light of the possible role of chromosomal disposition in the maintenance of the differences between heterochromatic DNA from different chromosomal regions and the homogenization of DNA sequences from equilocal chromosomal domains.

Changes in acridine orange binding and its use in the characterisation of heterochromatic regions

Heterochromatic regions of the mountain grasshopper Arcyptera fusca have been stained with acridine orange (A.O.) under different conditions. Results show a certain amount of differentiation between centromeric and telomeric regions as regards their staining properties. Thus, A.O. seems able to differentiate homogeneous C-bands. In this response not only the DNA but also the fraction of proteins which remain in the chromatin may be involved.

The role of sister chromatid cohesiveness and structure in meiotic behaviour

Abstract. Sister chromatid cores, kinetochores and the connecting strand between sister kinetochores were differentially silver stained to analyse the behaviour of these structures during meiosis in normal and two spontaneous desynaptic individuals of Chorthippus jucundus (Orthoptera). In these desynaptic individuals most of the chromosomes appear as univalents and orient equationally in the first meiotic division. Despite this abnormal segregation pattern, the changes in chromosome structure follow the same timing as in normal individuals and seem to be strictly phase dependent. Chromosomes in the first prometaphase have associated sister kinetochores and sister chromatid cores that lie in the chromosome midline; we propose that this promotes the initial monopolar orientation of chromosomes. However, the requirements of tension for stable attachment to the spindle force the autosomal univalents to acquire amphitelic orientation. Sister kinetochores behave in a chromosome orientation-dependent manner and, in the first metaphase, they appear to be interconnected by a strand that can be detected by silver impregnation, as seen in the second metaphase of wild-type individuals. The disappearance of the sister kinetochore-connecting strand, needed for equational chromatid segregation, however, can only take place in the second meiotic division. This connecting strand is ultimately responsible for the inability of chromosomes to segregate sister chromatids in the first anaphase.

Effects of supernumerary heterochromatin on chiasma formation and chromosome segregation in Dociostaurus genei (Orthoptera)

Dociostaurus genei is a grasshopper in which most of the autosomes present polymorphisms for distal heterochromatic supernumerary segments. C-banding, DAPI banding and fluorescence in situ hybridization (FISH) of a probe, which is representative of the highly repetitive DNA sequences comprising the segments, have been used to analyse the effects of the segments on chiasma formation. Results show that the supernumerary segments are associated with two major effects. First, a higher content of supernumerary heterochromatin is correlated with the formation of a lower number of chiasmata within a given chromosome complement. Secondly, because the methods used strongly suggest that the distal segments are truly terminal, they could be considered to be involved in the maintenance of achiasmate associations that persist up to the first anaphase migration. These terminal associations occurred in about 50 per cent of homozygous and heterozygous bivalents and did not impair the correct segregation of homologues at anaphase I.

Chromosome differentiation in the Triturus alpestris complex (Amphibia: Caudata)

Several individuals from distinct populations of 7 subspecies included in the Triturus alpestris complex have been analysed to obtain characteristic C-banding patterns.The differences found among the populations as regards the amount, complexity and distribution of heterochromatin bands have been employed to test a pattern of chromosome evolution.Relevant similarities and dissimilarities of these populations are discussed in the light of the tentative evolutionary history inferred from a rooted tree.

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.