Leptysma argentina is polymorphic for a centric fusion between pairs 3 and 6 of the basic complement, for interstitial (S1) and proximal (Sn) supernumerary segments in the smallest member of the complement, and for a B-chromosome. Fusion 3/6 is associated with several effects that would lead to a drastic reduction of recombination (Colombo 1987(Colombo , 1988. Nine populations from the Argentine provinces of Buenos Aires, Entre RIos and Santa Fe were analysed for total (T), proximal (P), interstitial (I) and distal (D) chiasma frequencies, and for the morphometrical characters tegmina length, total length, femur length, tibia length, thorax length and thorax height. There was a strong negative correlation of T, P and I frequencies, and a significant positive correlation of D, with fusion 3/6 frequency. Fusion carriers were significantly bigger than basic homozygotes, leading to highly significant correlations of all morphometrical variables with fusion frequency.The frequencies of the fusion, segment S and the B-chromosome follow a geographical pattern. The uneven distribution of the fusion is explained by effects on several exo-and endophenotypic traits. The nonrandom distribution of the B-chromosome is attributed to differential tolerance under differing chromosomal, genetical and/or ecological environments. No significant effects were detected for segments but S, is geographically widespread, arguing against its neutrality, while the low frequency and even frequency distribution of S, suggests maintenance by mutation.

Abstract. C. aquaticum (Acrididae: Leptysminae) inhabits water-hyacinths in the Neotropical region. The blue-flowered waterhyacinth, Eichhornia crassipes, has been introduced elsewhere without its natural enemies and has become a weed; recently C. aquaticum was considered as a possible biological control agent. In this work, six populations were sampled and cytologically studied. C. aquaticum has 2n = 23 chromosomes in males and 24 in females, with an X0/XX sex determination system. All chromosomes are acro-telocentric and the basic karyotype includes three Robertsonian (Rb) translocations between pairs 1/6, 2/5, and 3/4. These polymorphisms are restricted to the lower course of the Paraná river, between Rosario and Buenos Aires. Fusion frequencies increase southwards, thus showing a geographical cline. The polymorphisms mostly accord with Hardy-Weinberg and gametic phase equilibria. The rearrangements cause a drastic chiasma repatterning in the fusion bivalents (or trivalents) which reduces the proximal chiasma frequency. Recombination is also reduced due to the loss of independent segregation. A recombination index that takes into account both these factors correlates negatively with the number of pairs of chromosomes affected by fusions among populations, which does not accord with the "central-marginal" pattern.

Six populations of Trimerotropis pallidipennis located along an altitudinal gradient in the Antinaco-Los Colorados valley (La Rioja, Argentina), were cytologically analysed. The male karyotype consists of 23 chromosomes (22+XO) with three long pairs submetacentric, the X-chromosome metacentric and the remaining ones basically acrocentric. Populations from La Rioja were polymorphic for seven pericentric inversions. Correlations between chromosome frequencies and altitude were statistically tested. In most cases, variables were either positively or negatively related in a significant fashion. As a consequence, mean populational heterozygosis (measured as the mean number of heteromorphic bivalents per individual per population) was also positively correlated with altitude. These results suggest that altitude (or other correlated environmental variable) may exert a differential selective pressure on coadapted gene blocks in the mutually inverted sequences. The possible relation of these results with the centralmarginal model is discussed.

It has long been assumed, since the early works of Mather, that the centromere plays a central role in chiasma position determination, so much so that in all sequential models chiasma determination was supposed to start or finish at this point. More specifically, it has been asssumed that the centromere acts as a barrier to the transmission of interference, so that a chiasma in the vicinity of a centromere would not affect the probability of chiasma formation across this point. Some statistical analyses seemed to ratify this supposition. However, a reassessment of the literature led us to the conclusion that the statistical analyses that were not flawed were consistent in showing that interference may act across the centromere. Using large sets of chiasma data from the grasshoppers Lepysma argentina and Chorthippus brunneus and applying statistical approaches that involved either the calculation of coincidence or correlating the distances between the centromere and the nearest chiasma in either arm, it is concluded: 1 that interference acts across the centromere; 2 that the action of interference is not changed by the presence of an intervening centromere.

Abstract. The New World grasshopper Cornops aquaticum (Leptysminae: Acrididae) shows a geographical pattern for three Robertsonian polymorphisms in its southernmost area of distribution in Argentina and Uruguay. The frequency and distribution of chiasmata were analysed in five Argentinian populations. This study reveals a strong redistribution of chiasmata in fusion carriers, with a reduction in proximal and increase of distal chiasma frequency in fusion bivalents and trivalents, when all three karyotypes were compared. However, when only fusion bivalents and trivalents were compared, chiasma frequency was significantly higher in the former than in the latter. This higher chiasma frequency in fusion bivalents is due to an increase in proximal chiasma frequency. It is argued that the reduction in proximal chiasma frequency (relative to unfused bivalents) in fusion bivalents may be due to interference across the centromere. Proximal chiasma reduction in trivalents may be attributed either to a physical effect of structural heterozygosity or to an adaptation to the polymorphic condition. Therefore the differences in the distribution of chiasmata in trivalents and Robertsonian bivalents have different causes. 653* Affiliated to CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas).

Correlation of a trait with environmental factors is a common means of measuring natural selection in natural populations. If the same correlation is seen in independent groups of populations over a wide area, selection is then a likely explanation. Natural populations of T. pallidipennis (2n = 23, X0) — polymorphic for 3‐7 pericentric inversions in 4 medium‐sized pairs — show altitudinal clines for 9 chromosome sequences over a wide area in Argentina. Joint analysis has shown correlations with latitude and longitude. In order to identify the climatic variables responsible for this clinal variation, 7 populations sited on an altitudinal gradient were scored for inversion frequency. Multiple regression analyses among chromosome frequencies and environmental variables do not only confirm the consistency of altitudinal gradients for all sequences but also identify minimum temperature (Tmin) as the main climatical variable responsible for this pattern, to such extent that populations with extreme Tmin values are monomorphic. In addition, some sequences are correlated with humidity. These results do not only reinforce the hypothesis of a selective origin for cline maintenance but are also a new example of monomorphism associated to marginal environments. Three inversions (4A1, 7SM2, 8SM4) correlate simultaneously with altitude, latitude, and minimum temperature; two other inversions (6M and 8SM4) correlated with longitude and humidity, because this variable decreases westward in Argentina. The identification of the causes of clinal variation leads to a predictive pattern of inversion distribution for still unexplored regions.

The neotropical genus Dichroplus and related genera are characterized by a relatively uniform external morphology and a remarkably divergent male genitalia and hence its taxonomy is controversial. It also shows an extreme karyotypic diversification. In this study we used molecular and morphological characters to test the monophyly of the genus and to evaluate chromosome evolution. Twenty-seven species from Dichroplus and related genera were included in the analysis. Morphological characters refer to the general morphology, male genitalia and female structures. Molecular studies were performed, sequencing part of two mitochondrial genes, cytochrome oxidase I and II. Independent and combined phylogenetic analyses of the data were performed under maximum parsimony. The karyotypic characters (rearrangements) were either mapped onto the combined topology or combined with the other data sets. While the molecular analysis confirms some results attained with morphology, some others do not. All point towards the paraphyly of the genus. Our results show the relevance of morphological data in phylogenetic studies because morphology and molecules supply complementary evidence. The mapping of chromosome characters on the combined tree shows that the most extreme karyotype, in D. silveiraguidoi, is a derived condition, probably reached through several centric fusions, and that X-autosome centric fusions were recurrently fixed during the evolution of the group.

Leptysma argentina is a highly polymorphic South-American grasshopper from the cytological point of view; all populations so far studied carry a polymorphic fusion between pairs 3 and 6. In heterozygotes, the trivalent 3-3/6-6 shows alternate orientation in 97.17°70 of the cells. Trivalent chiasma frequency is significantly lower than in the combined 3 and 6 bivalents of the standard homozygote; besides, there is a marked displacement of chiasmata to a distal position. In structural homozygotes the same effects, but not so marked, were observed.One individual was a double heterozygote for both the polymorphic centric fusion and a spontaneous one between pairs 5 and 7. The presence of a fragment, sometimes associated with the centromeric region of the nonfused 5 chromosome, was detected in more than 50°70 of the cells. The orientation of trivalent 5-5/7-7 in metaphase I was highly irregular (36°70 linear orientation). Neither frequency nor position of chiasmata were altered in trivalent 5-5/7-7 when compared with bivalents 5 +7 of normal individuals.The results suggest that proximal and interstitial chiasma reduction observed in trivalent 3-3/6-6 of L.argentina is due to a later adaptation to the polymorphic condition or a fortuitous genetic condition present in the original mutant, rather than to a direct effect of the fusion itself on chiasma distribution.