C. aquaticum (Acrididae: Leptysminae) inhabits water-hyacinths in the Neotropical region. The blue-flowered water-hyacinth, 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.
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.
The water-hyacinth grasshopper, Cornops aquaticum, occurs in freshwater environments in the New World between latitudes 23°N and 35°S. At the southernmost margin of this distribution the populations are polymorphic for three centric fusions (Robertsonian translocations). The frequencies of these chromosome rearrangements increase southwards and the recombination in structural homozygotes and heterozygotes diminishes both along the middle and lower courses of the Paraná River. In the present paper we report a similar cline along the southward flowing Uruguay River. In addition, we report the morphological effects of two of these centric fusion polymorphisms, namely the fusions between chromosomes 2 and 5 of the standard complement (fusion 2/5) and chromosomes 3 and 4 (fusion 3/4) and extend this study to the Uruguay River. There is a strong inverse correlation of fusion frequency with temperature, which indicates that these polymorphisms may be related to increased tolerance of colder climates in this originally tropical species, or some other correlated variable. This study is a further example of chromosomal clines correlated with latitude and is one of a few examples of chromosome polymorphisms associated with phenotypic effects. Finally, it indicates ways of using this species for controlling pests., Pablo C. Colombo, María I. Remis., and Obsahuje bibliografii