Abstract
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 stud- ied. 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 Parana 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 equi- libria. The rearrangements cause a drastic chiasma repatterning in the fusion bivalents (or trivalents) which reduces the proximal chi- asma 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.
Highlights
The water-hyacinth grasshopper Cornops aquaticum (Orthoptera: Acrididae) is a Neotropical grasshopper that lives in close association with plants of the genera Eichhornia and Pontederia, on which this species feeds and lays eggs (Adis & Junk 2003, Adis et al, 2004)
C. aquaticum was considered as a possible biological control agent, and is the main one for many workers, because of ecological considerations
Its cytogenetics remains largely unknown; only two reports describe the chromosome number of this species (Mesa, 1956; Mesa et al, 1982), both of which indicate that the chromosome number for C. aquaticum is the typical one for acridoid grasshoppers, modified by the occurrence of three centric fusions; it is not known whether the centric fusions are polymorphic or polytypic
Summary
The water-hyacinth grasshopper Cornops aquaticum (Orthoptera: Acrididae) is a Neotropical grasshopper that lives in close association with plants of the genera Eichhornia and Pontederia, on which this species feeds and lays eggs (Adis & Junk 2003, Adis et al, 2004). C. aquaticum was considered as a possible biological control agent, and is the main one for many workers, because of ecological considerations. Nowadays, it is being studied for possible release in South Africa (Oberholzer & Hill, 2001). As Rb polymorphisms are likely to be important features of the genetic system of a species, and given the ecological importance of Cornops aquaticum, it was decided to undertake a population cytogenetic study of this species. In a previous study (Colombo, 2007), the chiasma effects of the Robertsonian rearrangements were described, and in the present contribution behaviour and geographical distribution of these polymorphisms is analysed
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