Chromosome, genetic and morphometric variation of the aquatic grasshopper Cornops aquaticum (Bruner) (Acrididae: Leptysminae) in the Middle and Lower Paraná River, Argentina.

Cornops aquaticum inhabits and feeds on water-hyacinths of the genera Eichhornia and Pontederia in the Neotropical region. The blue-flowered waterhyacinth, Eichhornia crassipes, has been introduced elsewhere and it has become a weed; recently C. aquaticum was considered as a possible biological control agent. Analysis of phenotypic, chromosome and genetic variability is indispensable to infer the population dynamics of this grasshopper species proposed for pest management. In the lower course of the Paraná River, the karyotype of C.aquaticum includes three centric fusions (1/6, 2/5 and 3/4).. Fusion frequencies increase southwards showing a geographical cline. The present study analyzes four chromosomally differentiated populations assessing morphometric variation and genetic diversity through microsatellite markers. Morphometric analysis in two chromosomally polymorphic populations revealed that there is a considerable variation in adult male size and part of this variation was related to 1/6 y 3/4 fusions. Positive and significant correlations were detected between fusion 1/6 and femur, tibia and tegmina length (p<0.05 in all cases). Fusion 3/4 also showed a positive and significant relationship with tegmina length (p<0.05). Microsatellite analysis in three chromosomally differentiated populations revealed high levels of genetic diversity estimated through number of alleles per locus (6,5-12,6), mean expected heterozygosity (0,89-0,98) and mean allelic richness (8,35-10,22). Genetic differentiation assessed through analyses of molecular variance (AMOVA) showed that most of the genetic variation was found within populations (97.49 %) whereas there is low variation between populations (2.51 %), suggesting high levels of gene flow among them. The disagreement between chromosomal and molecular differentiation may reflect different evolutionary histories for both markers and point out the importance of the adaptive effects of chromosomal polymorphisms and coadapted supergenes. The present results improve our knowledge of the population structure in the water-hyacinth grasshopper which will be useful to propose appropriate methods for its release in biological control programs.