Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2011
Publication Date: 4/1/2011
Citation: Telesnicki, M.C., Sosa, A.J., Greizerstein, E., Julien, M. 2011. Cytogenetic effect of Alternanthera philoxeroides (alligator weed) on Agasicles hygrophila (Coleoptera: Chrysomelidae) in its native range. Biological Control. Vol(57)138-142. Interpretive Summary: Genomes of living organisms are inside the cell in chromosomes. Sometimes the number of chromosomes can vary among and within a species. Plants in the same group or species can have different chromosome numbers and the species could be characterized according to this parameter. Several aspects of plant genetics and plant-insect interaction have been explored but what happens with insects in relation to plant-chromosome numbers is still largely unexplored. The amphibian plant alligator weed invaded aquatic and terrestrial habitats of USA, Australia and other countries. In Argentina, where it is native, a complex of hybrids (populations with different chromosome numbers) was identified, and on the basis of this information it is believed that the plant evolved gaining more sets of chromosomes throughout its history. Populations with 4 sets of about 16 chromosomes (total 66), called tetraploids, and 6 sets of about 16 chromosomes (total 100), called hexaploids, were found in Argentina. A flea beetle was introduced, from this region, into several countries to control alligator weed, successfully in most of the situations. Several hypotheses tried to explain the failure to control the weed in some locations, from stem characteristics to abiotic factors such as temperature or humidity. The different chromosome numbers of alligator weed were never considered as a factor that could play a role on insect performance. In this work we evaluated the development and biology of this insect considering the genetic characteristics of alligator weed, in this case chromosome numbers, in relation to insect fecundity and survival. We found that this affected insect survival and fecundity, but no other characteristics such as insect life cycle length nor larval mortality, pupae mortality or female proportion. We concluded that the effect of the ploidy level (chromosome numbers) of alligator weed on its biological control seems negligible.
Technical Abstract: Plant polyploidy potentially affects plant-insect interactions; however, its effect on insect fitness remains largely unexplored. Alternanthera philoxeroides is a South American amphibious Amaranthaceae, which invades aquatic and terrestrial habitats. Different morphotypes and cytotypes were identified and further associated with a polyploid hybrids complex in Argentina, which is probably the centre of the origin and diversification of alligator weed. The current model of the hybrid complex formation proposes that diploid ancestors gave rise to allotetraploids and allohexaploids. Agasicles hygrophila is a monophagous insect that successfully controls A. philoxeroides in regions with mild winters and where the weed grows in aquatic habitats. However, both the ploidy level and the genomic composition of the plants may constrain insect development and reproduction, providing an alternative explanation to the restricted success of biological control of alligator weed with A. hygrophila. We investigated the effect of plant ploidy of four cytotypes of A. philoxeroides on plant-insect interaction with A. hygrophila in the native range of the weed. The life cycle of the insect on each host population was studied. In addition, the genetic relationships among the weed populations were explored using dot blot analysis. Plant ploidy level affected insect survival and fecundity, but no effect could be attributed to plant genomic composition. On the other hand, plant cytotype did not affect life cycle length nor third larval instar mortality, pupae mortality or female proportion. Plant internode diameter was not a restriction for insect pupation. All hexaploid populations of A. philoxeroides, as well as the close related A. aquatica, have the same affinity with the only tetraploid population considered. We conclude that the effect of the ploidy level of alligator weed on its biological control seems negligible.