Pianificazione di strategie di controllo della zanzara Ochlerotatus caspius in Nord Italia attraverso un approccio genetico molecolare
Author(s)
Porretta, Daniele
Date Issued
November 5, 2008
Type
Doctoral Thesis
Abstract
Ochlerotatus caspius is a mosquito of great public health and economic impact in Mediterranean countries such as Spain, France, Italy and Greece where it is widely spread. Given its marked anthropophily, it significantly impairs the liveability of large areas and has thus become a major target of mosquito control programs implemented by local authorities. The total yearly cost of control programs in the Po Delta area, Po plain in the Veneto and Emilia regions and rice-growing areas in Lombardy and Piedmont was estimated in about 10 million euros in 2004. In Northern Italy Oc. caspius is widespread both in the coastal plain and inland, where rice fields constitute optimal breeding sites.
The overall objective of this study was to improve our knowledge of the biology, genetic and ecology of Oc. caspius in the Po plain area in order to planning optimal control strategies of this mosquito species. Firstly, we used both mitochondrial and nuclear markers to investigate the population genetic structure and diversity of Oc. caspius. Our results suggested that the migration pattern among populations of Oc. caspius in Northern Italy appears to be high enough to maintain an extensive genetic homogeneity. At a local geographical scale, evidence of a recent population size growth was found in several populations. The increased availability of large breeding sites recently offered by rice paddies seem to have played a major role in giving rise to the observed demographic expansions. Indeed, these sites have become more numerous and undergone changes in the management technique (increased number of dry/flood cycles), which enhanced their suitability as breeding habitats. Therefore, satisfactory control programmes should: i) be mainly focused on rice paddies as breeding habitats, ii) favour agronomical techniques involving a reduced number of dry/flood cycles, iii) be carried out on a wide (regional) geographical scale, iv) periodically monitor the susceptibility of populations to the compounds used in control campaigns and adopt strategies aimed to avoid the possible spread of resistance alleles.
In the second part of this research we studied in Oc. caspius the possible existence of a defence mechanism against insecticides due to P-glycoproteins (P-gps) efflux transporters or multidrug transporters. P-gps are cell membrane transporters and are found in all living cells protecting them from multiple structurally dissimilar toxic compounds. Their action consist in transporting toxic substances outside the cell, reducing the concentration inside cells as much as possible. The purpose of this study was to examine in the mosquito Oc. caspius the possible role of P-gps in the defence against insecticides temephos, ivermectin and diflubenzuron and to isolate genomic DNA sequences encoding for P-gp transporters. Mosquito larvae were treated with insecticides alone and in conjunction with a sublethal dose of P-gp inhibitor verapamil. The inhibition of P-gps reduced the LD50 values of temephos and diflubenzuron by a factor of 3.5 and 16.4 respectively, suggesting the potential involvement of P-gps in insecticide defence. Using a polymerase chain reaction (PCR)-based approach, a 476-base-pair sequence was isolated, whose inferred aminoacidic sequence shows high homology with the C-terminal sequence of known P-gps.
The existence of such a mechanism protecting insects from a broad spectrum of chemical products and thus the possibility that other sites are involved in endowing them with resistance poses an additional challenge to pest control planners. However, identifying P-gp sites with the ultimate aim of inhibiting them could represent a new control strategy. By increasing the susceptibility of organisms to insecticides, it would be possible to reduce their doses and frequency of application. The isolation and preliminary characterization of a P-gp putative sequence from Oc. caspius is the first approach to a better molecular understanding of the role played by multidrug transporters in the defence of this species against insecticides, which could open the way to novel control strategies based on the inhibition of pest defences.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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