Pascal Chavigny

Ecological specialization of the aphid Aphis gossypii Glover on cultivated host plants

Many plant‐feeding insect species considered to be polyphagous are in fact composed of genetically differentiated sympatric populations that use different hosts and between which gene flow still exists. We studied the population genetic structure of the cotton‐melon aphid Aphis gossypii that is considered as one of the most polyphagous aphid species. We used eight microsatellites to analyse the genetic diversity of numerous samples of A. gossypii collected over several years at a large geographical scale on annual crops from different plant families. The number of multilocus genotypes detected was extremely low and the genotypes were found to be associated with host plants. Five host races were unambiguously identified (Cucurbitaceae, cotton, eggplant, potato and chili‐ or sweet pepper). These host races were dominated by asexual clones. Plant transfer experiments using several specialized clones further confirmed the existence of host‐associated trade‐offs. Finally, both genetic and experimental data suggested that plants of the genus Hibiscus may be used as refuge for the specialized clones. Resource abundance is discussed as a key factor involved in the process of ecological specialization in A. gossypii.

Host‐based genetic differentiation in the aphid Aphis gossypii Glover, evidenced from RAPD fingerprints

Samples of the aphid Aphis gossypii (Glover) were collected from different host plants at 18 locations in southern France, La Réunion, Portugal and Laos. RAPD (random amplified polymorphic DNA) patterns of the 480 aphids were obtained using three random primers. A large number of RAPD bands were shared by all aphids of the 18 populations, but some RAPD bands appeared to be population specific. Over all aphids, a total of 37 polymorphic bands were identified and defined 142 RAPD phenotypes. A cluster analysis based on genetic distance revealed that the 18 aphid populations were divided into two groups, depending on whether they were collected on a cucurbit host plant. An analysis of molecular variance (AMOVA) was also performed and confirmed the differentiation into two groups. Several RAPD bands that were obtained using random primer A11 could be considered as diagnostic loci as they were fixed in populations collected on cucurbits and were always absent in those collected on noncucurbit host plants. These results represent the first evidence for genetic structuring within the species A. gossypii, according to host‐plant type.

Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France.

Aphis gossypii is an aphid species that is found throughout the world and is extremely polyphagous. It is considered a major pest of cotton and cucurbit species. In Europe, A. gossypii is assumed to reproduce exclusively by apomictic parthenogenesis. The present study investigates the genetic diversity of A. gossypii in a microgeographic, fragmented habitat consisting of eight glasshouses of cucurbit crops. This analysis, which was based on the results from seven microsatellite loci, has confirmed that A. gossypii populations in southern France are primarily asexual, as only 12 nonrecombinant genotypic classes (clones) were identified from 694 aphids. Moreover, a high proportion of the aphids (87%) had one of three common genotypes. No significant correlation was found between genotypic class and host plant species. Within a glasshouse population of A. gossypii, a significant reduction in clonal diversity was observed as the spring/summer season progressed. The final predominance of a clone could result from interclonal competition. At the microgeographic level (i.e. glasshouses within a 500‐m radius), significant genetic subdivision was detected and could be attributed to founder effects and the limitation of gene flow imposed by the enclosed nature of the glasshouse structure. Finally, the three common clones of A. gossypii detected in 1996 reappeared in spring 1997 following the winter extinction, together with rare clones that had not previously been seen. The probability that A. gossypii overwinters within refuges at a microgeographic scale from which populations are renewed each spring is discussed.

Characterization of microsatellite loci in the aphid species Aphis gossypii Glover

Baldwin BS, Black M, Sanjur O et al. (1996) A diagnostic molecular marker for zebra mussels (Dreissena polymorpha) and potentially co-occurring bivalves: mitochondrial COI. Molecular Marine Biology and Biotechnology, 5, 9Ð14. Claxton WT, Martel A, Dermott RM, Boulding EG (1997) Discrimination of field-collected juveniles of two introduced dreissenids (Dreissena polymorpha and Dreissena bugensis) using mitochondrial DNA and shell morphology. Canadian Journal of Fisheries and Aquatic Sciences, 54, 1280Ð1288. Hebert PDN, Muncaster BW, Mackie GL (1989) Ecological and genetic studies on Dreissena polymorpha (Pallas): a new mollusc in the Great Lakes. Canadian Journal of Fisheries and Aquatic Sciences, 46, 1587Ð1591. Lincoln S, Daly M (1991) Primer, Version 0.5. Whitehead Institute for Biomedical Research, Cambridge, MA. Marsden JE, Spidle AP, May B (1996) Review of genetic studies of Dreissena spp. American Zoologist, 36, 259Ð270. Mills EL, Dermott RM, Roseman EF et al. (1993) Colonization, ecology, and population structure of the ÔquaggaÕ mussel (Bivalvia: Dreissenidae) in the lower Great Lakes. Canadian Journal of Fisheries and Aquatic Sciences, 50, 2305Ð2314. Mills EL, Rosenberg G, Spidle AP et al. (1996) A review of the biology and ecology of the quagga mussel (Dreissena bugensis), a second species of freshwater dreissenid introduced to North America. American Zoologist, 36, 271Ð286. Naish K-A, Skibinski DOF (1998) Tetranucleotide loci for Indian Major Carp. Journal of Fish Biology, 53, 886Ð889. Spidle AP, Marsden JE, May B (1994) Identification of the Great Lakes quagga mussel as Dreissena bugensis from the Dneiper River, Ukraine, on the basis of allozyme variation. Canadian Journal of Fisheries and Aquatic Sciences, 51, 1485Ð1489.

Genotypic diversity of the cotton-melon aphid Aphis gossypii (Glover) in Tunisia is structured by host plants

The study of intraspecific variation with respect to host plant utilization in polyphagous insects is crucial for understanding evolutionary patterns of insect-plant interactions. Aphis gossypii (Glover) is a cosmopolitan and extremely polyphagous aphid species. If host plant species or families constitute selective regimes to these aphids, genetic differentiation and host associated adaptation may occur. In this study, we describe the genetic structure of A. gossypii collected in six localities in Tunisia on different vegetable crops, on citrus trees and on Hibiscus. The aim was to determine if the aphid populations are structured in relation to the host plants and if such differentiation is consistent among localities. The genetic variability of A. gossypii samples was examined at eight microsatellite loci. We identified only 11 multilocus genotypes among 559 individuals. Significant deviations from Hardy-Weinberg equilibrium, linkage disequilibria and absence of recombinant genotypes, confirmed that A. gossypii reproduces by continuous apomictic parthenogenesis. Genetic differentiation between localities was not significant, whereas a strong differentiation was observed between host plant families (0.175<FST<0.691). The great majority of aphids exhibited one of three predominant multilocus genotypes that were repeatedly and respectively associated to the three plant families, Cucurbitaceae, Solanaceae and Rutaceae, demonstrating host specialization in A. gossypii. These specialized genotypes were simultaneously found with other clones on Hibiscus, suggesting that this perennial host could act as a refuge plant between two vegetable crop seasons.

Inter-strain competition and dispersal in aphids: evidence from a greenhouse study

1. Crops are often colonised by aphids having different life‐history traits, and the aphids and their offspring compete for the shared resource. The intraspecific competition and dispersal characteristics of two strains (A and B) of the cotton aphid Aphis gossypii on cucumber were examined in a greenhouse study. Strain A normally feeds on cucumber, whereas strain B originated from melon but develops on cucumber. Both strains reproduce exclusively via parthenogenesis and can be discriminated using molecular markers.

A single molecular marker to distinguish between strains of Trichogramma cacoeciae

Abstract A single microsatellite marker was used to distinguish between strains of Trichogramma cacoeciae Marchal collected from the carob moth (Ectomyelois ceratoniae) in Tunisia and from the grapevine moth (Lobesia botrana) and a hawk moth in France. This marker can be used as an identification and monitoring tool in pest management programs.

Spatial and Temporal Genetic Variation in Tunisian Field Populations of Macrosiphum euphorbiae (Thomas)

ABSTRACT The potato aphid Macrosiphum euphorbiae (Thomas) is a major pest of several economic crops in Tunisia. Using 10 microsatellites, we analyzed five populations of M. euphorbiae sampled during the 2004–2005 solanaceous and cucurbit season (April through September) from five geographic origins. From 235 aphids, 61 different multilocus genotypes were identified of which three genotypes MLG1, MLG2, and MLG42 were predominant on all host plants and regions. MLG1 and MLG2 genotypes were detected in 2004 and did not reappear in spring 2005, while the genotype MLG42 was detected only in 2005. All populations showed significant deviation from Hardy—Weinberg equilibrium even in data sets including one individual per genotype, &khgr; 2 independence tests and analysis of molecular variance showed no significant differentiation among populations collected on different host plant and from different geographic origin, but differences between populations from two successive years were significant. Factorial component analysis corroborates these results. The probable causes of this seasonal variation were discussed.