Perfect knowledge of the taxonomy and biology is a prerequisite for all research projects using aphids as biological model. But in return, no modern taxonomic studies could be conducted without relying on the results of some of these researches (i.e. phylogenetic reconstructions, knowledge on the evolution of morphological and biological characters, genetic divergence between populations and species). Moreover, most of the new specimens that enrich the collection are collected during these research programs. We are currently involved in the following research programs:
Cinara wahtolca © Hotte, 1953
This program aims at measuring the relative weight of ecological factors and geographical isolation on the speciation processes inside the genus Cinara.
The conifer aphid genus Cinara Curtis, 1835 comprises 243 described species. Approximately 154 of them are native to North Americ , 47 have originated in Europe and the Mediterranean area and about 40 occur in the Far East. Cinara species feed exclusively on the two conifer families that are found primarily in temperate and subtropical regions of the Northern hemisphere: Pinaceae and Cupressaceae.
Most Cinara feed on a single or a few species of conifers, while others are less discriminatory, feeding on several species within a genus (e.g. C. pinea (Mordvilko, 1895) and C. pergandei (Wilson, 1919) on Pinus spp.) and sometimes even on several unrelated species of conifers (e.g. C. confinis (Koch, 1856) on Abies spp. and Cedrus spp.). Species of Cinara have specific feeding sites on their host plants. Some species feed only on young shoots, while others feed exclusively on large trunks. Host plants are considered as the main ecological factor involved in the speciation process of phytophagous insects. Speciation within genus Cinara is thought to be mostly driven by host switch.
However, some of their biological features also make Cinara species susceptible to geographic reproductive isolation. First, they have limited dispersal ability, compared to other aphid species. Their weight/wing length ratio is high and some species are not recorded as producing winged morphs, which are the only ones able to disperse over large distances. Then, because of their association with some conifer species living at higher altitude (some Pinus, Picea and Abies spp.), some species are restricted to mountain ranges. Hence, many Cinara species show disjunct distributions and can encompass several allopatric populations. Allopatric speciation is a possible scenario for the diversification of Cinara with limited dispersal ability and/ or associated with plants that have patchy distributions.
The objectives of this study are to reconstruct the phylogeny and the biogeographical histories of Cinara species and that of their host, in order to investigate general trends in the diversification of this genus.
Brachycaudus helichrysi (Kaltenbach, 1843) is a polyphagous, cosmopolitan aphid. It has been reported to have a heteroecious lifecycle, alternating between several Prunus (Rosaceae) species as primary hosts and numerous herbaceous secondary hosts, mostly from the Asteraceae and Boraginaceae. This species is known as the leaf-curl plum aphid and is considered a major pest of, plum orchards and ornamental flowers, such as chrysanthemums. Brachycaudus helichrysi colonies cause damage by curling the leaves of their host plants and transmitting viruses, including the plum-pox virus, which causes a very serious viral disease of stone fruits.
We conducted a phylogeography study on B. helichrysi specimens collected around the world using several genetic markers. We showed that B. helichrysi actually comprises two lineage, hereafter called B. helichrysi H1 and B. helichrysi H2, separated by a genetic distance similar to that generally found between sister species within the genus Brachycaudus. Our phylogenetic studies suggest that these lineages could be two well differentiated sibling species. As B. helichrysi H1 could not be distinguished from B. helichrysi H2 on the basis of morphological features, we have developed a discrimination test, based on RFLP (restriction fragment length polymorphism).We used this test to investigate the patterns of host plant association and geographical distribution of these lineages, with a large sample of individuals collected from numerous host plants and locations worldwide. We found that H2 lineage was never present on plum trees and that individuals from both lineages coexisted on many secondary herbaceous hosts at almost all the sites studied. We also determined whether the distribution of the two lineages could be explained by climatic features. We found that H2 was not found in regions with harsh winters. This suggests that this lineage may be an obligate parthenogenetic aphid.
We used further ecological and genotypic data (14 microsatellite loci) to investigate B. helichrysi H1 and B. helichrysi H2 biological features (e.g. host range, life cycle) and evolutionary history.
B. helichrysi H1 life cycle is mainly cyclical parthenogenesis,. Winter hosts belong to the genus Prunus subsection Prunus. In accordance with these assumptions, our genetic data show that H1 specimens are grouped according to their geographical origin. These geographic clusters are the signatures of species undergoing sexual reproduction, with reproductive isolation being associated with geographic distance. These genetic data suggest also that H1 populations sampled in West coast of North America have a European origin. This validates the hypothesis that this species was introduced to the USA from the European continent at the beginning of the century, perhaps with the culture of the plum.
B. helichrysi H2 is mainly constituted of 8 obligate asexual lineages that are distributed world-wide (superclones) , and some elusive sexual lineages, probably heteroecious, found on Prunus persica in India. Analyses of the geographic distribution of B. helichrysi H2 relative to climatic variables and host plant association do not show any adaptation of these superclones to particular environmental condition or to different host plant ranges. These superclones seem to be generalist genotypes. Their distributions (some of them are present in three continents) suggest long distance dispersion events probably by the mean of horticultural/ornamental exchanges (i.e. Chrysantemum, Achillea).
Several questions remain about the history of the colonization of the continents by the two lineages of B. helichrysi. What is the impact of the history of domestication of Prunus and, more recently, that of the transport of plants (trees and grasses) infested by B. helichrysi on the spread of the two lineages within this species?
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COPYRIGHT: Coeur d'acier et al. (2014). DNA barcoding and the Associated PhylAphidB@se Website for the identification of European Aphids (Insecta: Hemiptera: Aphididae). Plos One, 9 (6) to use the database in a publication. DOI:10.1371/journal.pone.0097620
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