Systématique du genre Essigella (Hemiptera : Sternorrhyncha) au moyen de données moléculaires

Background: Host plant specificity is a common phenomenon in phytophagous insects and notably in aphids. Host identity is therefore often used in species recognition, for its practicality but also for groups for which few morphological characters are reliable. This issue is notably important in the case of cryptic species that, by definition, exhibit no morphological difference with related species. However, populations of one same insect species living on different host plants can sometimes show host-mediated phenotypic differences. As a consequence, although host plant association can be a practical diagnostic character, it can also be misleading when recognizing and circumscribing species. The use of host association as a diagnostic character is most reliable when the species in question are well described and delimited using other diagnostic criteria such as DNA sequences. Essigella (Insecta, Hemiptera, Sternorrhyncha, Aphididae, Lachninae) is an aphid genus encompassing thirteen species, two of which have two subspecies each. They are phloem-feeders on the needles of true pines (Pinus) and Douglas firs (Pseudotsuga). Most Essigella species are monophagous and develop on a single or a few closely-related host species. Two species of Essigella, E. californica and E. pini, are oligophagous, however, and found on several pine species. Essigella species are naturally Nearctic. One species, E. californica, has however been accidentally introduced in several countries around the world, becoming in some instances an important pest in pine plantations. Essigella, Cinara, Eulachnus and Pseudessigella comprise the tribe Eulachnini, although the phylogenetic relationships between these genera have not yet been clarified. Essigella species exhibit high intra- and interspecific morphological variability. Moreover, there exists few or no diagnostic reliable characters, making the taxonomy of the genus difficult. Several species were circumscribed by subtle morphological variations only detectable with morphometric measurements and multivariate analyses. Thus, identification of Essigella species has traditionally been based on a combination of morphometry and host plant identity. Because Essigella systematics has not been tested with molecular means, it is likely that some morphologically close species living on related host plants actually correspond to populations of a single species. At the same time, it is also possible that different populations of one apparent species may actually correspond to different cryptic species. This thesis presents a revision of Essigella systematics using molecular data and more particularly DNA sequences of four genes: ATP6, COI, EF-1α and Gnd.  Methods: I estimated a phylogeny of Essigella using maximum likelihood and Bayesian inference using DNA sequences of four genes: ATP6 and COI (mitochondrial), EF-1α (nuclear), and Gnd (from the obligate endosymbiont Buchnera aphidicola). Representative species of the three other eulachnine genera were used as outgroups. I employed five species delimitation methods in order to test the species taxonomy of Essigella: the traditional 2% COI barcode threshold, the Automatic Barcode Gap Discovery (ABGD), the General Mixed Yule Coalescent (GMYC), the Bayesian Poisson Tree Process (bPTP), and the Refined Single Linkage (RESL) via the BOLD (Barcode of Life Data Systems) database. I also compared ATP6, COI, EF-1α and Gnd sequences from exotic E. californica populations with sequences of North American ones to confirm they actually belongs to that species.  Results: The Essigella phylogeny showed that Pseudessigella is the sister-group of Essigella and that Eulachnus is the sister group of Essigella + Pseudessigella. That phylogeny, combined with two species delimitation methods, ABGD and COI barcoding, confirmed that all known Essigella species were valid and, as a result, morphological variations were truly specific and not linked to the host plant. Thus, respective specificity between Essigella species and their host plant were confirmed. The analyses also revealed that the taxa known as E. californica and E. pini actually include four and two cryptic species, respectively, and that the BOLD database contained sequences of three other unknown species. In the case of E. pini, the additional cryptic species corresponds to its synonym, E. patchae, which I thus re-established. In the case of E. californica, the three new species do not correspond to any known synonym of the species. Thus E. californica sensu lato includes at least four distinct species, E. californica sensu stricto and three others described as new: Essigella domenechi, E. gagnonae and E. sorenseni. Because of the strong morphological proximity between the four species, and for the first time in aphid taxonomy, nucleotide differences in DNA sequences (ATP6, COI and Gnd), are used in the respective diagnoses of the three new species. Despite discovery of these three species, I confirmed that only E. californica sensu stricto was introduced outside North America, and that the introductions occurred independently and at least four times.  Conclusion: This thesis largely confirmed the morphometrics-based species delimitation of Essigella and the narrow host plant specificity of most of the species. However, it also uncovered the existence of several cryptic species and suggested that other such species may exist within Essigella, confirmation requiring further work and substantial additional material. This study also highlights that the use of DNA data as diagnostic characters is essential in delimiting but also in recognizing cryptic species.

Keyphrases: Eulachnini, barcode, populations, Eulachnini, host plant, cryptic species, phylogeny, species delimitation, taxonomy, speciation, barcoding, populations