Molecular Phylogenetic Analysis and Species Delimitation in the Pine Needle-feeding Aphid Genus Essigella (Hemiptera, Sternorrhyncha, Aphididae)

Species of the genus Essigella Del Guercio, 1909 (Hemiptera: Aphididae) are known to be specific to one or a few related species of Pinaceae hosts. The current Essigella classification includes 15 species-group taxa, inferred with morphological and ecological data. We present a phylogeny of Essigella using maximum likelihood and Bayesian inference using DNA sequences from three genomes: mitochondrial (COI, ATP6), nuclear (EF-1 alpha), and endosymbiont (Gnd). We also challenged the taxonomy of Essigella species using four species delimitation methods: the 2% COI barcode threshold, the ABGD (Automatic Barcode Gap Discovery), the GMYC (General Mixed Yule Coalescent), and the bPTP (Bayesian Poisson Tree Process) methods. Fifty-three populations of Essigella were studied, with the eulachnine genera Cinara Curtis, 1835, Eulachnus Del Guercio, 1909 and Pseudessigella Hille Ris Lambers, 1966 as outgroups. Phylogenetic analyses support Pseudessigella as sister-group of Essigella. They confirm that all the known species are valid and mostly linked to a specific host plant, but also that E. pini Wilson, 1919 encompasses two species, the second probably being E. patchae Hottes, 1957, currently considered a synonym. ABGD and COI barcoding species delimitation analyses were partially congruent, although the barcoding threshold was less than 2% for the latter. They suggest the existence of several cryptic species also supported by ecological data. Methods of bPTP and GMYC gave conflicting results, possibly due to inadequate sampling. Our results highlight that substantial data are often required to delimit species with confidence.