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Research Article
Molecular data and species diagnosis in Essigella Del Guercio, 1909 (Sternorrhyncha, Aphididae, Lachninae)
expand article infoThomas Théry, Mariusz Kanturski§, Colin Favret
‡ University of Montreal, Montreal, Canada
§ University of Silesia in Katowice, Katowice, Poland
Open Access

Abstract

Morphological and molecular data are used to describe three new species of Essigella (Sternorrhyncha: Aphididae: Lachninae): Essigella domenechi sp. n., Essigella gagnonae sp. n., and Essigella sorenseni sp. n.; and to re-establish as valid Essigella patchae Hottes, 1957, stat. n., until now considered a synonym of E. pini Wilson, 1919. The catalogue of Essigella species is updated. This study highlights the need and utility to use discreet DNA characters in aphid species diagnoses.

Keywords

Cryptic species, DNA sequences, Hemiptera, taxonomy

Introduction

Morphological characters remain the commonest way to separate animal species, and they are conspicuously used in diagnoses and descriptions of new taxa. However, in the case of cryptic species, no or few morphological differences are available, and other kinds of taxon-related attributes must be employed as valuable diagnostic characters. DNA sequences permit the discovery of cryptic species and are used to separate them from their relatives (Hebert et al. 2003a, b, Cœur d’acier et al. 2014, Lukhtanov and Dantchenkoet 2017, Morinière et al. 2017). However, despite their reliability, they are seldom used specifically in diagnoses of new species, notably because they are not specifically recommended in the International Code of Zoological Nomenclature (Renner 2016).

Essigella (Sternorrhyncha: Aphididae: Lachninae) is an aphid genus found on the needles of various pinaceous hosts. Most species feed on true pines, Pinus Linnaeus, but E. wilsoni Hottes, 1957, is found only on Douglas firs, Pseudotsuga Carrière. Essigella alyeska Sorensen, 1988 is recorded on spruce, Picea A. Dietrich, although its typical host is Pinus banksiana Lamb. (Sorensen 1994). Most species of Essigella are considered monophagous except E. californica (Essig, 1909) and E. pini Wilson, 1919 which are oligophagous on Pinus (Sorensen 1994). Although all species are Nearctic in origin, E. californica was accidently introduced in several countries around the world (Théry et al. 2017). Essigella currently encompasses 15 valid taxa, with an additional 13 synonyms (Wilson 1919, Gillette and Palmer 1924, Hottes 1957, 1958, Sorensen 1988, 1994). Species are variable and show few diagnostic characters (Sorensen 1994). The genus was revised by Sorensen (1994) using morphometric data and multivariate analyses. Besides the 15 taxa he recognized, Sorensen (1994) notably divided Essigella into three subgenera: Archeoessigella, Essigella and Lambersella, two species series, and three species complexes. A recent molecular phylogenetic study did not support the validity of the three subgenera and of one of the species series (Théry et al. in press). Moreover, the phylogenetic results, combined with molecular species delimitation methods, revealed that two species, Essigella californica and E. pini, actually encompass four and two species, respectively. In the case of E. pini, one of the two species is suspected to be E. patchae Hottes, 1957, considered a synonym of E. pini by Sorensen (1994). Examination of type material of E. californica and E. pini, as well as that of their respective synonyms and reference specimens, indicates that the three cryptic species found within E. californica are new to science and confirms the validity of E. patchae.

In the present work, we describe as new the three cryptic species revealed by Théry et al. (in press): Essigella domenechi sp. n., E. gagnonae sp. n. and E. sorenseni sp. n. In addition, we re-establish E. patchae stat. n. and provide diagnostic characters to separate it and E. pini. Because these four species are difficult to distinguish morphologically, discreet DNA sequence data supplement classical morphological characters in the diagnoses.

Materials and methods

Abbreviations used

CTT Private Collection of T. Théry, Fleury les Aubrais, France;

EMEC Essig Museum of Entomology, University of California, Berkeley, CA, USA;

QMOR Ouellet-Robert Entomological Collection, University of Montreal, QC, Canada;

UMSP University of Minnesota Insect Collection, St Paul, MN, USA;

USNM National Aphid Collection, National Museum of Natural History, Beltsville, MD, USA.

Taxon sampling

All Essigella specimens published here were collected recently in the USA and Canada (TT and CF), or are found in the Sorensen Collection at EMEC. Specimens studied were mainly viviparous apterae. Some viviparous alatae also were studied in the case of E. patchae for which the holotype is an alate. Recently collected specimens were preserved in 95% ethanol after collecting and subsequently kept at -20 °C or -80 °C. DNA extraction of at least one specimen per population was realized. It was non-destructive (Favret 2005), permitting us to keep the specimen as voucher. Those specimens were identified using the keys of Sorensen (1994) and Blackman and Eastop (2017). We compared our material with the type specimens of the valid species, E. californica (EMEC) and E. pini (UMSP), as well as those of their synonyms E. claremontiana Hottes, 1957, E. cocheta Hottes, 1957, E. monelli Hottes, 1957, E. pineti Hottes, 1957, E. swaini Hottes, 1957, for E. californica, and E. patchae for E. pini (EMEC, USNM). We also compared specimens of new taxa and of E. patchae with other E. californica specimens from the Sorensen Collection (EMEC), and of E. pini from UMSP and USNM.

Preparation, measurements, and photographs

All new material was slide-mounted in Canada balsam and deposited in QMOR, CTT, and USNM, in the case of holotypes. Preparations were thick to reduce deformation due to compression. As far as possible, appendages were placed so that they be strictly horizontal permitting correct length and width measurements as well as to ascertain the correct location of dorsal and ventral setae of the hind femora and tibiae. Body length was measured from the frontal margin of the head to the posterior margin of the 7th abdominal segment. The abdominal tergum being sclerotized with most segments fused, the cauda and 8th segment sometimes telescope into the 7th, making standardized measurements difficult across specimens. Because of the likely deformation of the body due to a variable number of embryos, width measurements were taken only of the head, between the frontal interior margins of the compound eyes. Lengths of appendages were measured at their longest, including condyles, widths were measured at the widest part of the appendages. The length of the processus terminalis was taken from the distal margin of primary rhinarium to the apex of the antenna. The following abbreviations are applied:

BL body length;

LAIII length of third antennal segment;

LAIV length of fourth antennal segment;

LAV length of fifth antennal segment;

LPRIV length of primary rhinarium on fourth antennal segment;

LPRV length of primary rhinarium on fifth antennal segment;

LPT length of processus terminalis;

HWE head width at eyes;

LURS length of ultimate rostral segment;

LMF length of metafemur;

WMF width of metafemur;

LMT length of metatibia;

WMT width of metatibia;

WS width of siphunculus at external edges;

LMB length of metabasitarsus;

LMD length of metadistitarsus;

LFS length of longest frontal seta;

LDMFS length of longest dorsal metafemoral seta;

LVMFS length of longest ventral metafemoral seta;

LDMTS length of longest dorsal metatibial seta;

LVMTS length of longest ventral metatibial seta.

Entire non-prepared specimens were photographed with a Carl Zeiss Discovery.V20 stereoscope using an AxioCam HRc camera and a Zen 2012 Carl Zeiss Software, version 1.1.1.0. Pictures of slide-mounted specimens were realized using light microscope Nikon Eclipse E600 with differential interference contrast (DIC) and photographed by Nikon DS-Fi camera. Scanning electron microscope (SEM) photos were taken at the University of Silesia in Katowice (Poland) using a Hitachi SU8010 Field Emission Scanning Electron Microscope (FE-SEM) (Hitachi High-Technologies Corporation, Tokyo, Japan) at 5, 10 and 15 kV accelerating voltage with a secondary electron detector (ESD). For specimen preparation for SEM pictures, we followed the protocol of Kanturski et al. (2015). Measurements in diagnoses and descriptions are given in microns (µm) with standard deviation.

Molecular data

The three new species were primarily revealed in the study of Théry et al. (in press) using DNA sequences of the genomes of the mitochondrion (ATP6, COI) and the obligate bacterial endosymbiont Buchnera aphidicola (Gnd) within populations of E. californica sensu lato. Indeed, ATP6 and Gnd show similar properties as COI in species discrimination in barcoding (Hebert et al. 2003a, b, Chen et al. 2013, Lee et al. 2014). Sequence lengths were 663 base pairs (bp), 658 bp and 749 bp for ATP6, COI and Gnd, respectively (see Théry et al. in press, for GenBank accession numbers and other details).

Taxonomy

The following species, including E. patchae, belong to the E. californica species complex, which also includes E. hoerneri Gillette & Palmer, 1924 (Sorensen 1994) (see discussion). All of these species, as well as E. pini, exhibit six dorsal setae at their 3rd and 4th abdominal segments (Sorensen 1994). However, this character is homoplastic within Essigella as E. pini and the E. californica complex are not closely related (Théry et al. in press); it is used here to distinguish the species of the E. californica complex and E. pini from the other species of the genus. Morphological and ecological (host plant identity) comparisons of specimens of the new species with type material of synonym species of E californica and E. hoerneri permitted to reject the possibility that our new species correspond to one of those synonyms.

Essigella domenechi sp. n.

Figure 1d

Holotype

viviparous aptera, USA, California, Alpine Co., 38.328°N 119.637°W, 10.vii.2013, on Pinus albicaulis, T. Théry & C. Favret leg. (USNM). Paratypes. 8 viviparous apterae, same data as holotype (QMOR, CTT).

Figure 1. 

Habitus of viviparous apterae of a Essigella pini b E. patchae c E. californica d E. domenechi sp. n. e E. gagnonae sp. n. f. E. sorenseni sp. n.

Diagnosis

Like species of the E. californica complex and E. pini, E. domenechi sp. n. has its 3rd and 4th abdominal dorsal terga usually bearing six setae. The species can be distinguished from E. patchae by the presence of rows of spinules on the URS (absent or faint in E. patchae; Fig. 2b, d); from E. pini by a relatively elongate URS with subparallel lateral margins (URS with margins rounded and convergent at base in E. pini; Fig. 2a, c); from E. gagnonae sp. n. and E. sorenseni sp. n. with the following characters: tibiae and femora more or less concolorous showing almost or same color as that of body (pro- and metatibiae and metafemora conspicuously darkened in E. sorenseni sp. n., pro- and metatibiae sometimes slightly darkened in E. gagnonae sp. n.), dorsal tegument thick; width of head between eyes = 300.7 ± 14.2 (289.0 ± 13.3 for E. gagnonae sp. n., and 353.6 ± 15.3 for E. sorenseni sp. n.); ratio of 3rd / 5th antennal segments < 1.6 (< 1.6 for E. gagnonae sp. n. but > 1.6 in E. sorenseni sp. n.); overall pubescence short or medium-sized with average length of the longest dorsal seta of metafemora = 29.7 ± 4.2 (59.8 ± 9.8 for E. gagnonae sp. n., and 51.2 ± 10.7 for E. sorenseni sp. n.; average length of the longest ventral seta of metafemora = 32.6 ± 4.5 (43.1 ± 5.4 for E. gagnonae sp. n., and 54.4 ± 5.6 for E. sorenseni sp. n.); average length of the longest dorsal seta of metatibiae = 44.0 ± 8.1 (85.7 ± 10.8 for E. gagnonae sp. n., and 76.4 ± 15.8 for E. sorenseni sp. n.); average length of the longest ventral seta of metatibiae = 37.5 ± 7.0 (49.4 ± 9.5 for E. gagnonae sp. n., and 67.7 ± 12.0 for E. sorenseni sp. n.); average length of the longest frontal seta = 32.6 ± 7.5 (58.7 ± 8.3 for E. gagnonae sp. n., and 53.4 ± 11.9 for E. sorenseni sp. n.); average number of setae of the genital plate = 22.0 ± 2.1 (23.6 ± 2.1 for E. gagnonae sp. n., and 31.6 ± 1.7 for E. sorenseni sp. n.). Essigella domenechi sp. n. is morphologically not distinguishable from E. californica, the latter being highly variable, nor from E. hoerneri. Essigella domenechi sp. n. can be separated from E. californica, E. gagnonae sp. n., E. hoerneri and E. sorenseni sp. n. with the DNA characters shown in Table 1.

Figure 2. 

Morphological structures in Essigella pini and in E. patchae: a URS in E. pini (slide-mounted specimen) b URS in E. patchae (slide-mounted specimen) c URS in E. pini (SEM) d URS in E. patchae (SEM) e genital plate in E. pini (SEM) f genital plate in E. patchae (SEM) g details in genital plate in E. pini (SEM) h details in genital plate in E. patchae (SEM).

Diagnostic nucleotide differences between E. domenechi sp. n. and E. californica, E. gagnonae sp. n., E. hoerneri, and E. sorenseni sp. n. for ATP6, COI, and Gnd.

Gene ATP6 (663 bp) COI (658 bp) Gnd (749 bp)
Site 4 71 227 324 190 229 334 386 418 565 619 625 219 621
E. domenechi sp. n. C C C G G G A G C G G G C C
E. gagnonae sp. n. T T T A A A T A T A A A A A
E. sorenseni sp. n. T T T A A A T A T A A A A A
E. californica T T T A A A T A T A A A A A
E. hoerneri T T T A A A T A T A A A A A

Description

Viviparous apterae (prepared specimens): body with pale tegument, with visible pigmented scleroites; dorsal tegument visibly thicker, sclerotized. Legs quite pale, concolorous, more or less the same color than that of body. Antennae pale, the 5th, the 4th and the apical third part of the 3rd segment of antennae darkened. URS elongated, with lateral margins subparallel, bearing rows of spinules. Overall pubescence short to medium-sized, dorsal setae of appendages incrassate, ventral ones acute. Terga of abdominal segments 3 and 4 with six dorsal setae. Genital plate with 19–25 setae (22.0 ± 2.1) (n = 6). Cauda obvious but not too protruding, apically rounded, slightly turned upward. BL: 1600–2100 (1800 ± 170) (n = 7). HWE: 283.2–326.0 (300.7 ± 14.2) (n = 7), LAIII: 162.2–184.6 (171.8 ± 6.5) (n = 13), LAIV: 96.4–106.7 (101.0 ± 4.1) (n = 9), LAV: 113.7–124.4 (120.2 ± 4.1) (n = 5), LPRIV: 20.8–25.8 (23.0 ± 1.6) (n = 9), LPRV: 17.6–21.9 (19.6 ± 1.4) (n = 9), LPT: 8.5–14.5 (11.9 ± 1.9) (n = 9), LURS: 71.4–79.2 (75.2 ± 2.8) (n = 6), LMF: 675.7–728.8 (708.3 ± 24.2) (n = 6), WMF: 68.2–77.3 (74.9 ± 2.9) (n = 11), LMT: 975.1–1074.4 (1027.8 ± 38.2) (n = 9), WMT: 36.8–43.9 (41.4 ± 2.2) (n = 12), WS: 36.7–43.5 (40.4 ± 2.2) (n = 9), LMB: 107.2–114.4 (110.9 ± 2.3) (n = 11), LMD: 189.4–206.4 (194.9 ± 6.9) (n = 11), LFS: 18.7–39.5 (32.6 ± 7.5) (n = 7), LDMFS: 25.2–36.8 (29.7 ± 4.2) (n = 12), LVMFS: 26.1–44.0 (32.6 ± 4.5) (n = 12), LDMTS: 33.7–61.8 (44.0 ± 8.1) (n = 12), LVMTS: 24.7–48.1 (37.5 ± 7.0) (n = 12).

Comments

USA, California, on Pinus albicaulis Engelmann, known from Stanislaus National Forest at high elevation (type series). The species probably occurs in other high mountains where P. albicaulis is present. This species corresponds to the E. californica population living on P. albicaulis (cluster H3) shown in Théry et al. (in press).

Etymology

This species is dedicated to Boris Domenech, PhD student at the University of Montreal (QC, Canada) for his comments in genetic analyses with which the species was discovered.

Essigella gagnonae sp. n.

Figs 1e, 3b

Holotype

viviparous aptera, USA, Nevada, Douglas Co., 38.999°N 119.896°W, 10.vii.2013, on Pinus monticola, T. Théry & C. Favret leg. (USNM). Paratypes. 1 viviparous aptera, same data as holotype (QMOR); 12 viviparous apterae, California, El Dorado Co., 38.834°N 120.042°W, 09.vii.2013, on Pinus monticola, T. Théry & C. Favret leg., specimens on 10 slides (QMOR, CTT); 5 viviparous apterae, California, Lassen Co., HWY 89, 6 km N Jct HWY 36 & 89, 6600’, S of Lassen Nat’l Park (77G20), 10.vii.1977, on Pinus monticola, J. T. Sorensen leg., specimens on 1 slide (EMEC); 5 viviparous apterae, Californica, Alpine Co., E side Ebbett’s Pass, HWY 4, 3 km E summit (77G41), 17.vii.1977, on Pinus monticola, J. T. Sorensen leg., specimens on 1 slide (EMEC); 13 viviparous apterae, Washington, Kitsap Co., 8 km S Hood Canal Bridge, HWY 3 (78G49), 09.vii.1978, on Pinus monticola, J. T. Sorensen leg., specimens on 3 slides (4 + 4 + 5) (EMEC); 8 viviparous apterae, Washington, Grays Harbor Co., 16 km W Amanda Park, HWY 101 (78G54), 10.vii.1978, on Pinus monticola, J. T. Sorensen leg., specimens on 2 slides (4 + 4) (EMEC); 5 viviparous apterae, Nevada, Washoe Co., Mt Rose, Summit, Cmpgd, Toiyabe Nat’l Forest (78H9), 02.viii.1978, on Pinus monticola, J. T. Sorensen leg., specimens on 2 slides (2 + 3) (EMEC).

Diagnosis

Like species of the E. californica complex and E. pini, E. gagnonae sp. n. has its 3rd and 4th abdominal dorsal terga usually bearing six setae. It can be distinguished from E. patchae by the presence of spinules on the URS (absent or faint in E. patchae; Fig. 2b, d); from E. pini by a relatively elongate URS with subparallel lateral margins (URS with margins rounded and convergent at base in E. pini; Fig. 2a, c); from E. domenechi sp. n. and E. sorenseni sp. n. with the following characters: legs ranging from concolorous and slightly darker than body, to pro- and metatibiae slightly darkened with mesotibiae lighter and metafemora pale (tibiae concolorous in E. domenechi sp. n., pro- and metatibiae, and metafemora conspicuously darkened in E. sorenseni sp. n.); width of head between eyes = 289.0 ± 13.3 (300.7 ± 14.2 for E. domenechi sp. n., and 353.6 ± 15.3 for E. sorenseni sp. n.); ratio of 3rd / 5th antennal segments < 1.6 (< 1.6 for E. domenechi sp. n. but > 1.6 in E. sorenseni sp. n.); overall pubescence medium-sized to long with average length of the longest dorsal setae of metafemora = 59.8 ± 9.8 (29.7 ± 4.2 for E. domenechi sp. n., and 51.2 ± 10.7 for E. sorenseni sp. n.); average length of the longest ventral seta of metafemora = 43.1 ± 5.4 (32.6 ± 4.5 for E. domenechi sp. n., and 54.4 ± 5.6 for E. sorenseni sp. n.); average length of the longest dorsal seta of metatibiae = 85.7 ± 10.8 (44.0 ± 8.1 for E. domenechi sp. n., and 76.4 ± 15.8 for E. sorenseni sp. n.); average length of the longest ventral seta of metatibiae = 49.4 ± 9.5 (37.5 ± 7.0 for E. domenechi sp. n., and 67.7 ± 12.0 for E. sorenseni sp. n.); average length of the longest frontal seta = 58.7 ± 8.3 (32.6 ± 7.5 for E. domenechi sp. n., and 53.4 ± 11.9 for E. sorenseni sp. n.); average number of setae of the genital plate = 23.6 ± 2.1 (22.0 ± 2.1 for E. domenechi sp. n., and 31.6 ± 1.7 for E. sorenseni sp. n.). Essigella gagnonae sp. n. is for now morphologically not distinguishable from E. californica, the latter being highly variable, nor from E. hoerneri. Essigella gagnonae sp. n. can be separated from E. californica, E. domenechi sp. n., E. hoerneri, and E. sorenseni sp. n. with the DNA characters shown in Table 2.

Description

Viviparous apterae (prepared specimens): body with pale tegument sometimes slightly yellowish, with visible pigmented scleroites. Legs ranging from concolorous, slightly darker than body, to pro- and metatibiae slightly darkened, darker than body and mesotibiae. Antennae pale, the 5th, the 4th and the apical third part of the 3rd segment darkened. URS elongated, with lateral margins subparallel, bearing rows of spinules. Overall pubescence medium-sized to long, dorsal setae of appendages incrassate, ventral ones acute, in specimens with very long dorsal setae in metafemora and metatibiae (> 100 µm), these setae almost acute to acute (Fig. 3b), straight to sinuated. Terga of abdominal segments 3 and 4 with six dorsal setae. Genital plate with 21–26 setae (23.6 ± 2.1) (n = 9). Cauda obvious but not too protruding, apically rounded, slightly turned upward. BL: 1600–2000 (1800 ± 130) (n = 19). HWE: 271.0–311.9 (289.0 ± 13.3) (n = 13), LAIII: 157.6–197.4 (178.1 ± 11.1) (n = 29), LAIV: 90.2–111.6 (99.7 ± 6.3) (n = 33), LAV: 116.0–141.6 (125.4 ± 5.8) (n = 20), LPRIV: 21.5–29.1 (24.3 ± 1.8) (n = 21), LPRV: 18.5–22.6 (20.6 ± 1.2) (n = 18), LPT: 7.6–16.8 (12.1 ± 2.5) (n = 23), LURS: 64.5–79.8 (72.0 ± 3.8) (n = 18), LMF: 650.3–798.5 (707.3 ± 38.6) (n = 22), WMF: 69.5–104.6 (87.0 ± 10.8) (n = 29), LMT: 876.1–1104.2 (999,9 ± 67.4) (n = 25), WMT: 33.8–52.5 (42.3 ± 4.1) (n = 40), WS: 34.4–42.6 (38,9 ± 2.5) (n = 18), LMB: 101.8–131.0 (116.1 ± 8.0) (n = 36), LMD: 180.3–209.9 (195.0 ± 8.6) (n = 34), LFS: 44.4–80.2 (58.7 ± 8.3) (n = 26), LDMFS: 42.0–82.9 (59.8 ± 9.8) (n = 43), LVMFS: 31.5–52.6 (43.1 ± 5.4) (n = 42), LDMTS: 60.9- 107.7 (85.7 ± 10.8) (n = 46), LVMTS: 30.5–74.5 (49.4 ± 9.5) (n = 46).

Comments

USA, California, Nevada, and Washington, on Pinus monticola Douglas ex D. Don. The species occurs in elevated places where P. monticola is present. This species corresponds to the E. californica population living on P. monticola (cluster H2) shown in Théry et al. (in press).

Etymology

This species is dedicated to Édeline Gagnon, PhD student at the University of Montreal (QC, Canada) for her help in genetic analyses with which the species was discovered.

Diagnostic nucleotide differences between E. gagnonae sp. n. and E. californica, E. domenechi sp. n., E. hoerneri and E. sorenseni sp. n. for ATP6, COI, and Gnd.

Gene ATP6 (663 bp) COI (658 bp) Gnd (749 bp)
Site 260 28 235 271 665
E. gagnonae sp. n. G G C C C
E. domenechi sp. n. A A T A T
E. sorenseni sp. n. A A T A A
E. californica A A T A A
E. hoerneri A A T A T

Essigella sorenseni sp. n.

Figs 1f, 3a

Holotype

viviparous aptera, USA, California, Sonoma Co., 38.534°N 123.276°W, 02.vii.2013, on Pinus muricata, T. Théry & C. Favret leg. (QMOR). Paratypes. 14 viviparous apterae, same data than holotype, specimens on 14 slides (QMOR, CTT); 3 viviparous apterae, California, Mendocino Co., 38.984°N 123.696°W, 03.vii.2013, on Pinus muricata, T. Théry & C. Favret leg., specimens on 3 slides (QMOR, CTT); 6 viviparous apterae, California, Mendocino Co., HWY 1, 5 km of Albion, Little River Road, 23.vii.1977, on Pinus muricata, 77G52, J. T. Sorensen leg., specimens on 3 slides (2 + 2 + 2) EMEC); 13 viviparous apterae, California, Humbodlt Co., nr Little River State Beach, 17 km N Arcata, HWY 101, 04.vii.1978, on Pinus muricata, 78G3, J. T. Sorensen leg., specimens on 4 slides (4 + 4 + 4 + 1) (EMEC).

Figure 3. 

Details of dorsal setae of metatibia in a E. sorenseni sp. n. (slide-mounted specimen) b E. gagnonae sp. n. (slide-mounted specimen).

Diagnosis

Like species of the E. californica complex and E. pini, E. sorenseni sp. n. has its 3rd and 4th abdominal dorsal terga usually bearing six setae. It can be distinguished from E. patchae by the presence of spinules on the URS (absent or faint in E. patchae; Fig. 2b, d); from E. pini by a relatively elongate URS with subparallel lateral margins (URS with margins rounded and convergent at base in E. pini; Fig. 2a, c); from E. domenechi sp. n. and E. gagnonae sp. n. with the following characters: usually pro- and metatibiae conspicuously darkened with mesotibiae always lighter, metafemora darkened (tibiae concolorous, metafemora pale in E. domenechi sp. n., concolorous or with pro- and metatibiae slightly darkened with mesotibiae lighter, metafemora pale in E. gagnonae sp. n.); width of head between eyes = 353.6 ± 15.3 (300.7 ± 14.2 for E. domenechi sp. n., and 289.0 ± 13.3 for E. gagnonae sp. n.); ratio of 3rd / 5th antennal segments > 1.6 (< 1.6 for E. domenechi sp. n. and E. gagnonae sp. n.); overall pubescence medium-sized to long with average length of the longest dorsal setae of metafemora = 51.2 ± 10.7 (29.7 ± 4.2 for E. domenechi sp. n., and 59.8 ± 9.8 for E. gagnonae sp. n.); average length of the longest ventral seta of metafemora = 54.4 ± 5.6 (32.6 ± 4.5 for E. domenechi sp. n., and for 43.1 ± 5.4 for E. gagnonae sp. n.); average length of the longest dorsal seta of metatibiae = 76.4 ± 15.8 (44.0 ± 8.1 for E. domenechi sp. n., and 85.7 ± 10.8 for E. gagnonae sp. n.); average length of the longest ventral seta of metatibiae = 67.7 ± 12.0 (37.5 ± 7.0 for E. domenechi sp. n., and 49.4 ± 9.5 for E. gagnonae sp. n.); average length of the longest frontal setae = 53.4 ± 11.9 (32.6 ± 7.5 for E. domenechi sp. n., and for 58.7 ± 8.3 E. gagnonae sp. n.); average number of setae of the genital plate = 31.6 ± 1.7 (22.0 ± 2.1 for E. domenechi sp. n., and 23.6 ± 2.1 for E. gagnonae sp. n.). Essigella sorenseni sp. n. is for now morphologically not distinguishable from E. californica, the latter being highly variable, nor from E. hoerneri. E. sorenseni sp. n. can be separated from E. californica, E. domenechi sp. n., E. gagnonae sp. n., and E. hoerneri with the DNA characters shown in Table 3.

Description

Viviparous apterae (prepared specimens): body with a yellowish tegument more or less darkened at joints depending on the specimens, with conspicuous and pigmented scleroites. Legs usually with pro- and metatibiae conspicuously darkened, much darker than body and mesotibiae. Antennae pale, the 5th, the 4th and the apical third part of the 3rd segment darkened. URS elongated, with lateral margins subparallel, bearing rows of spinules. Overall pubescence medium-sized to long, dorsal setae of appendages incrassate, ventral ones acute, in specimens with very long dorsal setae on metafemora and metatibiae (> 100 µm), these setae not acute or seemingly acute but still incrassate (Fig. 3a), the setae sometimes curved at base. Terga of abdominal segments 3 and 4 with six dorsal setae. Genital plate with 29–34 setae (31.6 ± 1.7) (n = 10). Cauda obvious but not protruding, apically rounded, slightly turned upward. BL: 1900–2300 (2200 ± 110) (n = 21). HWE: 322.3–376.1 (353,6 ± 15.3) (n = 17), LAIII: 207.5–256.3 (233.6 ± 12.8) (n = 25), LAIV: 98.3–130.0 (112.2 ± 7.1) (n = 34), LAV: 120.1–139.8 (127.9 ± 4.7) (n = 23), LPRIV: 19.9–27.8 (24.1 ± 1.8) (n = 28), LPRV: 17.4–23.4 (19.5 ± 1.5) (n = 21), LPT: 11.6–15.7 (13.8 ± 1.4) (n = 21), LURS: 74.1–86.4 (80.5 ± 3.2) (n = 21), LMF: 702.3–927.8 (810.8 ± 58.9) (n = 26), WMF: 87.5–128.9 (103.1 ± 11.3) (n = 36), LMT: 1064.2–1450.4 (1233.4 ± 95.1) (n = 26), WMT: 49.5–76.0 (55.1 ± 5.1) (n = 37), WS: 39.0–44.6 (41.4 ± 1.7) (n = 22), LMB: 118.5–140.3 (130.3 ± 6.4) (n = 38), LMD: 183.4–212.5 (198.1 ± 7.9) (n = 34), LFS: 31.9–82.7 (53.4 ± 11.9) (n = 25), LDMFS: 34.2–79.4 (51.2 ± 10.7) (n = 45), LVMFS: 43.4–66.0 (54.4 ± 5.6) (n = 44), LDMTS: 47.7–113.8 (76.4 ± 15.8) (n = 46), LVMTS: 45.9–92.2 (67.7 ± 12.0) (n = 45).

Comments

USA, California, on Pinus muricata D. Don, known from Humboldt, Mendocino, and Sonoma counties (type series), but probably present everywhere on the coastal range in California where P. muricata occurs. This species corresponds to the E. californica population living on P. muricata (cluster H1) shown in Théry et al. (in press).

Etymology

This species is dedicated to John T. Sorensen, aphid specialist who eminently revised the genus Essigella in 1994, for his advice and hospitality accorded to the authors (TT and CF) in California.

Diagnostic nucleotide differences between E. sorenseni sp. n. and E. californica, E. domenechi sp. n., E. gagnonae sp. n., and E. hoerneri for ATP6, COI, and Gnd.

Gene ATP6 (663 bp) COI (658 bp) Gnd (749 bp)
Site 110 399 247 198 407 431
E. sorenseni sp. n. C C T T C G
E. domenechi sp. n. T T C C T T
E. gagnonae sp. n. T T C C T T
E. californica T T C C T T
E. hoerneri T T C C T T

Essigella patchae Hottes, 1957, stat. n.

Figs 1b, 2b, d, f, h

Essigella patchae Hottes, 1957: 98 (Type locality: “Stillwater, Maine”). Holotype viviparous alate in USNM. Sorensen 1994: 49 [synonymy with E. pini Wilson]. Status re-established.

Other examined material

1 viviparous alate and 1 viviparous aptera, Canada, Québec, Saint-Hippolyte, N45.991 - W74.009, ix.2015, on Pinus strobus, C. Favret leg. (QMOR); 1 viviparous aptera, Saint-Hippolyte, N45.989 - W74.005, ix.2016, on Pinus strobus, T. Théry leg. (QMOR); 1 viviparous aptera, Saint-Hippolyte, N45.989 - W74.005, ix.2017, on Pinus strobus, T. Théry leg. (QMOR).

Diagnosis

Like species of the E. californica complex and E. pini, E. patchae has its 3rd and 4th dorsal abdominal terga usually with six setae. Essigella patchae can be distinguished from the other species of the E. californica complex species and from E. pini by its ultimate rostral segment (URS) exhibiting no or barely visible rows of spinules (Fig. 2b, d), which are clearly visible in other species of the E. californica complex and also in E. pini (Fig. 2a, c). Essigella patchae can also be differentiated from E. pini by having the general shape of the URS more elongated with lateral margins almost parallel (Fig. 2b, d) (margins more rounded and convergent at base in E. pini; Fig. 2a, c); shorter cauda than that of E. pini which can be elongated and acute; genital plate with fewer setae (15–20 vs 26–30 in E. pini), longer in E. patchae in comparison with E. pini (Fig. 2e, f), with spinules of the genital plate tegument more developed in E. patchae (Fig. 2g, h).

Host plant and distribution

The species is currently known from its type locality in Maine (USA) and from one locality in Quebec (Canada) on Pinus strobus Linnaeus (see discussion).

Simplified key to species of the Essigella californica complex, for viviparous apterae

Due to the variability of preparation, notably cover slip-induced deformations, teneral specimens, and general morphological variability, several specimens and the identity of the host plant are required to best use this key.

1 Dorsal terga 3 and 4 usually with six setae E. californica complex, E. pini...2
Dorsal terga 3 and 4 usually with more than six setae other Essigella species (see Sorensen 1994)
2 Western North American species 3
Eastern North American species 7
3 On pinyon pines (Pinus cembroides, P. edulis, P. monophylla, P. quadrifolia) E. hoerneri
Not on pinyon pines 4
4 On Pinus albicaulis, P. monticola, or P. muricata 5
On other pine species E. californica
5 Ratio of LAIII / LAV > 1.6 (1.66–1.94), number of setae on genital plate > 27 (29–34), on P. muricata E. sorenseni sp. n.
Ratio of LAIII / LAV < 1.6 (1.29–1.54), number of setae on genital plate < 27 (19–26) 6
6 Dorsal setae of metafemora (25.2–36.8 µm) and of metatibiae (33.7–61.8 µm) short, on P. albicaulis E. domenechi sp. n.
Dorsal setae of metafemora (42.0–82.9 µm) and of metatibiae (60.9–107.7 µm) long, on P. monticola E. gagnonae sp. n.
7 Ultimate Rostral Segment (URS) with rows of spinules; sides of URS convex, convergent at base (Fig. 2a, c), number of setae on genital plate > 24 (26–30) E. pini
Ultimate Rostral Segment (URS) without or with barely visible rows of spinules (Fig. 2b, d); sides of URS subparallel, not convergent at base, number of setae on genital plate < 24 (15–20) E. patchae

Catalogue of Essigella species

Genus Essigella del Guercio, 1909: 329

Type species : Lachnus californicus Essig, 1909: 1

= Archeoessigella Sorensen, 1994: 21; [new synonym]

= Lambersella Sorensen, 1994: 29; [new synonym]

Essigella alyeska Sorensen, 1988: 118; Sorensen 1994: 72

Essigella californica (Essig), 1909: 1; Sorensen 1994: 53

= Lachnus californicus Essig, 1909: 1

= Essigella claremontiana Hottes, 1957: 79 [synonymy by Sorensen 1994: 53]

= Essigella cocheta Hottes, 1957: 82 [synonymy by Sorensen 1994: 53]

= Essigella monelli Hottes, 1957: 95 [synonymy by Sorensen 1994: 53]

= Essigella pineti Hottes, 1957: 101 [synonymy by Sorensen 1994: 53]

= Essigella swaini Hottes, 1957: 105 [synonymy by Sorensen 1994: 53]

Essigella critchfieldi Sorensen, 1994: 75

Essigella domenechi sp. n.

Essigella eastopi Sorensen, 1994: 30

Essigella essigi Hottes, 1957: 84; Sorensen 1994: 45

Essigella fusca fusca Gillette & Palmer, 1924: 6 ; Sorensen 1994: 34

= Essigella fusca Gillette & Palmer, 1924: 6

= Essigella agilis Hottes, 1957: 71 [synonymy by Sorensen 1994: 34]

= Essigella palmerae Hottes, 1957: 96 [synonymy by Sorensen 1994: 34]

Essigella fusca voegtlini Sorensen, 1994: 39

Essigella gagnonae sp. n.

Essigella hillerislambersi Sorensen, 1994: 41

Essigella hoerneri Gillette & Palmer, 1924: 5; Sorensen 1994: 62

= Essigella gillettei Hottes, 1957: 88 [synonymy by Sorensen 1994: 62]

= Essigella maculata Hottes, 1957: 93 [synonymy by Sorensen 1994: 62]

Essigella kathleenae Sorensen, 1988: 115; Sorensen 1994: 26

Essigella kirki Sorensen, 1988: 121; Sorensen 1994: 22

Essigella knowltoni braggi Hottes, 1957: 73; Sorensen 1994: 84

= Essigella braggi Hottes, 1957: 73 [new status by Sorensen 1994: 84]

= Essigella robusta Hottes, 1957: 103 [synonymy by Sorensen 1994: 84]

Essigella knowltoni knowltoni Hottes, 1957: 92 [new status by Sorensen 1994: 78]

= Essigella knowltoni Hottes, 1957: 92

Essigella patchae Hottes, 1957: 98; Sorensen 1994: 49; [stat. n.]

Essigella pini Wilson, 1919: 2; Sorensen 1994: 49

Essigella sorenseni sp. n.

Essigella wilsoni Hottes, 1957: 106; Sorensen 1994: 67

= Essigella pergandei Hottes, 1957: 100 [synonymy by Sorensen 1994: 67]

= Essigella oregonensis Hottes, 1958: 155 [synonymy by Sorensen 1994: 67]

Discussion

Essigella californica

Sorensen, in his revision of the genus Essigella (1994) had already documented the existence of different host-associated groups within E. californica. He notably mentioned populations living on Pinus flexilis E. James and P. lambertiana Douglas, populations that he nevertheless considered as exhibiting intraspecific variation (Sorensen 1983, 1994). Populations from those two pine species were not considered in the study of Théry et al. (in press) and it is possible that they correspond to yet two more cryptic species. Essigella californica is known to live on at least 34 Pinus species (Kimber et al. 2013) and it is likely that other cryptic species await discovery. We are unable to fully evaluate the species complex here due to a lack of material. The taxonomic nature of Essigella californica continues to be a complex issue meriting further study. Such a study would require substantial material of representative populations from as many known host plants as possible. A redescription of this species and the members of its complex would require morphometric data and multivariate analyses as per Sorensen (1994), combined with molecular phylogenetic and species delimitation methods as per Théry et al. (in press).

Essigella patchae and E. pini

Essigella pini is known to be oligophagous on Pinus and according to Sorensen (1994), it can be found on pine species of the subgenus Pinus, section Trifoliae, subsection Contortae (notably on P. virginiana Miller), subsection Australes (notably on P. taeda Linnaeus), and on pine species of the subgenus Strobus, section Quinquefoliae, subsection Strobus (notably on P. strobus). It may be also found on species of subsection Sylvestres (Sorensen 1994). The type specimen of E. pini was collected in Maryland on P. virginiana (Wilson 1919; Sorensen 1994) whereas that of E. patchae was collected in Maine on P. strobus (Hottes 1957; Sorensen 1994). Genetic material analysed by Théry et al. (in press) came from a Canadian specimen of E. patchae collected on P. strobus and a US specimen of E. pini collected on P. rigida (subsection Australes). Our first suspicions are that E. patchae could be a more northern species that would feed on pines of subsection Strobus whereas E. pini would be more southern developing on pines of both subsections Australes and Contortae. It could appear curious that Sorensen did not discriminate both species, even though they are morphologically very close. Actually, Sorensen himself collected only species occurring in the western part of USA. Because E. pini and E. patchae are the only species occurring in the East, all E. pini and E. patchae specimens that Sorensen studied came from other collections and represented a smaller specimen sample in comparison with other species. Considering the list of specimens Sorensen (1994) studied and those we verified from both USNM and UMSP collections, it is likely that he studied no more than two specimens identified as E. patchae, notably the type specimen in poor condition. Those conditions made revelation of significant differences between the two species difficult.

Molecular data in aphid diagnoses

Aphids represent a relatively well-studied insect group mostly because of their economic importance. Molecular data are most often used in population genetics (Wongsa et al. 2017; Medina et al. 2017). They are used also in works linked with species recognition using barcodes because of their small size and their difficult systematics (Cœur d’acier et al. 2014; Lee et al. 2011). As in other animal groups, new aphid species can be discovered or confirmed using DNA analyses (Depa et al. 2012; Chen et al. 2015; Jiang et al. 2015). The present paper represents the first time that DNA sequence characters have been used in an aphid species diagnosis. Indeed, use of this kind of data and especially substitutions of nucleotides as characters is rare in animal diagnoses (Renner 2016), and rarer in insects. The precedent was established 8 years ago (Brower 2010). The International Code of Zoological Nomenclature does not explicitly recommend DNA sequence data to establish animal taxa, yet nor does it forbid it (ICZN 1999). Other kinds of non-morphological characters are commonly used in other groups. For example, songs or acoustic signals are used to differentiate species in several animal groups and can be considered good diagnostic characters in frogs (Brown and Richards 2008) or in Orthopteran insects (Hertach et al. 2015; Iorgu et al. 2017). In consequence, we judge that the absence, the presence, or the identity of a nucleotide or of a DNA sequence fragment are the molecular equivalent to the absence, the presence, or the shape of a seta, a puncture, or of any other morphological character. We thus support that this kind of DNA character can be used unambiguously in a diagnosis.

Acknowledgments

We are grateful to G. L. Miller (USDA Systematic Entomology Laboratory, Beltsville, MD), P. T. Oboyski (Essig Museum of Entomology, University of California, Berkeley, CA), and R. E. Thomson (University of Minnesota, St Paul, MN) for specimen loans. We extend our thanks to J. T. Sorensen for his advice and assistance in the field. We also thank the two reviewers, Susan Halbert and Roger Blackman, for the helpful comments on previous versions of the manuscript. Mariusz Kanturski gratefully acknowledges the Scholarship for Outstanding Young Scientists from the Ministry of Science and Higher Education of Poland (1165/E-340/STYP/12/2017).

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