Rhopalosiphum laconae (Taber, 1993)

Skvarla, Michael J., Miller, Gary L., Bauchan, Gary, Lewis, Matthew, Foottit, Robert & Maw, Eric, 2018, Taxonomy and Natural History of Cattail Aphids, Rhopalosiphum enigmae (Hemiptera: Aphidomorpha: Aphididae), Including a New Synonymy and Notes on Ant and Parasitoid Associates of Rhopalosiphum, Insect Systematics and Diversity 2 (2018), No. 2, pp. 1-14 : 8-9

R. laconae Types

Holotype and paratypes of R. laconae were reported to be deposited in the USNM ( Taber 1993); however, an extensive search, including correspondence with the author, did not find any such specimens. It is unclear whether the specimens were ever deposited or perhaps lost after deposition, but they apparently no longer exist. In order to avoid future confusion about the identity of R. laconae due to the lack of a name-bearing specimen, an apterous female paratype collected from the type locality previously housed at NCSU was designated as the neotype ( Fig. 6) and deposited in the USNM collection.

Collections and Phylogenetic Analysis

Forty collections of cattails aphids were made across nine U.S. states, including within a few miles of the type locality of R. laconae ; specimens with morphology corresponding to R. enigmae , R. laconae , and forms with intermediate morphology were found ( Fig. 7 , Table 3). R. laconae is present along the east coast of the United States from Delaware south through Georgia, a much larger range than originally reported. However, forms with morphology intermediate between R. enigmae and R. laconae exist throughout much of the range and especially near areas where R. enigmae occurs. These intermediate forms include specimens with small abdominal marginal tubercles 1 and 7 but abdominal marginal tubercles 2–6 always present, specimens with large abdominal marginal tubercles 1 and 7 but abdominal marginal tubercles 2–6 sporadically present or absent, R. laconae specimens with a long processus terminalis (up to a ratio of pt: base of antenna VI of 6.3), and specimens with long and short dorsal abdominal setae. Examining morphology alone, it was unclear whether R. enigmae and R. laconae are separate species with large hybrid zones or if they are a single species that exhibits a continuum of morphology across a large geographic area.

COI sequence data were obtained for 69 R. enigmae or R. laconae individuals from 49 localities ( Table 3), 49 of which were included in the analyses. Additionally, COI sequence data were obtained for six R. musae and four R. nymphaeae individuals from two and three localities, respectively. The resultant phylogenetic hypothesis had well-supported (posterior probably>95%) clades that corresponded to R. nymphaeae , R. musae , and R. enigmae + R. laconae ( Fig. 8 ). Within the R. enigmae + R. laconae clade there was some structure, including three clades that were well supported, two of which contain either R. enigmae or R. laconae exclusively. However, R. enigmae , R. laconae , and intermediate forms were interspersed throughout the larger R. enigmae + R. laconae clade, so the two well-supported subclades are better explained by their geographic closeness than by morphological similarity.

R. nymphaeae , R. musae , and R. enigmae + R. laconae exhibited less than 1% difference in COI within each clade and 4.6–7% difference between clades ( Table 4). This level of variation is typical of within- and between-species difference in COI reported in Rhopalosiphum ( Valenzuela et al. 2009) and other aphids ( Foottit et al. 2008, Foottit et al. 2009, Wang et al. 2011, Rebijith et al. 2013).

The lack of genetic differentiation within COI, lack of phylogenetic structure within the R. enigmae + R. laconae clade, and morphological gradation from R. enigmae through intermediate forms into R. laconae along a geographic gradient strongly suggest that R. enigmae and R. laconae are not separate species. We therefore declare that R. laconae is a junior synonym of R. enigmae .

Figures

Fig. 7. Map of cattail aphid collections.Closed symbols represent collections included in the phylogenetic analysis, open symbols represent locality records without corresponding DNA.

Fig. 8. Phylogenetic hypothesis inferred using Baysean analysis based on COI sequence data. Posterior probabilities greater than 95% are represented by black circles.