The utility of wing morphometrics for assigning type specimens to cryptic bumblebee species

Abstract

Since the beginning of taxonomy, species have been described based on morphology, but the advent of using semio‐chemicals and genetics has led to the discovery of cryptic species (i.e. morphologically similar species). When a new cryptic species is described, earlier type specimens have to be re‐evaluated, although this process can be challenging as only nondestructive methods ought to be used in order to preserve the integrity of the type specimens. Methods should allow comparison with recently collected specimens clustered based on chemical, ethological and/or genetic traits with old specimens (i.e. type specimens) where only morphological traits are available. Here we develop a method based on geometric morphometric analyses of wing shape for a taxonomically challenging group of bumblebees, the subgenus Alpinobombus Skorikov. We consider nine monophyletic taxa (including several cryptic species) to assess the accuracy of this method to discriminate the taxa based on their wing shape and then to attribute type specimens using a leave‐one‐out cross‐validation procedure. We show that, for these bees, wing shape is taxon‐specific, except for two sister taxa for which the species status is still debated. Moreover, for most of the taxa, type specimens were correctly attributed with high posterior probabilities of attribution, except for a few type specimens corresponding to the same two sister taxa where taxa delimitation based on wing shape was previously the subject of discussion. Our study highlights the potential of geometric morphometric analyses to help in the re‐attribution of type specimens when the existence of cryptic species is revealed.

Resolving the species status of overlooked West‐Palaearctic bumblebees

“…The highly polytypic nature of bumblebees makes their taxonomy especially complex (Williams, 1998). Although the increasing use of genetic markers (Ghisbain, Lozier, et al., 2020; Williams et al., 2020), semio‐chemical traits (Martinet et al., 2018, 2019) sometimes combined with other tools (e.g., geometric morphometrics on the wings, Gérard et al., 2020) is significantly refining our global comprehension of this diverse group of bees, some taxa have remained overlooked. Here, we clarified the taxonomic status of several uncommon bumblebee taxa belonging to the former subgenera Eversmannibombus , Laesobombus and Mucidobombus , now gathered in the monophyletic genus Thoracobombus (Williams et al., 2008).…”

“…While traditional taxonomy is mainly based on discrete morphological traits (Cipola et al., 2014; Ji & Du, 2014; Rampini et al., 2012), such traits can fail to detect species in taxon groups with low or no morphological differentiation (i.e., cryptic species) or in groups exhibiting large morphological variability at the intraspecific level (e.g., in some bumblebee species complexes: Carolan et al., 2012; Ghisbain, Lozier, et al., 2020; Williams et al., 2012, 2020). Subsequently, many attempts have been made to improve species delimitation by using alternative features such as shapes (Aytekin et al., 2007; Gérard et al., 2020), genetic markers (White et al., 2014), or semio‐chemical markers (Martin et al., 2008). Nevertheless, each of these approaches presents its own limitations (for bumblebees see review in Lecocq, Dellicour, et al., 2015; Williams et al., 2015).…”

A worthy conservation target? Revising the status of the rarest bumblebee of Europe

“…Wing shape has been an increasingly utilised discriminant character for insect taxonomy (e.g. Grimaldi & Engel, 2006) and especially in bumblebee taxonomy (Dehon et al ., 2019), although limitations of this approach have also been discussed (Lecocq et al ., 2015a; Gérard et al ., 2020). Briefly, closely related but heterospecific taxa are not always expected to present significantly distinct wing shape.…”

“…Briefly, closely related but heterospecific taxa are not always expected to present significantly distinct wing shape. However, the technique is non‐destructive and has the advantage to allow quantified morphological comparisons with older material from which no genetic sequences can be obtained (Dehon et al ., 2019; Gérard et al ., 2020).…”

“…(2019) and Gérard et al . (2020) (complete protocol detailed in Appendix 4 of the Supporting Information). Prior to the assignment, shape variation within the reference dataset and species‐level discrimination were assessed by linear discriminant analyses (LDA) on the projected aligned configuration of landmarks.…”