Nylanderia, Emery, 1906

Williams, Jason L., Zhang, Y. Miles, LaPolla, John S., Schultz, Ted R. & Lucky, Andrea, 2022, Phylogenomic Delimitation of Morphologically Cryptic Species in Globetrotting Nylanderia (Hymenoptera: Formicidae) Species Complexes, Insect Systematics and Diversity 6 (1), pp. 1-15 : 3

Nylanderia Taxonomy

Species within Nylanderia and six other closely-related genera— collectively called the ‘ Prenolepis genus-group’—have vexed taxonomists for more than a century because most species have few to no readily distinguishable morphological characters ( LaPolla et al. 2010, 2012; Williams and LaPolla 2016; Williams and Lucky 2020). In Nylanderia , globetrotting species co-occur alongside morphologically identical endemic species on nearly every continent, leading to difficulty distinguishing species. Historical classifications based mainly on morphology are insufficient for identifying species that concurrently exhibit high intraspecific variation and low interspecific variation. Often, the characters cited for distinguishing workers of Nylanderia species are subtle and include differences in color, size, cuticular sculpturing/microstructure, the presence or absence of pubescence or pilosity on parts of the body, or minor differences in macrosetal counts ( Kallal and LaPolla 2012, LaPolla et al. 2011, LaPolla and Kallal 2019, Trager 1984). Male characters—especially genitalia—are often cited as useful ( Gotzek et al. 2012, LaPolla and Kallal 2019, Trager 1984), though males are relatively rare in collections, not known at all for many species, and difficult to associate with worker conspecifics if not collected as part of a nest series. Queens, at least externally, appear to be the least useful among workers, queens, and males for distinguishing Nylanderia species.

The potential ecological and economic consequences of taxonomic confusion in this genus are significant considering the widespread co-occurrence of globetrotting Nylanderia species with morphologically identical endemic species. For example, three range-limited endemic Caribbean species— Nylanderia lucayana LaPolla and Kallal , Nylanderia coveri LaPolla and Kallal , and Nylanderia esperanza LaPolla and Kallal —are very similar morphologically to the globetrotting N. guatemalensis ( LaPolla and Kallal 2019) , rendering conservation of endemics and management of the invasive species extremely difficult. Similarly, in the southeastern United States in the early 2000s, early records of the now-economically devastating introduced species Nylanderia fulva (Mayr) were misidentified as Nylanderia pubens (Forel) . The confusion resulted from identification based solely on examination of workers; currently, these two species can be distinguished only by differences in male genitalia. Males of N. fulva are distinguished from N. pubens by genitalic shape and pilosity: in profile, the secondary gonopods (formerly called parameres; see Boudinot (2018)) of N. fulva are triangular, less sclerotized, and have fewer macrosetae arising from the margin, while those of N. pubens are more rounded, well sclerotized, and have a dense, fan-like projection of long macrosetae arising from the margin ( Gotzek et al. 2012, Trager 1984).

Species boundaries—along with the full extent of intraspecific and interspecific morphological variation of all castes and sexes in the fulva complex—remain unclear and the taxonomic utility of male genitalia in Nylanderia remains untested. Because the molecular phylogeny of Gotzek et al. (2012) did not provide conclusive evidence that N. pubens is distinct from N. fulva, Deyrup (2016) asserted that N. pubens could be a more ‘hairy-tailed’ form of N. fulva , perhaps with a different geographic origin. Investigation of genitalic variation in N. pubens has been particularly difficult because male specimens are relatively rare in collections.

As the specific example of N. fulva and N. pubens illustrates, the interpretation of intra- versus interspecific morphological variation in the context of an inclusive molecular phylogeny is necessary for integrative delimitation of cryptic Nylanderia species. Previous studies have sequenced five to six nuclear markers and COI barcodes to reconstruct the phylogeny of Nylanderia ( Gotzek et al. 2012, LaPolla et al. 2010, Matos-Maraví et al. 2018), while Williams et al. (2020) were the first to employ UCE sequences from Nylanderia species representing all major biogeographic regions. However, methods of molecular species delimitation have never been employed for resolving some of the most difficult questions involving globetrotting and invasive Nylanderia species. Some species as currently defined, such as N. bourbonica , appear to be highly morphologically variable and to have broad distributions. In these cases, it is necessary to determine whether populations sampled across the full geographic range are most closely related to each other (i.e., truly constitute a single, monophyletic species) and to document the limits of morphological and genetic variation expressed across the species. Simultaneously, it is also necessary to identify cases in which morphological variation overlaps in distantly related species.