“Forms” of water mites (Acari: Hydrachnidia): intraspecific variation or valid species?

Abstract

In many groups of organisms, especially in the older literature, it has been common practice to recognize sympatrically occurring phenotypic variants of a species as “forms”. However, what these forms really represent often remains unclear, especially in poorly studied groups. With new algorithms for DNA-based species delimitation, the status of forms can be explicitly tested with molecular data. In this study, we test a number of what is now recognized as valid species of water mites (Hydrachnidia), but have in the past been treated as forms sympatrically occurring with their nominate species. We also test a form without prior taxonomical status, using DNA and morphometrics. The barcoding fragment of COI, nuclear 28S and quantitative analyses of morphological data were used to test whether these taxa merit species status, as suggested by several taxonomists. Our results confirm valid species. Genetic distances between the form and nominate species (Piona dispersa and Piona variabilis, COI 11%), as well as likelihood ratio tests under the general mixed-Yule coalescent model, supported that these are separately evolving lineages as defined by the unified species concept. In addition, they can be diagnosed with morphological characters. The study also reveals that some taxa genetically represent more than one species. We propose that P. dispersa are recognized as valid taxa at the species level. Unionicola minor (which may consist of several species), Piona stjordalensis, P. imminuta s. lat., and P. rotundoides are confirmed as species using this model. The results also imply that future studies of other water mite species complexes are likely to reveal many more genetically and morphologically distinct species.

A new species group in Megaselia, the lucifrons group, with description of a new species (Diptera, Phoridae)

“…However, our molecular results show that Megaselia subnitida is genetically quite distinct from Megaselia lucifrons . The COI genetic distance is 15%, which is far above the threshold usually considered as indicating species-level differentiation (e.g., Stålstedt et al 2013). This essentially confirms Schmitz’s original view on the species.…”

Bryobia abyssiniae(Prostigmata: Tetranychidae), a new species from the highlands of Ethiopia

“…Although the region we examined is a different COI fragment than typically used in barcoding studies of other taxa (i.e., the Folmer fragment; Folmer et al 1994), we chose this particular fragment because it has been subject to extensive analysis in the genus Bryobia (Ros et al 2008). In addition, amplification failed in multiple preliminary trials in our study that used the universal primers (Stålstedt et al 2013) for the Folmer fragment.…”

“…Where such morphological variants are observed, DNA-barcoding has the potential to help researchers determine whether different morphological "forms" represent intraspecific variation or separate species. In some cases, genetic differences are consistent with morphological differences (Stålstedt et al 2013), while in others morphological distinctions are better explained by phenotypic plasticity (Glowska et al 2013). For example, among tetranychid mites, molecular markers showed that despite a large phenotypic plasticity T. urticae and T. cinnabarinus belong to the same species (Auger et al 2013).…”

“…Differences of a similar degree were found recently in a species of quill mite, another member of the Prostigmata; despite morphological variation in this species, COI sequences differed by only 1.6-2.4% between variants (Glowska et al 2013). In order to confirm that different forms of water mite are valid species (Acari: Hydrachnidia), a minimum threshold of 17-23% sequence variation has been proposed, which is much higher than the sequence variability observed within populations of a single known species (Martin et al 2010;Stålstedt et al 2013).…”

Geometric and traditional morphometrics for the assessment of character state identity: multivariate statistical analyses of character variation in the genusArrenurus(Acari, Hydrachnidia, Arrenuridae)

“…They analysed 15 morphological characters, five of which allowed for the recognition of five presumed cryptic species; additionally, phylogenetic analyses of molecular data indicated that Uroobovella nova consisted of at least four morphologically similar species. Similarly, Stålstedt, Bergsten & Ronquist () evaluated the species status of five water mites, Unionicola minor (Soar, 1900) (Unionicolidae), Piona stjordalensis (Thor, 1897) (Pionidae), Piona imminuta (Piersig, 1897), Piona rotundoides (Thor, 1897), and Piona dispersa Sokolow, 1926. They analysed traditional morphometric data to compare with the results of a separate phylogenetic analysis of nuclear and mitochondrial sequences.…”