The importance of being integrative: a remarkable case of synonymy in the genus Viridiscus (Heterotardigrada: Echiniscidae)

Abstract

There are two predominant sources of taxonomically useful morphological variability in the diverse tardigrade family Echiniscidae: the internal structure and surface sculpture of the cuticular plates covering the dorsum (sculpturing) and the arrangement and morphology of the trunk appendages (chaetotaxy). However, since the appendages often exhibit intraspecific variation (they can be reduced or can develop asymmetrically), sculpturing has been considered more stable at the species level and descriptions of new echiniscid species based solely on morphology are still being published. Here, we present a case study in which a detailed analysis of the morphology and multiple genetic markers of several species of the genus Viridiscus shows that cuticular sculpture may also exhibit considerable intraspecific variation and lead to false taxonomic conclusions. In a population collected from the eastern Nearctic, in the type locality of the recently described species V. miraviridis, individuals with transitional morphotypes between those reported for V. viridissimus and V. miraviridis were found. Importantly, all morphotypes within the viridissimus-miraviridis spectrum were grouped in a single monospecific clade according to rapidly evolving markers (ITS-1, ITS-2 and COI). Given the morphological and genetic evidence, we establish V. miraviridis as a junior synonym of V. viridissimus. This study explicitly demonstrates that a lack of DNA data associated with morphological descriptions of new taxa jeopardizes the efforts to unclutter tardigrade systematics. Additionally, V. perviridis and V. viridissimus are reported from Lâm Đồng Province in southern Vietnam, which considerably broadens their known geographic ranges.

Keywords: DNA barcoding; Dorsal sculpturing; Integrative taxonomy; Junior synonym; Nearctic.

Fig. 1

Dorsal plate sculpturing of sexually mature females of (a) Viridiscus aff. viridianus from North America and (b) Viridiscus perviridis from Madeira. PCM photomicrographs; scale bars in μm, both images in the same scale

Fig. 2

The variability (morphotypes) of dorsal plate sculpturing in V. viridissimus; in every row (a–d), the first column shows all dorsal plates, whereas the second, third and fourth columns show close up views of the scapular, second paired, and the caudal plates, respectively: (a) the most common morphotype described in the original description of the species with epicuticular granules present only in the anterior portions of the paired segmental plates, the anterior portion of median plate 2 and on the median plate 3 (the viridissimus morphotype; Indochina), (b) epicuticular granules extend toward the posterior portion of paired segmental plate 2 (indicated by the white frame) (morphotype intermediate between the viridissimus and miraviridis morphotypes; North America), (c) compared to morphotype “a”, epicuticular granules present on both median and paired segmental plates (indicated by filled arrowheads; intermediate morphotype; North America), (d) epicuticular granules dominate the dorsal armor, and 2–4 neighboring pores merge into large irregular pores, especially on the caudal plate (indicated by empty arrowheads; miraviridis morphotype/V. miraviridis syn. nov.; North America). PCM photomicrographs; scale bars in μm, scale in all columns the same as indicated in the first row

Fig. 3

Phylogenetic relationships of the genus Viridiscus: left panel: Bayesian tree based on the concatenated ITS-1 + ITS-2 dataset (1064 bp); vertical bars denote different delineation methods used in the formulation of the primary species hypotheses: sculpture (dorsal plate sculpture observed in PCM), ASAP, ABGD, and bPTP; right panel: Bayesian tree based on COI (611 bp). Asterisks indicate the maximal (1.00) posterior probability value; (v) – a hologenophore was secured for post hoc PCM analysis [the miraviridis morphotype and an intermediate morphotype between viridissimus and miraviridis are indicated in square brackets]. Echiniscus succineus was used as an outgroup