First evidence of cryptic species diversity and significant population structure in a widespread freshwater nematode morphospecies (Tobrilus gracilis)

Abstract

Free-living nematodes are ubiquitous and highly abundant in terrestrial and aquatic environments, where they sustain ecosystem functioning by mineralization processes and nutrient cycling. Nevertheless, very little is known about their true diversity and intraspecific population structure. Recent molecular studies on marine nematodes indicated cryptic diversity and strong genetic differentiation of distinct populations, but for freshwater nematode species, analogous studies are lacking. Here, we present the first extensive molecular study exploring cryptic species diversity and genetic population structure of a widespread freshwater nematode morphospecies, Tobrilus gracilis, from nine postglacially formed European lakes. Taxonomic species status of individuals, analysed for fragments of the mitochondrial COI gene and for the large (LSU) and small (SSU) ribosomal subunits, were determined by morphological characteristics. Mitochondrial and nuclear markers strongly supported the existence of three distinct genetic lineages (Tg I-III) within Tobrilus gracilis, suggesting that this morphospecies indeed represents a complex of highly differentiated biological species. High genetic diversity was also observed at the population level. Across the nine lakes, 19 mitochondrial, and seven (LSU) and four (SSU) nuclear haplotypes were determined. A phylogeographical analysis revealed remarkable genetic differentiation even among neighbouring lake populations for one cryptic lineage. Priority and persistent founder effects are possible explanations for the observed population structure in the postglacially colonized lakes, but ask for future studies providing direct estimates of freshwater nematode dispersal rates. Our study suggests therefore that overall diversity of limnetic nematodes has been so far drastically underestimated and challenges the assumed ubiquitous distribution of other, single freshwater nematode morphospecies.

Cryptic species of hairworm parasites revealed by molecular data and crowdsourcing of specimen collections

“…Discovery of crypsis is particularly important within groups of morphologically simple invertebrates with few taxonomically diagnostic characters. Recent examples of groups in which cryptic species have been found include cnidarians (Schuchert, 2014), marine nematodes (Derycke et al, 2005), freshwater nematodes (Ristau et al, 2013), and parasitic nematodes (Tan et al, 2012).…”

How to tackle the molecular species inventory for an industrialized nation—lessons from the first phase of the German Barcode of Life initiative GBOL (2012–2015)

“…Among the best lead stories for the media are discoveries of new, invasive or other enigmatic species (for example, parasitic living lampreys, or the notion of the "highest DNA barcode" from a moth from the top of Germany's highest mountain). Several scientific publications have resulted from the first phase of GBOL and the Barcoding Fauna Bavarica project, including, for example, the demonstration of extreme levels (up to 16.7% p-distance) of cryptic diversity in the centipede Stenotaenia linearis (Wesener et al 2015) or a widespread freshwater nematode morphospecies (Ristau et al 2013). Analyzing more than 400 species of true bugs, other colleagues showed that DNA barcodes are highly effective in distinguishing over 90% of Heteroptera species (Raupach et al 2014), and a similar high level of identification success was observed in neuropterids and Lepidoptera (Hausmann et al 2011a(Hausmann et al , 2011b.…”

Species Delimitation and Description of Mesocriconema nebraskense n. sp. (Nematoda: Criconematidae), a Morphologically Cryptic, Parthenogenetic Species from North American Grasslands

“…Geographically, M. curvatum has been reported from six continents in both tropical and temperate climates. Judging from these reports, either this species is an extreme generalist, adapted to a multitude of environments and plant hosts, or M. curvatum as recorded in the literature is actually a composite of morphologically similar but genetically distinct lineages, possibly cryptic species (Fontaneto et al, 2008;Nadler and De Le on, 2011;Ristau et al, 2013;Palomares-Rius et al, 2014).…”