Date Published: October 31, 2018
Publisher: Public Library of Science
Author(s): Azar Kordbacheh, Robert L. Wallace, Elizabeth J. Walsh, Ulrike Gertrud Munderloh.
Microorganisms, including rotifers, are thought to be capable of long distance dispersal. Therefore, they should show little population genetic structure due to high gene flow. Nevertheless, substantial genetic structure has been reported among populations of many taxa. In rotifers, genetic studies have focused on planktonic taxa leaving sessile groups largely unexplored. Here, we used COI gene and ITS region sequences to study genetic structure and delimit cryptic species in two sessile species (Limnias melicerta [32 populations]; L. ceratophylli [21 populations]). Among populations, ITS region sequences were less variable as compared to those of the COI gene (ITS; L. melicerta: 0–3.1% and L. ceratophylli: 0–4.4%; COI; L. melicerta: 0–22.7% and L. ceratophylli: 0–21.7%). Moreover, L. melicerta and L. ceratophylli were not resolved in phylogenetic analyses based on ITS sequences. Thus, we used COI sequences for species delimitation. Bayesian Species Delimitation detected nine putative cryptic species within L. melicerta and four putative cryptic species for L. ceratophylli. The genetic distance in the COI gene was 0–15.4% within cryptic species of L. melicerta and 0.5–0.6% within cryptic species of L. ceratophylli. Among cryptic species, COI genetic distance ranged 8.1–21.9% for L. melicerta and 15.1–21.2% for L. ceratophylli. The correlation between geographic and genetic distance was weak or lacking; thus geographic isolation cannot be considered a strong driver of genetic variation. In addition, geometric morphometric analyses of trophi did not show significant variation among cryptic species. In this study we used a conservative approach for species delimitation, yet we were able to show that species diversity in these sessile rotifers is underestimated.
Microorganisms are capable of long distance dispersal thus it has been suggested that they have cosmopolitan distributions with little geographic structure [1–4]. However, biogeographical patterns have been documented for many taxa such as soil  and marine bacteria [6–8], protists [9,10], fungi , and rotifers [12,13]. One of the reasons that microorganisms are often considered ubiquitous is the failure to identify cryptic species [14,15]. However, cryptic species complexes commonly occur in these taxa (e.g., [16–19]).
With the advent of molecular tools, detecting cryptic species in rotifers has become a common occurrence, which has improved our knowledge of their diversity. All the previous studies about genetic variation and cryptic species in rotifers are on non-sessile taxa. Here, using two molecular markers we found cryptic diversity within two morphospecies of sessile rotifers, Limnias melicerta and L. ceratophylli. Using the BSD delimitation method, nine putative cryptic species for L. melicerta and four putative cryptic species for L. ceratophylli were identified based on COI gene sequences.