An updated look at the uneven distribution of cryptic diversity among parasitic helminths

Abstract

Cryptic parasite diversity is a major issue for taxonomy and systematics, and for attempts to control diseases of humans, domestic animals and wildlife. Here, we re-examine an earlier report that, after correcting for sampling effort, more cryptic species of trematodes are found per published study than for other helminth taxa. We performed a meta-analysis of 110 studies that used DNA sequences to search for cryptic species in parasitic helminth taxa. After correcting for study effort and accounting for the biogeographical region of origins, we found that more cryptic species tend to be uncovered among trematodes, and fewer among cestodes and animal-parasitic nematodes, than in other helminth groups. However, this pattern was only apparent when we included only studies using nuclear markers in the analysis; it was not seen in a separate analysis based only on mitochondrial markers. We propose that the greater occurrence of cryptic diversity among trematodes may be due to some of their unique features, such as their mode of reproduction or frequent lack of hard morphological structures, or to the way in which trematode species are described. Whatever the reason, the high frequency of cryptic species among trematodes has huge implications for estimates of parasite diversity and for future taxonomic research.

A fine‐scale phylogenetic assessment of digenean trematodes in central Alberta reveals we have yet to uncover their total diversity

“…From a recent meta-analysis of 110 studies, it has been noted that there is a trend, particularly among trematodes, that sequencing effort positively correlates with more cryptic species as opposed to any other group of helminths. This has been attributed to differences in trematode biology, and our ability as taxonomists to identify them by their morphological characters, or lack thereof (Pérez-Ponce de León & Poulin, 2018).…”

“…Because of high saturation within the cox1 gene for this family, nad1 has been the gene of choice in the literature (Detwiler et al, ; Georgieva et al, ; Morgan & Blair, ) and best represented the samples within GenBank for comparisons. For all other families, partial cox1 was used (Gordy et al, ; Moszczynska, Locke, McLaughlin, Marcogliese, & Crease, ; Van Steenkiste, Locke, Castelin, Marcogliese, & Abbott, ). Nucleotide sequence inspection, trimming, alignments, model testing for best‐fit substitution models, and maximum‐likelihood (ML) and Bayesian inference (BI) phylogenetic analyses were as described in Gordy et al ().…”

“…Furthermore, the onset of molecular methods for species identifications has widened the knowledge gap through revelations of prior undetected diversity that cannot always be traced to a described species. In fact, the revelation of cryptic species is enhanced with greater sequencing effort, and more so for trematodes than any other group of parasitic helminth (Pérez-Ponce de León & Poulin, 2018). This, overall, can make it incredibly difficult to understand the larger picture when it comes to parasite ecology because we are lacking long-term, field studies, and precision in data collection.…”

Incipient genetic divergence or cryptic speciation? Procamallanus (Nematoda) in freshwater fishes (Astyanax)

“…Solórzano-García, Nadler & Pérez-Ponce de León, 2016;Mockett et al, 2017;Lagunas-Calvo et al, 2019). In many cases, relatively low levels of genetic divergence are accompanied by the absence of conspicuous morphological changes, leading to the recognition of cryptic species complexes (see Pérez-Ponce de León & Nadler, 2010;Nadler & Pérez-Ponce de León, 2011;Pérez-Ponce de León & Poulin, 2018). Our results indicate an incipient genetic divergence of P. (S.) neocaballeroi, accompanied by the lack of conspicuous of morphological differences among the three lineages ( Figure 4); detailed observations of the specimens through light and scanning electron microscopy yielded no evident morphological differences among the studied specimens; we acknowledge, however, that we could not detect such differences at this time because some specimens were found as immature adults, and our sampling size was relatively small given the low prevalence and abundance of the nematode (Table S4).…”

Systematics and diversification of Anindobothrium Marques, Brooks & Lasso, 2001 (Eucestoda: Rhinebothriidea)

“…The morphological homogeneity of these three marine species of Anindobothrium and their distribution in two species of batoid fishes draws attention to a component on the diversity of cestodes that might have been neglected in the past, that is cryptic species—those assumed to be only recognized by the examination of molecular data. Reports of cryptic species have been published for cestodes before (see reviews in [ 82 , 83 ]), but to the best of our knowledge, no accounts of cryptic species exists for cestodes infecting elasmobranchs. Even so, compared to other groups of Metazoa, there is a small number of publications addressing cryptic speciation in cestodes.…”