Advances in Entomopathogenic Nematode Taxonomy and Phylogeny. Nematology Monographs and Perspectives.
The rise of molecular methods caused a revolution in the systematics of many organisms, including entomopathogenic nematodes (EPN). From the 1990s, we experienced an enormous increase in the number of described EPN species. However, many of these taxa were inadequately supported by morphology and molecular data. Thus, in 2007 two respected nematode taxonomists, K. B. Nguyen and D. J. Hunt, prepared their excellent book entitled "Entomopathogenic Nematodes: Systematics, Phylogeny and Bacterial Symbionts" (Nguyen & Hunt 2007) that clarified the EPN taxonomy and became a vital tool for any EPN taxonomist. In his review, Ralf Ehlers wrote (Ehlers 2008) that he expected the 2nd edition of the book would come out in 10 yr and expressed the wish that the book would be published with a hard cover. Indeed, the growth in the number of EPN species being discovered has not stopped, and from the year 2007 about 50 new EPN species have seen the light of day. The new book, "Advances in Entomopathogenic Nematode Taxonomy and Phylogeny" by the same authors, came out in 2016 and in a nice hard cover. Just in time.
The book is divided into 6 main chapters. The introductory section gives the perspective of the authors and briefly describes the current situation in EPN systematics. The 2nd chapter is focused on taxonomy and systematics. At the beginning, the authors engagingly summarize the milestones of EPN systematics achieved in the last 100 yr. In the next part, the authors examine all described EPN species and select 95 valid species of Steinernema (Rhabditida: Steinernematidae) and 16 of Heterorhabditis (Rhabditida: Heterorhabditidae). The authors also include an updated list of species inquirendae and nomina nuda and discuss the taxonomic status of problematic species in the form of notes at the end of the chapter.
Notably, 11 species have been proposed as junior synonyms of existing taxa based on molecular data: Steinernema anatoliense, Hazir, Stock & Keskin, 2003; Steinernema meghalayense Ganguly, Rathour & Singh, 2011; and Steinernema websteri Cutler & Stock, 2003 are regarded as conspecific with Steinernema carpocapsae Weiser, 1955; Steinernema everestense Khatri-Chhetri Waeyenberge, Spiridonov, Manandhar & Moens, 2011 is a junior synonym of Steinernema akhursti Qiu, Hu, Zhou, Mei, Nguyen & Pang, 2005; Steinernema dharanai Kulkarni, Rizvi, Kumar, Paunikar & Mishra, 2012 is a junior synonym of Steinernema hermaphroditum Stock, Griffin & Chaerani, 2004; Steinernema maqbooli Fayyaz, Khanum, Gulsher & Javed, 2013 is a junior synonym of Steinernema pakistanense Shahina, Anis, Reid, Rowe & Maqbool, 2001; Steinernema tbilisiense Gorgadze, Lortkhipanidze, Ogier, Tailliez & Burjanadze, 2015 is a junior synonym of Steinernema thesami (Gorgadze, 1988) Gorgadze & Lortkipanidze, 2004; Heterorhabditis gerrardi Plichta, Joyce, Clarke, Waterfield & Stock, 2009 is a junior synonym of Heterorhabditis indica Poinar, Karunakar & David, 1992; Heterorhabditis sonorensis Stock, Rivera-Orduno & Flores-Lara, 2009 is a junior synonym of Heterorhabditis taysearae Shamseldean, Abou El-Sooud, Abd-Elgawad & Saleh, 1996; Heterorhabditis somsookae Maneesakorn, An, Grewal & Chandrapatya, 2015 is a junior synonym of Heterorhabditis baujardi Phan, Subbotin, Nguyen & Moens, 2003; and Heterorhabditis pakistanensis Shahina, Tabassum, Salma, Mehreen & Knoetze, 2016 is a junior synonym of H. indica.
The problem with the original descriptions of these junior synonyms was usually the use of non-edited sequences that created false differences from the existing species or an inadequate selection of reference sequences. This shows the extreme importance of molecular data in EPN systematics but also the necessity of good practice in interpretation. It is obvious that the genetic markers currently used in EPN taxonomy, the ITS and D2-D3 expansion segments of the 28S ribosomal RNA (rRNA), are sufficient for distinguishing species of EPN. What is also evident is that the level of 95% and lower similarity in the ITS sequence for the separation of new species, as suggested in the 1st edition (Nguyen & Hunt 2007), cannot be sustained because some species regarded as valid are separated by no more than 3%. The authors left this topic without comment; however, the status of EPN species will probably need further clarification in the future.
It might seem that nematode morphology is out of date. However, besides being a source of joy for nematologists, the morphology can be useful in many other ways. Excellent examples are the tabular keys to species of Steinernema and Heterorhabditis that form another section of the new book. Each key is easy to follow because the nematode species are ordered by body length of the infective juvenile stage, and the key provides the most important characters of the infective juvenile stage and male, including a photograph of the male spicules. The key can be very useful, especially when dealing with EPN from a particular geographic region with known EPN diversity (e.g., the major part of Europe or the USA), but it can also be used to rank any EPN to a particular phylogenetic group.
The major part of the book is occupied by species descriptions of Steinernema and Heterorhabditis from 2007. From my own experience with the previous edition, I can state that this compilation is probably the most valuable part of the book. For any taxonomic work, it is extremely useful to have all the information to hand, instead of searching for particular manuscripts of which only some may be available online.
The last chapter was written by 2 renowned experts in nematode molecular biology, S. E. Spiridonov and S. A. Subbotin, and is focused on the phylogenies of Heterorhabditis and Steinernema nematodes. The phylogenetic reconstructions generally correspond to current knowledge but are definitely among the best published so far. The phylogenetic analysis of the genus Heterorhabditis, performed using the ITS region of the rDNA, without doubt confirmed the closer relationship of the "bacteriophora" and "megidis" groups. In the genus Steinernema, the analysis based on the sequences of the D2-D3 of 28S and ITS rRNA genes did not support the traditional division into 5 main clades but gave 3 clearly supported superclades. The position of the "affine" clade and several smaller groups is unclear. Thus, it is evident that although the relationships on the lower phylogenetic level are satisfactorily resolved with the regularly used rDNA genes, for the clarification of the deeper phylogeny we will have to wait for new genes or whole genome data. Within the steinernematid phylogenetic tree, the authors propose 12 strongly supported subclades named after flagship member species. The use of this terminology for the subclades within the genus Steinernema would, if accepted by other researchers, simplify orientation within the taxonomy of the group.
The biogeography of entomopathogenic nematodes is a largely unexplored area. The phylogeographic analysis presented within this chapter is the first of its kind ever made for EPN and shows very interesting patterns. Clearly, many steinernematid groups originated on the Asian continent, which corresponds to the fact that more than half of all known EPN species have been recovered from this area, including a large number of indigenous species. We can only hope that the phylogeographic approach will become a more frequent tool in EPN research.
My conclusion is very short. Together with the previous edition, the present book is a must-have for anyone dealing with the taxonomy of entomopathogenic nematodes, but it is also valuable for any research or student nematologist. And finally, shall we see a 3rd edition? Considering the rate of new EPN species descriptions and the still relatively large unexplored areas (most of Africa and South America), the answer seems clear.
The review first appeared in "Nematology" but should be of interest to entomologists, too. Used with permission of the author.
Ehlers R-U. 2008. Book review of Entomopathogenic Nematodes: Systematics, Phylogeny and Bacterial Symbionts. (Nguyen KB & Hunt DJ, eds.). Nematology 10: 449-450.
Nguyen KB & Hunt DJ. 2007. Entomopathogenic Nematodes: Systematics, Phylogeny and Bacterial Symbionts. Nematology Monographs and Perspectives 5. Brill, Leiden, Netherlands.
Biology Centre CAS, Institute of Entomology Branisovska 31, 37005 Ceske Budejovice Czech Republic
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|Author:||Frank, J. Howard|
|Article Type:||Book review|
|Date:||Jun 1, 2017|
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