Twenty Years after De Ley and Blaxter—How Far Did We Progress in Understanding the Phylogeny of the Phylum Nematoda?

Abstract

Molecular phylogenetics brought radical changes to our understanding of nematode evolution, resulting in substantial modifications to nematode classification implemented by De Ley and Blaxter and widely accepted now. Numerous phylogenetic studies were subsequently published that both improved and challenged this classification. Here we present a summary of these changes. We created cladograms that summarise phylogenetic relationships within Nematoda using phylum-wide to superfamily-wide molecular phylogenies published in since 2005, and supplemented with the phylogenetic analyses for Enoplia and Chromadoria with the aim of clarifying the position of several taxa. The results show which parts of the Nematode tree are well resolved and understood, and which parts require more research, either by adding taxa that have not been included yet (increasing taxon coverage), or by changing the phylogenetic approach (improving data quality, using different types of data or different methods of analysis). The currently used classification of the phylum Nematoda in many cases does not reflect the phylogeny and in itself requires numerous improvements and rearrangements.

Comparative reconstruction of the predatory feeding structures of the polyphenic nematodePristionchus pacificus

“…Shared among Rhabditina representatives, in contrast to Tylenchina and Myolaimina, are: (9) pm1 as three syncytia; (10) pm3 as three syncytia (“paired cells”); (11) pm4 as one syncytium; (12) pm5 as one syncytium; (13) absence of HypD. Asterisk indicates equivocal character polarity, given the unresolved position of Myolaimina (Ahmed & Holovachov, 2021 ), missing states in Plectida, and (for 13) incomplete data also in Myolaimina…”

“…Shared among Rhabditina representatives, in contrast to Tylenchina and Myolaimina, are: (9) pm1 as three syncytia; (10) pm3 as three syncytia ("paired cells"); (11) pm4 as one syncytium; (12) pm5 as one syncytium; (13) absence of HypD. Asterisk indicates equivocal character polarity, given the unresolved position of Myolaimina (Ahmed & Holovachov, 2021), missing states in Plectida, and (for 13) incomplete data also in Myolaimina derived from the telostegostom (Fürst von Lieven & Sudhaus, 2000). However, the telostegostom had still remained undefined in Diplogastridae: in A. halicti, cells of two classes ("m b ," "m c ") were found to line the tooth, the posterior of which (m c ) lined the DGO, while a third class ("m d ") consisted of cells lining the most posterior region of the mouth.…”

Introductory Chapter: Roundworms from Past to Present

“…More than 30,000 members of the phylum Nematoda, also known as roundworms, live in different habitats. This number increases even more with phylogenetic studies [1,2].…”

“…Various types of phylogenetic research on nematode systematics are carried out by continuously changing molecular techniques. These studies have added many new species to the nematode group [1][2][3].…”

Aonchotheca (Nematoda: Capillariidae) is validated as a separated genus from Capillaria by both mitochondrial and nuclear ribosomal DNA

“…The nuclear small subunit of the ribosomal gene (18S rRNA) has been frequently used to analyze phylogenetic relationships among nematodes at different taxonomic levels, especially of some representative capillariids [ 5 , 10 – 14 ]. Genome-based phylogenetic analyses such as mitochondrial (mt) genome (mitogenome) are increasingly utilized by the taxonomist for a better understanding of nematodes’ relationships at various taxonomic levels and/or judgment of the discordances and congruences with morphology-based and nuclear rDNA-based phylogenies due to the shortcomings of 18S rRNA [ 15 , 16 ]. Within the family Capillariidae, the currently available mitogenomes are representatives of the genus Capillaria Zeder, 1800 (MH665363), Eucoleus Dujardin, 1845 (NC_056391), and Pseudocapillaria Freitas, 1959 (MZ708958).…”