Aphelenchoididae Panagrolaimorpha

Rhabditomorpha Diplogasteromorpha

■ Clade 8 Spirurina

■ Clade 7 Teratocephalidae

■ Clade 6 Plectida

Monhysterida Araeolaimida

■ Clade 4 Desmodorida

■ Clade 3 Chromadorida

■ Clade 3 Chromadorida

Clade 2 Dorylaimida Mononchida Trichinellida Mermithida

Fig. 3.1 Schematic overview of the evolution of the phylum Nematoda derived from SSU rDNA sequence data (based on Holterman et al. 2006). Major lineages of plant-parasites are indicated by dotted boxes (Tylenchomorpha, Dorylaimida and Triplonchida). It is noted that the infraorder Tylenchomorpha is possible a polyphyletic group; it includes the members of Clade 12 and the Aphelenchoididae, a family within Clade 10

Fig. 3.1 Schematic overview of the evolution of the phylum Nematoda derived from SSU rDNA sequence data (based on Holterman et al. 2006). Major lineages of plant-parasites are indicated by dotted boxes (Tylenchomorpha, Dorylaimida and Triplonchida). It is noted that the infraorder Tylenchomorpha is possible a polyphyletic group; it includes the members of Clade 12 and the Aphelenchoididae, a family within Clade 10

1,215 small subunit ribosomal DNA sequences covering a wide range of nematode taxa was presented by van Megen et al. (2009). The overall topology of this phylo-genetic tree resembles that of Holterman et al. (2006). However, the support values for the backbone tend to be lower. The deep subdivision of the phylum Nematoda should be regarded as a 'work in progress', and a multi loci approach will be required for a more definitive framework.

The extensiveness of convergent evolution is one of the most striking phenomena observed in the phylogenetic tree of nematodes—it is hard to find a morphological, ecological or biological characteristic that has not arisen at least twice during nematode evolution. Convergent evolution appears to be an important additional explanation for the seemingly persistent volatility of nematode systematics. One of the peculiarities of the phylum Nematoda is the multitude of times that (animal- or plant-) parasitic lifestyles have arisen. Understanding the phylogenetic history of the acquisition of particular phenotypes associated with successful parasitism permits fuller appreciation of the evolutionary constraints experienced by organisms adapting to new hosts. Plant-parasitism has evolved independently in each of three major clades in the phylum Nematoda (Fig. 3.1). The plant-parasitic Tylenchomor-pha, Dorylaimida, Triplonchida have acquired their ability to parasitize plants independently. The infraorder Tylenchomorpha comprises manifestly the economically most relevant plant-parasites and within this chapter we will mainly focus on this particular group.

Apart from the scientific merits of studying of the phylogeny of the Nematoda, the underlying molecular framework can be used for DNA barcode-based nematode detection and community analysis. It is (in most cases) possible to define species-specific sequence signatures and to design simple and cheap PCR primers that allow real-time PCR-based detection and quantification of pathogenic nematodes in complex DNA backgrounds. At the same time, the SSU rDNA alignment has been used to design many family-specific PCR primers (see for example Holterman et al. 2008b) and quantitative DNA barcode-based nematode community analyses under field conditions are currently being tested.

3.3 Phylogeny of Tylenchomorpha 3.3.1 Overview

The Tylenchomorpha, the most intensively investigated infraorder within the Ty-lenchina, comprises the largest and most economically important group of plant-parasitic nematodes. Although there are examples of nematodes that exploit all plant organs including flowers and seeds, they mostly attack roots. The evolution of plant-parasitic Tylenchomorpha is of particular interest because associations range from transitory grazing by root-hair feeders to the highly complex host-pathogen interactions of gall-inducing nematodes and their hosts. Non plant-parasitic Tylen-chomorpha feed on fungi, algae, lichens, mosses, insects, mites, leeches or frogs

(Siddiqi 2000). However, the evolution of this diversity of complex feeding traits is not yet fully understood. In recent years LSU and SSU rDNA sequences have been used to infer relationships among Tylenchomorpha (Subbotin et al. 2006; Bert et al. 2008; Holterman et al. 2009). A multiple gene approach derived from an EST mining strategy has been used to characterise the relationships between the plant-parasitic genera Meloidogyne, Heterodera and Globodera (Scholl and Bird 2005). Other phylogenetic studies within Tylenchomorpha, mainly based on rDNA sequences or the internal transcribed spacers, have been restricted to individual (super) families or genera. Recent studies include analyses of Heteroderidae (Subbotin et al. 2001), Anguinidae (Subbotin et al. 2004), Criconematoidea (Subbotin et al. 2005), Hoplolaimidae (Subbotin et al. 2007), Meloidogyne (Tandingan De Ley et al. 2002; Tenente et al. 2003; Tigano et al. 2005) and Pratylenchus (Subbotin et al. 2008).

In Fig. 3.2, a schematic phylogenetic framework of the Tylenchomorpha based on Bert et al. (2008) and Holterman et al. (2009) is shown. The families Hoplo-laimididae (including Heteroderinae), Pratylenchidae (except Pratylenchoides) and Meloidogynidae, which comprises the economically most important plant-parasites, plus the genera Tylenchorhynchus and Macrotrophurus form a well supported clade. A robust sister relationship between Meloidogyne (root-knot nematodes) and representatives of the migratory endoparasitic Pratylenchidae (Pratylenchus, Zy-gotylenchus and Hirschmanniella) can be observed. The Hoplolaimidae, which include the Heteroderinae, Hoplolaiminae, Rotylenchoidinae and Rotylenchulinae according to the classification of de Ley and Blaxter (2002), appear as a monophyletic group. Remarkably, the migratory endoparasitic Radopholus, a notorious pest in banana and citrus, has a well-supported sister relationship with the Hoplolaimidae. Thus, the cyst-forming (Heteroderinae) and root-knot nematodes (Meloidogyne) are likely to have arisen independently and the migratory endoparasitic Pratylenchidae appears to be polyphyletic.

Between the earliest divergences within the Tylenchomorpha (Aphelenchoidea) and the top parts of the tree, a number of branching points remain unresolved. Although we cannot define the relationship between the suborder Criconematoi-dea and other Tylenchomorpha, its members clearly constitute a separate and well supported clade. Also, the tylenchid nematodes with supposedly ancestral morphological characters, including Tylenchidae and Sphaerularioidea, do not have an established phylogenetic relationship. Nevertheless, the tylenchid nematodes—those Tylenchomorpha that are characterized by a tylenchid stylet (Tylenchomorpha without Aphelenchoididae)—appear to be clearly monophyletic. The Aphelen-choidea or "aphelenchs" comprising the mainly fungal-feeding Aphelenchidae and Aphelenchoididae are appointed as polyphyletic in all molecular analyses to date. However, the morphology based hypothesis of their monophyly could not be significantly rejected based on statistical analysis of molecular data (Bert et al. 2008). Several studies have confirmed the sister relationship of the predominantly plant-parasitic Tylenchomorpha (without Aphelenchoidea) with the bacteriovorous Cephalobidae (Blaxter et al. 1998). However, this was not unequivocally supported by van Megen et al. (2009).

Trophic ecology | sedentary endoparasitic migratory endoparasitic | migratory ectoparasitic | sedentary ectoparasitic ■ lower plant-epidermal cell root hair feeder fungivore | insect parasitic bacteriovore I vertebrate parasite












Radopholus Criconematoidea

Belonolaimidae Pratylenchoides





Aphelenchidae Cephalobomorpha



Strongyloides Steinernema le el h p

0 0

Post a comment