1 Taxonomy and Systematics

1 Taxonomy and Systematics BYRON J. ADAMS AND KHUONG B. NGUYEN Entomology and Nematology Department, University of Florida, PO Box 110620, Gainesville, Florida, USA 1.1. Introduction 1 1.2. General Biology 2 1.3. Origins 2 1.4. Molecular Systematics 4 1.4.1. Species identification 4 1.4.2. Phylogenetic analysis 6 1.5. Taxonomy and Systematics of Family Steinernematidae (Chitwood & Chitwood, 1937) 7 1.5.1. Morphological review 7 1.5.2. Taxonomy 10 1.5.3. Phylogenetic relationships 13 1.6. Taxonomy and Systematics of Family Heterorhabditidae (Poinar, 1976) 21 1.6.1. Morphological review 21 1.6.2. Taxonomy 23 1.6.3. Phylogenetic relationships 24 1.7. Conclusions and future prospects 25 Acknowledgements 28 References 28 1.1. Introduction The discovery of entomopathogenic nematode species and the rate at which they have been described is correlated with the historical need for biological alternatives to man- age insect pests.After the initial discovery and subsequent development of Steinernema glaseri as a biological control agent in the early 20th century, research on entomopathogenic nematodes remained somewhat dormant as chemical-based pest control measures remained cheap, effective and relatively unregulated.Upon recognition of their negative environmental effects in the 1960s, pesticides gradually became more CAB International 2002. Entomopathogenic Nematology (ed. R. Gaugler) 1 2 B.J. Adams and K.B. Nguyen restricted, less effective, and much more costly.Consequently, the search for biological alternatives to chemical-based pest management programmes received renewed atten- tion from scientists.By the 1980s, fuelled by an enormous infusion of resources from government and industry, research on entomopathogenic nematodes rapidly expanded.The search for new species of nematodes that could provide effective control of a persistent pest – and be marketed as such – was on. 1.2. General Biology The invasive stage (= infective juveniles) of Steinernema carries cells of a symbiotic bacterium in the anterior part of the intestine (genus Xenorhabdus).When infective juveniles enter the haemocoel of a host, they release the bacterium.The bacterium multiplies rapidly in the insect blood and kills the insect by septicaemia, usually within 24–48 h.The nematodes then change to feeding third-stage juveniles, feed on the bacteria, and moult in succession to the fourth stage where they become adult females and males of the first generation.After mating, the females lay eggs that hatch as first-stage juveniles and moult successively to the second, third and fourth stages. The fourth-stage juveniles develop into adult females and males of the second generation.The adults mate and the eggs produced by the second-generation females hatch as first-stage juveniles that moult to the second stage.Nematode reproduction continues until resources in the cadaver are depleted, usually allowing for two to three genera- tions (long cycle, Fig.1.1).Ifthe food supply is limited, the eggs produced by the first-generation females develop directly into infective juveniles (short cycle, Fig.1.1). As resources in the cadaver are extinguished, late second-stage juveniles cease to feed and incorporate a pellet of bacteria in the bacterial chamber, or vesicle.They then moult to pre-infective and infective stages, retaining the cuticle of the second stage as a sheath.The infective juveniles then typically leave the cadaver in search of a new host. Infective juveniles do not feed and can survive in the soil for several months. The life cycle of Neosteinernema is similar to that of Steinernema, except that Neosteinernema has only one generation, females move out into the environment and the offspring become infective juveniles in the female body before emerging from it.A bacterial endosymbiont of Neosteinernema has been observed, but remains uncharacterized. Little is known of this nematode beyond its taxonomic description. The life cycle of Heterorhabditis is also similar to that of Steinernema, but the her- maphroditic female in Heterorhabditis replaces the first generation in Steinernema, and the symbiotic bacterium is Photorhabdus instead of Xenorhabdus. 1.3. Origins Poinar (Poinar, 1983, 1993) proposed a mid-Palaeozoic origin (375 million years ago) for Steinernema and Heterorhabditis.He also postulated that independent, convergent evolution produced their similar morphology and life history traits.Based on similarities of the buccal capsule and male tail morphology, Poinar suggested that Heterorhabditis evolved from a ‘Pellioditis-like’ ancestor in a sandy, marine environment, while Steinernema arose from terrestrial ‘proto-Rhabditonema’ ancestors (Poinar, 1993). A preliminary evolutionary tree of the Nematoda based on 18S ribosomal DNA (Blaxter et al., 1998) bolsters Poinar’s ideas concerning the origins of Heterorhabditis Taxonomy and Systematics 3 Short cycle (Food scarce) J2 (~2–3 days) J1 PI (~1 day) Penetration to production of G1 males and females (~ 3 days) Food scarce J3 to J4 G1 males & females IJ Penetration stages feed produce eggs J1 Food plentiful G1 Male spicule morphology J1 From eggs to PI J2 PI (~2–3 days) G2 male spicule morphology J2 J3 J1 G2 males & females J4 produce eggs Long cycle (Food plentiful) Fig. 1.1. Diagram of the life cycle of Steinernema with illustrations of different stages that are important for identification (based on S. scapterisci). G1 = first generation, G2 = second generation, J1 = first-stage juvenile, J2 = second-stage juvenile, J3 = third-stage non-infective juvenile, PI = pre-infective stage juvenile, IJ = third-stage infective juvenile, J4 = fourth-stage juvenile. Morphological differences between long and short cycles of Heterorhabditis have also been observed (Wouts, 1979). and Steinernema.The tree depicts Heterorhabditis as being most closely related to an order of vertebrate parasites, the Strongylida. Heterorhabditis and Strongylida share a most recent common ancestor with Pellioditis (Rhabditoidea, a superfamily of order Rhabditida).The same tree depicts Steinernema as being most closely related to the Panagrolaimoidea and Strongyloidae (both superfamilies of Rhabditida), and as a member of a larger clade that includes plant parasitic, fungivorous, and bactivorous species of the orders Aphelenchida, Tylenchida and the Cephalobidae (Order of the Rhabditida).Thus, preliminary molecular data suggest that Heterorhabditis arose from a free-living bactivorous ancestor, while reconstruction of the trophic habits of Steinernematid ancestors remains ambiguous. Poinar’s hypothesis that Heterorhabditis and Steinernema do not share an exclusive common ancestry is upheld by Blaxter et al. (1998) and is consistent with other analy- ses (Sudhaus, 1993; Liu et al., 1997; Adams et al., 1998).All suggest that Heterorhabditis and Steinernema independently evolved relationships with bacteria and insects from disparate, unrelated ancestors.However, Burnell and Stock (2000) hint that these 4 B.J. Adams and K.B. Nguyen findings may be premature, and offer an astonishing scenario – that the genus Steinernema is paraphyletic, with Heterorhabditis arising from within the Steinernematidae. 1.4. Molecular Systematics Hampering efforts to describe new species of entomopathogenic nematodes is a rapidly diminishing pool of taxonomists.To a non-specialist, different species of Steinernema, and especially Heterorhabditis, look similar.Although excellent morphol - ogy-based taxonomic keys exist (Hominick et al., 1997), the task of sorting through numerous field-collected nematode samples and making species level taxonomic iden- tifications can be frustrating.Recent advances in molecular biology and evolutionary theory, while no substitute for the expertise of a taxonomist, can provide nematol- ogists with additional tools for identifying, delimiting and representing species of entomopathogenic nematodes (Liu et al., 2000).Several recent species descriptions integrate morphological and molecular genetic data (Gardner et al., 1994; Liu and Berry, 1996a; Stock et al., 1996; Stock, 1997; Stock et al., 1998; Van Luc et al., 2000). Molecular characters can be used to identify, diagnose and delimit species in the same way as morphological characters.The allure of molecular data is their clear genetic basis, a compelling advantage given the significant amount of environmental and host-induced morphological variation displayed by entomopathogenic nematodes.Potentially sensitive, objective and powerful, molecular characters can also be dangerous when wielded carelessly.Below we discuss the utility of some common molecular markers used to identify and diagnose species of entomopathogenic nematodes. 1.4.1. Species identification In theory, a nematode’s entire genome can be drawn upon for molecular genetic markers to discriminate a particular taxonomic rank.Nevertheless, for ease of comparison, markers that have the greatest representation in genetic databases are the most convenient starting points for thoroughly investigating the taxonomic status of an unidentified entomopathogenic nematode.Consequently, markers that are not easily databased or cannot produce consistently replicable results among comparable taxa and investigators are falling out of favour.Often overlooked are those that give an indication of population structure (subspecific variation), vital information for deter- mining the extent of species boundaries. Based on representation in genetic databases, the most popular marker for identifying unknown nematodes is the DNA sequence

Load more