Gerrit Karssen

Bio: Gerrit Karssen is an academic researcher from Ghent University. The author has contributed to research in topics: Phylogenetic tree & Population. The author has an hindex of 20, co-authored 79 publications receiving 2024 citations.

A phylogenetic tree of nematodes based on about 1200 full-length small subunit ribosomal DNA sequences.

Abstract: As a result of the scarcity of informative morphological and anatomical characters, nematode systematics have always been volatile. Differences in the appreciation of these characters have resulted in numerous classifications and this greatly confuses scientific communication. An advantage of the use of molecular data is that it allows for an enormous expansion of the number of characters. Here we present a phylogenetic tree based on 1215 small subunit ribosomal DNA sequences (ca 1700 bp each) covering a wide range of nematode taxa. Of the 19 nematode orders mentioned by De Ley et al. (2006) 15 are represented here. Compared with Holterman et al. (2006) the number of taxa analysed has been tripled. This did not result in major changes in the clade subdivision of the phylum, although a decrease in the number of well supported nodes was observed. Especially at the family level and below we observed a considerable congruence between morphology and ribosomal DNA-based nematode systematics and, in case of discrepancies, morphological or anatomical support could be found for the alternative grouping in most instances. The extensiveness of convergent evolution is one of the most striking phenomena observed in the phylogenetic tree presented here - 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.

Root-knot nematodes

Abstract: Plant-parasitic nematodes devastate crops worldwide, in turn impacting international trade, social and economic development. Effective control of nematodes is essential for crop protection, and requires an understanding of nematode biology, taxonomy, population dynamics and sampling methods. Providing a broad introduction to nematodes as plant parasites, this book begins by describing nematodes by genera, and builds on this foundation to detail nematode biology and pest management, including biological and chemical control. Chapters are authored by international experts and enhanced by extensive illustrations and focus boxes. Fully updated throughout, this new edition is an essential resource for postgraduate students, extension officers, researchers and crop protection scientists

Small subunit rDNA-based phylogeny of the Tylenchida sheds light on relationships among some high-impact plant-parasitic nematodes and the evolution of plant feeding.

Abstract: Cyst (Heteroderidae), root knot (Meloidogyne spp.), and lesion (Pratylenchus spp.) nematodes all belong to a single nematode order, Tylenchida. However, the relationships between and within these economically highly relevant groups, and their relatedness to other parasitic Tylenchida is unclear. We constructed a phylogeny of 116 Tylenchida taxa based on full length small subunit ribosomal DNA (small subunit [SSU] rDNA) sequences. Ancestral state reconstruction points at a gradual development of simple to more complex forms of plant parasitism. Good resolution was observed in distal clades that include cyst, root knot, and lesion nematodes, and monophyly of most families was confirmed. Our data suggest that root knot nematodes have evolved from an ancestral member of the genus Pratylenchus, but it remains unclear which species is closest to this branching point. Contrary to the notoriously polyphagous distal representatives, basal members of the genus Meloidogyne (and probably, their common ancestor) have narrow host ranges. Our analysis also shows that mitotic parthenogeny has arisen at least two times independently among root knot nematodes. In many cases resolution till species was observed, suggesting that SSU rDNA sequences have a potential for DNA barcode-based species identification with, due to the overall conserved nature of this gene, limited intra-species variation.

Invasive plants and their escape from root herbivory: a worldwide comparison of the root-feeding nematode communities of the dune grass Ammophila arenaria in natural and introduced ranges

Abstract: Invasive plants generally have fewer aboveground pathogens and viruses in their introduced range than in their natural range, and they also have fewer pathogens than do similar plant species native to the introduced range. However, although plant abundance is strongly controlled by root herbivores and soil pathogens, there is very little knowledge on how invasive plants escape from belowground enemies. We therefore investigated if the general pattern for aboveground pathogens also applies to root-feeding nematodes and used the natural foredune grass Ammophila arenariaas a model. In the late 1800s, the European A. arenariawas introduced into southeast Australia (Tasmania), New Zealand, South Africa, and the west coast of the USA to be used for sand stabilization. In most of these regions, it has become a threat to native vegetation, because its excessive capacity to stabilize wind-blown sand has changed the geomorphology of coastal dunes. In stable dunes of most introduced regions, A. arenaria is more abundant and persists longer than in stabilized dunes of the natural range. We collected soil and root samples and used additional literature data to quantify the taxon richness of root-feeding nematodes on A.␣arenaria in its natural range and collected samples from the four major regions where it has been introduced. In most introduced regions A. arenaria did not have fewer root-feeding nematode taxa than the average number in its natural range, and native plant species did not have more nematode taxa than the introduced species. However, in the introduced range native plants had more feeding-specialist nematode taxa than A. arenaria and major feeding specialists (the sedentary endoparasitic cyst and root knot nematodes) were not found on A. arenaria in the southern hemisphere. We conclude that invasiveness of A. arenaria correlates with escape from feeding specialist nematodes, so that the pattern of escape from root-feeding nematodes is more alike escape from aboveground insect herbivores than escape from aboveground pathogens and viruses. In the natural range of A. arenaria, the number of specialist-feeding nematode taxa declines towards the margins. Growth experiments are needed to determine the relationship between nematode taxon diversity, abundance, and invasiveness of A. arenaria.

Phylogenetic Analyses of Meloidogyne Small Subunit rDNA.

Abstract: Phylogenies were inferred from nearly complete small subunit (SSU) 18S rDNA sequences of 12 species of Meloidogyne and 4 outgroup taxa (Globodera pallida, Nacobbus abberans, Subanguina radicicola, and Zygotylenchus guevarai). Alignments were generated manually from a secondary structure model, and computationally using ClustalX and Treealign. Trees were constructed using distance, parsimony, and likelihood algorithms in PAUP* 4.0b4a. Obtained tree topologies were stable across algorithms and alignments, supporting 3 clades: clade I = [M. incognita (M. javanica, M. arenaria)]; clade II = M. duytsi and M. maritima in an unresolved trichotomy with (M. hapla, M. microtyla); and clade III = (M. exigua (M. graminicola, M. chitwoodi)). Monophyly of [(clade I, clade II) clade III] was given maximal bootstrap support (mbs). M. artiellia was always a sister taxon to this joint clade, while M. ichinohei was consistently placed with mbs as a basal taxon within the genus. Affinities with the outgroup taxa remain unclear, although G. pallida and S. radicicola were never placed as closest relatives of Meloidogyne. Our results show that SSU sequence data are useful in addressing deeper phylogeny within Meloidogyne, and that both M. ichinohei and M. artiellia are credible outgroups for phylogenetic analysis of speciations among the major species.

Top 10 plant-parasitic nematodes in molecular plant pathology

Abstract: The aim of this review was to undertake a survey of researchers working with plant-parasitic nematodes in order to determine a ‘top 10’ list of these pathogens based on scientific and economic importance. Any such list will not be definitive as economic importance will vary depending on the region of the world in which a researcher is based. However, care was taken to include researchers from as many parts of the world as possible when carrying out the survey. The top 10 list emerging from the survey is composed of: (1) root-knot nematodes (Meloidogyne spp.); (2) cyst nematodes (Heterodera and Globodera spp.); (3) root lesion nematodes (Pratylenchus spp.); (4) the burrowing nematode Radopholus similis; (5) Ditylenchus dipsaci; (6) the pine wilt nematode Bursaphelenchus xylophilus; (7) the reniform nematode Rotylenchulus reniformis; (8) Xiphinema index (the only virus vector nematode to make the list); (9) Nacobbus aberrans; and (10) Aphelenchoides besseyi. The biology of each nematode (or nematode group) is reviewed briefly.

Biotic interactions and plant invasions

Abstract: Introduced plant populations lose interactions with enemies, mutualists and competitors from their native ranges, and gain interactions with new species, under new abiotic conditions. From a biogeographical perspective, differences in the assemblage of interacting species, as well as in abiotic conditions, may explain the demographic success of the introduced plant populations relative to conspecifics in their native range. Within invaded communities, the new interactions and conditions experienced by the invader may influence both its demographic success and its effects on native biodiversity. Here, we examine indirect effects involving enemies, mutualists and competitors of introduced plants, and effects of abiotic conditions on biotic interactions. We then synthesize ideas building on Darwin’s idea that the kinds of new interactions gained by an introduced population will depend on its relatedness to native populations. This yields a heuristic framework to explain how biotic interactions and abiotic conditions influence invader success. We conclude that species introductions generally alter plants interactions with enemies, mutualists and competitors, and that there is increasing evidence that these altered interactions jointly influence the success of introduced populations. Ecology Letters (2006) 9: 726‐740

Soil biota and invasive plants

Abstract: Interactions between plants and soil biota resist invasion by some nonnative plants and facilitate others. In this review, we organize research and ideas about the role of soil biota as drivers of invasion by nonnative plants and how soil biota may fit into hypotheses proposed for invasive success. For example, some invasive species benefit from being introduced into regions of the world where they encounter fewer soil-borne enemies than in their native ranges. Other invasives encounter novel but strong soil mutualists which enhance their invasive success. Leaving below-ground natural enemies behind or encountering strong mutualists can enhance invasions, but indigenous enemies in soils or the absence of key soil mutualists can help native communities resist invasions. Furthermore, inhibitory and beneficial effects of soil biota on plants can accelerate or decelerate over time depending on the net effect of accumulating pathogenic and mutualistic soil organisms. These 'feedback' relationships may alter plant-soil biota interactions in ways that may facilitate invasion and inhibit re-establishment by native species. Although soil biota affect nonnative plant invasions in many different ways, research on the topic is broadening our understanding of why invasive plants can be so astoundingly successful and expanding our perspectives on the drivers of natural community organization.