Critical Thinking

Nematodes Up to now, approximately 4100 nematode species

Nematodes are one of the most numerous Metazoa on our
planet. They can be either free-living or plant-parasitic or animal parasites.
Although they occur in almost every habitat, they are essentially aquatic
animals. Soil structure, soil pH and other factors can affect nematodes by
different ways (Decraemer & Hunt,
2013).
Different groups of nematodes have adapted to different habitats through the
evolution over time.

Up to now, approximately 4100 nematode species have been
described as plant-parasites over the world (Decraemer & Hunt,
2013). In
Belgium, the nematofauna has been relatively well studied. However, a lot of
new species descriptions are being updated year by year. Therefore, In order to
obtain a more comprehensive overview of the nematode diversity, it is necessary
to investigate nematodes from various habitats.

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This thesis focuses on the investigation of plant-parasitic
nematodes from neglected biotopes that provide a more detailed description of
plant-parasitic nematode biodiversity in Belgium. The combination of molecular
and morphological data in classification will contribute the knowledge to
understand the controversial taxonomical problems as well as phylogenetic
relationships.

1.1. 
Nematodes in general

Nematodes are pseudocoelomate, unsegmented worm-like
animals, commonly described as filiform or thread-like, a characteristic
reflected by the taxon name nema
(Greek, nema= thread) and its
nominative plural nemata (Decraemer & Hunt,
2013).

The history of nematodes was marked with the oldest
reference from China in 2500 B.C. with the description of symptoms and
treatment of the relatively large intestinal roundworm Ascaris or Huei Ch’ung (Maggenti, 1981). Due to their small size and atypical symptom, the
reports of plant-parasitic nematodes were rarely found in ancient references.
It is suggested that the first awareness of plant-parasitic nematodes were
known in antiquity (235 B.C.) since the ancient Chinese symbol resembles in
shape an adult female soybean cyst nematode that was used to describe itself (Noel, 1992). Needham (1742) provided the first description of wheat seed plant-parasitic
nematodes. Currently, nematodes are generally regarded as a separate phylum
that is Nematoda or Nemata (De Ley & Blaxter,
2002). De Ley and Blaxter (2002) presented the systematic scheme that is based on the
higher classification proposed and reflect new taxa proposals with three basal
clades. Nonetheless, recent molecular phylogenetic analyses seemed to be more
precise with 12 clades within the Nematoda (Holterman et al., 2006).

The phylum Nematoda consists of about 27 000 described
species (Hugot et al., 2001). The prediction of nematode number can up to a hundred
million, but more accurate numbers can be about 100 000 species (Coomans, 2000) to ten million (Lambshead, 2004). To date, the number of described
plant-parasitic nematodes over the world is estimated by approximately 4100
species (Decraemer & Hunt,
2013).

According to recent studies, plant-parasitic nematodes
groups probably constitute several separate origins of parasitism (Quist et al., 2015; Sánchez-Monge
et al., 2017).
Strikingly, nowadays plant-parasitism has evolved several times independently
from fungivorous ancestors and plant-parasitic taxa located in the basic clade
1 (Trichodoridae), clade 2 (Longidoridae) and in the more advanced clade 12 by
Tylenchomorpha (Holterman et al., 2006).

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