Ageing Biology of a Rhabditid Nematode

LIST OF TABLES

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Table 1: Morphometric data of Teratorhabditis palmarum...............28

Table 2: Structure of ascarosides identified from C. elegans and other nematodes

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Table 3: Sub-behaviours in the mating process of Teratorhabditis palmarum..80

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Introduction

Nematodes are microscopic animals commonly known as eelworms or roundworms. They comprise the phylum Nematoda members of which are present in every habitat aquatic or terrestrial. Nematodes have varied life style and they occupy all trophic levels in the soil ecosystem. They may feed on bacteria (bacteriovorous), fungi (fungivores), other soil microorganism (predators), plants (herbivores) while some may have diverse feeding habitats (omnivores). Scientist conveniently classify them into two groups, parasitic (plants and animals including man) and free living (encompassing all other groups). A great majority of nearly 50% are marine inhabitants and a smaller proportion of about 25% are free-living. Approximately  10% are plant parasites and 15% animal parasites (Viglierchio, 1991). They are widespread in the polar seas, oceans, deserts, hot water springs, mountain tops and the frozen Antarctica. Nematodes are considered to be one of the most abundant groups among the invertebrates on the earth along with the arthropods. Their numbers in different habitats have been estimated by several  workers and range from 1.5 billion  in the upper 20 mm of an acre of marine beach sand to 3 billion in a single acre of soil (Chitwood & Chitwood, 1950). Nearly 16 million are estimated in a square metre of intertidal area (Teal & Wieser, 1966) and 380 million in a square metre of leaf litter (Wasilewska, 1979). Approximately 28,000 nematode species have been described so far and estimates of the probable number of species ranges from 100,000 to 10 million (Poinar, 2011). Their physiology, structure, adaptability and reproductive  patterns have helped them to colonize every habitat on land and in water.

Nematodes may range in size from 100 μm (Greefiella minutum) to about 8 meters (Placentonema gigantissima). However, most soil nematodes are microscopic and are on an average 1-2 mm long although some may be less than 1 mm others  reach almost 11 mm in length. Whatever their size, nematodes have a tubular body shape which is protected with a tough and flexible cuticle. Just beneath the cuticle is the epidermis, usually referred to as the hypodermis, followed by a layer of longitudinal muscles restricted to the interchordal quadrants. Between the hypodermis/longitudinal muscles and the digestive tract is the body cavity, the pseudocoelom which contains the pseudocoelomic fluid. Movement is brought about by a sinusoidal motion generated by coordinated contraction and relaxation of the longitudinal muscles in a dorso-ventral plane on opposite sides of the body. The pseudocoelomic fluid acts as a hydrostatic skeleton and facilitates the somatic muscle

contraction. Nematodes have a complete digestive system divisible into three regions, viz., stomodeum, mesentron and proctodeum. The stomodeum begins at the oral opening and includes a mouth cavity and pharynx. The mesentron consists of the intestine while the proctodeum is made up of the rectum and the anal/cloacal opening. The stomodeum consisting of the feeding apparatus and pharynx is highly variable. This variability is because of the extreme diversity in the modes of feeding that the nematodes have evolved, from bactriovores to predators to plant and animal parasites. When present, the excretory system consists of one or two renette cells that drain through and excretory pore. Mostly the renette cells are associated with longitudinal excretory canals that run along the length of the body in the lateral hypodermis. The nervous system consists of a circumpharyngeal nerve ring with dorsal and ventral nerve cords running the length of the body. Nerves extending from the circumpharyngeal ganglia also enervate the mouth, lips and anterior sense organs. Nematodes have diverse reproductive methods that play a key role to their success. Reproduction may be amphimictic (cross fertilization) or may be through autotoky (reproduction without males), including hermaphroditism, parthenogenesis and pseudogamy.

Their diversity and ubiquotous presence in all types of habitats have made themvery reliable ecological indicators of soil and aquatic ecosystems. Free-living nematodes are a major component of freshwater meiofaunal communities (Traunspurger, 2000; Traunspurger et al., 2012), and cover a body-size spectrum of several orders of magnitude (Traunspurger & Bergtold, 2006). Their varied feeding types is indicative of their trophic specialization (Traunspurger, 1997, 2002), whereas their extensive intraguild species diversity indicates trophic niche specialization based on diverse food sources (Moens et al., 2006). It also provides a food resource for larger benthos (Beier et al., 2004) and pelagic invertebrates and vertebrates (Muschiol et al., 2008; Spieth et al., 2011; Weber & Traunspurger, 2014), indicating their importance in freshwater food webs as trophic intermediaries between benthic microbial producers and macroscopic consumers. Traunspurger (1992) carried the first study on the littoral nematode fauna and its ecology in a Bavarian lake. Studies on vertical distribution of nematodes by (Holopainen & Paasivita, 1977; Traunspurger, 1996; Eyualem-Abebe et al., 2001) showed that the number of species is higher at or close to the surface sediment and decreases towards the deeper sediments.

Nematode communities are susceptible to changes in food supply (Yeates, 1987) and environment (Bongers et al., 1991; Ettema & Bongers, 1993; Freckman & Ettema, 1993; Samoiloff, 1987; Wasilewska, 1989). Hence communities an play role in monitoring decomposition and nutrient cycling (Anderson et al., 1983; Ingham et al., 1985). When community characteristics are evaluated using diversity index (Shannon & Weaver, 1949) or maturity index (Bongers, 1990; Yeates, 1994), the soil biological and ecological health can be assessed. Since nematodes are omnipresent in the ecosystem, they serve as excellent indicators of environmental disturbance (Bongers, 1990; Ferris et al., 2001; Yeates, 2003; Höss et al., 2004; Heininger et al., 2007). Several studies have been carried out to develop relationships between nematode community structure and successions of natural ecosystems or environmental disturbance (Ettema & Bongers, 1993; Freckman & Ettema, 1993; Freckman & Virginia, 1997; De Goede & Dekker, 1993; Wasilewska, 1994; Yeates & Bird, 1994).

Nematodes play an important role in nutrient cycling in soil. Additions of organic matter increase numbers of bacteriovores and fungivorous nematodes and decreases numbers of plant-parasitic nematodes (Bohlen & Edwards,  1994;  Freckman, 1988; Griffiths et al., 1994). Applications of manure provide both organic matter and microbes, a source of food for the nematodes (Andrén & Lagerlöf, 1983; Weiss & Larink, 1991). When bacteria are plentiful in soil, bacteriovorous nematodes may discharge amino acids in substantial amount. However, as bacterial population decrease, nematodes begin to starve, and protein catabolism for maintenance energy requirement leads to increased ammonium excretion by nematodes (Anderson et al., 1983). Nitrogen content appears to be an important measure of potential microbial activity and, subsequently, the rate of decomposition (Neely et al., 1991).

An important ecological contribution was the seven-year study on nematode ecology and their ecosystem services by Overgaard Nielsen (1949). Further notable ecological contributions emerged in the 1970s and 1980s. Centres of ecological study on nematodes developed in Sweden (Sohlenius, 1973b), Poland (Prejs, 1970; Wasilewska, 1970), Italy (Zullini, 1976), and Russia (Tsalolikhin, 1976). In the USA, there was a surge of activity in soil ecology at the National Recourse Ecology Laboratory in Colorado Springs, led by Coleman and others (Yeates & Coleman, 1982), and similar activity at the Institute of Ecology of the University of Georgia, led

by Crossley and colleagues (Stinner & Crossley, 1982). In the same time period, Yeates was developing a very productive programme on the ecology of soil  nematodes in New Zealand (Yeates, 1979). The functional significance of bacterivore and fungivore nematodes was established by the demonstration that their excretion of nitrogen in excess of structural and metabolic needs stimulated plant growth (Ingham et al., 1985).

The idea of using ecological indices as indicators of ecosystem quality (e.g. diversity, stability, and resilience) has received increased attention in recent times. Indices are useful tools as they provide quantitative means to characterize an ecosystem and also to compare different ecosystem (Ferris et al., 1996; Porazinska et al., 1998; Yeates & Bird, 1994; and Yeates et al., 1997). The densities of genera and trophic groups provide the ecological indices of the nematode community. The Shannon-Weaver diversity index (Shannon & Weaver, 1949) compares diversity of genera or trophic groups, and Simpson index (1949) is used to compare generic or trophic dominance. Maturity index Bongers (1990) and Yeates (1994) is a semi- quantitative measure taking into consideration biological and ecological  characteristics of individual nematode species comprising a particular community. These indices offer better prospects for detecting and sufficiently illustrating changes in the soil environment. Bongers (1990) defined two types of maturity indices: Maturity index (MI) which includes nematodes belonging to all feeding types except herbivores, and Plant parasitic index (PPI) which includes herbivores only. In general, the higher a maturity index value, the more mature and stable the ecosystem. Bongers et al., (1995) demonstrated that under certain conditions the PPI and MI may be diametrically opposite suggesting that an increase in the PPI/MI ratio might reflect ecosystem enrichment. It has been found that PPI/MI is an effective indicator of enrichment in agro-ecosystem (Ferris et al., 2001).

Several types of animals have been used as models in experimental biology. Some of the better known ones include the water flea Daphnia, the fruit fly, Drosophila melanogaster, nematodes, zebrafish, mice and primates. The prokaryote, Escherichia coli has also been an excellent model. Nematodes were used to study embryonic development in the later part of the nineteenth and early twentieth century. Some of the well known works includes those of zur Strassen (1896) on Ascaris megalocephala, Martini (1903) on Caenorhabditis elegans and Pai (1928) on T. aceti.

In the present time one