By: Javier Ovalles
Introduction to Nematoda
Nematoda, commonly known as Roundworms, include over 25,000 species that have been described. Of which more than half are parasitic. The total number of nematode species has been estimated to be about 1 million. Some species of roundworm may contain more than 27 million eggs at one time and lay more than 200,000 of them in a single day. Some scientists have estimated that there may be as many as half a million more unknown species of roundworm yet to be discovered. Nematodes were once classified with a very large and heterogeneous cluster of animals grouped together on the basis of their overall worm-like appearance, simple structure of an internal body cavity called a pseudocoelom, and the lack of features such as cilia and a well-defined head that are found in most animals.
Ascaris Lumbricoides (Ascaris Lumbricoides)
Male nematodes use chemotaxis to locate females. They have no visual abilities, and instead are attracted to specific sex pheromones which females release. Once the male has located a mate, it uses copulatory accessories such as papillae, spicules and its curved tail to direct sperm and stabilize the female during mating. Males possess two testes and a curved posterior end with spicules for copulation. Females possess ovaries which are continuous with an oviduct and a tubular uterus. The uteri join to form a vagina which opens into the vulva. Sperm is transferred into the vulva of the female, enters the ovum and forms a zygote. Females have been shown to lay as many as 234,000 eggs per day, and this daily egg output implies year-round mating with no specific breeding season. The eggs can survive in the soil for some time, and the larvae require 8-12 weeks after ingestion to reach reproductive maturity. The range number of offspring is 234,000 eggs laid daily at the high and an average number of 200,000 eggs laid daily.
Aphelenchoides Ritzemabosi (Aphelenchoides Ritzemabosi)
A. ritzemabosi multiplies bisexually and not by parthenogenesis; fertilized females go on reproducing for 6 months without further fertilization. A female lays about 25-30 eggs in a compact group; eggs hatch in 3-4 days and the juveniles take 9-10 days to reach maturity In susceptible chrysanthemum varieties the female remains in the leaf at one place, feeds on adjoining cells and lays many eggs which hatch into rapidly developing juveniles; however, in resistant varieties the female moves about rapidly through the tissues, feeding on cells, but lays only a few eggs whose emerging juveniles fail to reach maturity. A. ritzemabosi can reproduce on fungi, and soil fungi may therefore contribute to its survival in the absence of a host.
Pine Wood Nematode (Bursaphelenchus xylophilus)
The pine wilt nematode has a typical nematode life cycle, with four juvenile stages and an adult stage with both male and female individuals that reproduce sexually. In nature it reproduces most rapidly in the summer, producing large numbers of individuals that spread throughout the resin canal system of susceptible pines, into the trunk, the branches, and the roots. If living tree cells are no longer available the parasite feeds and reproduces on the fungal hyphae growing in the resin canals. In the fall and winter the parasite becomes inactive.