Changes Associated with Parasitism in Nematodes. I. Morphology and Physiology of Preparasitic and Parasitic Larvae of Meloidogyne javanica

Abstract

Both morphological and physiological changes associated with the onset of the parasitic mode of life were observed in living second-stage larvae of a plant-parasitic nematode. These changes which take place as the preparasitic or infective larvae of Meloidogyne javanica becomes parasitic in the root of its host were observed in living anesthetized larvae under the phase-contrast microscope at high magnification. Changes in the mobility and infectivity of these larvae during the same periods were also measured. There is an accumulation of granules in the ducts of the subventral esophageal glands shortly before hatching. These granules appear to be associated with penetration both of the eggshell and the plant cell wall and disappear completely within 1 to 3 days of entry into the host. Within this period of time there is an approximate threefold enlargement of the dorsal and subventral esophageal glands. At the same time there is a progressive loss in the ability of the larva to reinfect its host and to move through the soil. The morphological changes which take place in a nematode before and shortly after it infects its host can, in some respects, be studied more readily in forms that are endoparasitic in plants than in forms that parasitize vertebrates. For example, the larva is not spread throughout the body of its host by an active circulatory system or by peristalsis, but develops relatively close to its site of entry, where it can be dissected out. Furthermore, the transition from a freeliving stage to a parasitic stage does not depend on the completion of a molt; consequently the changes associated with the initiation of the parasitic life are not complicated by processes associated directly with molting. The genus Meloidogyne (root-knot nematodes) of the family Heteroderidae, whose female members are all sedentary endoparasites, feed on the tumorlike "giant cells" which they induce in their hosts. The histochemical and ultrastructural development of these structures from the time of larval invasion to the establishment of a close host-parasite relationship has been described for Meloidogyne javanica (Bird, 1961). Received for publication 15 March 1967. Molting in this genus usually takes place under optimal conditions about 2 weeks after larval invasion when three molts occur very close together without any feeding or growth between molts (Bird, 1959; Bird and Rogers, 1965). Nematodes have three esophageal glands, each of which is in fact a large cell. The universal occurrence of these glands in nematodes (Chitwood and Chitwood, 1950) as well as the absence of any other structures of similar size in this region of larval nematodes suggests that they could have various physiological functions. Changes in the morphology and physiology of nematode larvae during the initial stages of their parasitic life have not been recorded in any detail, presumably because it is difficult to remove large numbers of larvae from an animal host shortly after their entry, and to observe both infective and parasitic stages alive and unstained at high magnification. This paper describes observations on living anesthetized infective and parasitic M. javanica larvae examined under phase contrast and a comparison is made of some aspects of their