Investigations of the ultrastructure of the juveniles of a related genus, Meloidogyne (Baldwin and Hirschmann 1975a, Bird 1971, Endo and Wergin 1977, Wergin and Endo 1976) and of various other plant (De Grisse 1977, De Grisse et al. 1974, Yuen 1967) and animal (McLaren 1972, 1974, 1976; Wright 1974, 1975) parasitic species of nematodes provided a foundation for current study of the juveniles of the soybean cyst nematode. Likewise, ultrastructural investigations of the nervous system of a free-living soilborne nematode, Caenorhabditis elegans (Ward et al. 1975, Ware et al. 1975), provided insight into the anterior sensory system of a nematode having structural features similar to those of some plant and animal parasitic species. Conversely, structural differences between microbivorous and parasitic nematodes may provide clues to the function of certain sensory organs that relate to parasitism.
The anterior neurosensory organs of the infective juvenile (J2) of the soybean cyst nematode consist of two amphidial receptors, six inner and four outer labial receptors, four cephalic receptors, and two types of accessory receeptors. Each receptor has a characteristic number and arrangement of cilia. Seven cilia are present in the canals of each amphidial receptor, two in each inner labial receptor, and a single cilium in each of the outer labial and cephalic receptors.
The ciliary portions of the two types of accessory receptors are located internally and externally to the amphidial canal. The cilia of the inner accessory receptors have their basal regions just below the cephalic framework, whereas the basal regions of the cilia of the outer accessory receptors merge with the basal regions of the amphidial canal cilia. Some of the outer accessory receptor cilia branch as they ascend toward the cephalic framework. Both types of accessory receptors form swollen ciliary terminals that extend into the various sectors of the cephalic framework. The intermembranous spaces of the inner accessory receptors are electron-translucent, whereas spaces formed by cilia in the outer accessory and other receptors are filled with electron-opaque deposits.
Ventral and posterior to the dendrites supporting the amphidial canal cilia is a microvillar nerve process that extends into the amphidial sheath cell. Each nerve process dendrite terminates in a pair of stout cilia. The membrane of the dendrite that gives rise to these cilia evaginates to form numerous microvilli that characterize this nerve process.
Although observations of sensory organs were concerned with the morphology of Heterodera glycines, they provide a foundation for future studies of neuroresponses and neurosecretions. This information may be used to modify the behavior of plant parasitic nematodes such as H. glycines and would thus affect future strategies for plant pest control in agriculture.
Ultrastructure of the anterior neurosensory organs of juveniles of the soybean cyst nematode is shown in figure 43, figures 4447, figures 48-49, figures 50-51, figures 52-56, figures 57-58, figures 59-61, figures 62-64, and figures 65-67.
2 Reprinted in modified form with permission of Academic Press from Journal of Ultrastructure Research 72:349366, 1980.
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