4. ANALYSIS OF FACTORS INVOLVED IN SYMBIOSIS
81
Although in some instances artificial, the nutritional requirements
are being discussed under the headings of carbohydrates, lipids, and
proteins.
a. Carbohydrates
The occurrence of stored glycogen in the soma of a number of
intramolluscan parasitic helminths, especially pre-adult stages of
trematodes, is known (Axmann, 1947 ; Ginecinskij, 1960 ; Ginecinskij
and Dobrovalskij, 1962 ; Snyder and Cheng, 1961 ; Cheng and Snyder,
1962a; Cheng, 1963b,c; Palm, 1962a,b; Cheng and Burton, 1966;
and others). The source of this polysaccharide theoretically could be
from the metabolism of fatty acids via acetyl CoA or from amino acids
via pyruvic acid or acetyl CoA, with both processes involving reversal
of the Embden-Meyerhof glycolytic pathway, and some probably is
synthesized via these pathways. There is evidence, however, that most
of the glycogen is synthesized from glucose obtained from the molluscan
host. The series of studies which have led to this conclusion was
initiated by Faust (1920) who detected a reduction in stored glycogen
in trematode-infected snails. Hurst (1927) has confirmed this finding.
Von Brand and Files (1947), employing biochemical techniques, have
concurred by reporting a significant reduction in the amount of stored
glycogen in Australorbis gbbratus infected with Schistosoma mansoni.
These investigators stated that “whether this disease is due to an
impaired carbohydrate digestion and resorption, or to a toxic action by
the parasite is not clear ”. Furthermore, they have stated: “ The
possibility that it is due to the food consumption of the parasites has
not been ruled out completely.” Similarly, Zischke and Zischke (1965)
have reported a reduction in host glycogen in trematode-parasitized
snails.
An explanation for this reduction in host glycogen has been given by
Cheng and Snyder (1962a) who studied this aspect of the relationship
between the freshwater pulmonate Helisoma trivolvis and the trematode
G l y ~ € h e l m ~ n s
pennsylvaniensis. We have demonstrated that concurrent with the reduction of glycogen in the host’s hepatopancreatic
cells, there is an increase in the amount of stored glycogen in the
developing cercariae. This led to the postulation that the parasites
utilize the host’s glycogen and to further search for evidence. By
employing histochemistry, Cheng and Snyder (1963) have demonstrated that the gradual reduction of glycogen in the mollusc’s hepatopancreas is due to the breakdown of this polysaccharide to glucose
which in turn permeates the host’s hepatopancreatic cell membrane
and the sporocyst wall and becomes incorporated in the developing
A.P.B.--~
7
81
Although in some instances artificial, the nutritional requirements
are being discussed under the headings of carbohydrates, lipids, and
proteins.
a. Carbohydrates
The occurrence of stored glycogen in the soma of a number of
intramolluscan parasitic helminths, especially pre-adult stages of
trematodes, is known (Axmann, 1947 ; Ginecinskij, 1960 ; Ginecinskij
and Dobrovalskij, 1962 ; Snyder and Cheng, 1961 ; Cheng and Snyder,
1962a; Cheng, 1963b,c; Palm, 1962a,b; Cheng and Burton, 1966;
and others). The source of this polysaccharide theoretically could be
from the metabolism of fatty acids via acetyl CoA or from amino acids
via pyruvic acid or acetyl CoA, with both processes involving reversal
of the Embden-Meyerhof glycolytic pathway, and some probably is
synthesized via these pathways. There is evidence, however, that most
of the glycogen is synthesized from glucose obtained from the molluscan
host. The series of studies which have led to this conclusion was
initiated by Faust (1920) who detected a reduction in stored glycogen
in trematode-infected snails. Hurst (1927) has confirmed this finding.
Von Brand and Files (1947), employing biochemical techniques, have
concurred by reporting a significant reduction in the amount of stored
glycogen in Australorbis gbbratus infected with Schistosoma mansoni.
These investigators stated that “whether this disease is due to an
impaired carbohydrate digestion and resorption, or to a toxic action by
the parasite is not clear ”. Furthermore, they have stated: “ The
possibility that it is due to the food consumption of the parasites has
not been ruled out completely.” Similarly, Zischke and Zischke (1965)
have reported a reduction in host glycogen in trematode-parasitized
snails.
An explanation for this reduction in host glycogen has been given by
Cheng and Snyder (1962a) who studied this aspect of the relationship
between the freshwater pulmonate Helisoma trivolvis and the trematode
G l y ~ € h e l m ~ n s
pennsylvaniensis. We have demonstrated that concurrent with the reduction of glycogen in the host’s hepatopancreatic
cells, there is an increase in the amount of stored glycogen in the
developing cercariae. This led to the postulation that the parasites
utilize the host’s glycogen and to further search for evidence. By
employing histochemistry, Cheng and Snyder (1963) have demonstrated that the gradual reduction of glycogen in the mollusc’s hepatopancreas is due to the breakdown of this polysaccharide to glucose
which in turn permeates the host’s hepatopancreatic cell membrane
and the sporocyst wall and becomes incorporated in the developing
A.P.B.--~
7