Polypodium
hydriforme infection in the eggs of acipenseriform
fishes
E.V. Raikova-2002
Journal of
Applied Ichthyology, 18(4-6): 405-415
Abstract:
Polypodium hydriforme
is the only cnidarian adapted to intracellular parasitism in oocytes of
acipenserid and polyodontid fishes. Twelve species of the Acipenseridae and
one species of Polyodontidae are known to be infected. Acipenser ruthenus
(sterlet) is the most susceptible species. Investigations of the life cycle
of Polypodium reveal a unique system of adaptations of a
multicellular organism to prolonged (several years) parasitism inside a
single cell, the fish oocyte. The life cycle consists of parasitic and
free-living phases. Parasitic stages are found within pre-vitellogenic
oocytes (binucleate cell, cell-in-a-cell, cleavage stages including a
long-lasting morula) and in vitellogenic oocytes (planula; stolon without
buds and with internal buds). The binucleate cell gives rise to a generative
cell and to an enveloping polyploid cell, the trophamnion. All parasitic
stages develop inside a cavity formed by the trophamnion. The position of
the germ layers in planula and stolon is inverted: the endoderm faces yolk
of the host oocyte. Eversion of the stolon takes place inside the infected
oocytes before spawning, and the normal position of the germ layers is
restored. During the eversion, the yolk of the oocyte fills the gastral
cavities of the stolon buds supplying the future free-living specimens with
food. In mid-summer free-living specimens form gametophores filled with
binucleate cells and armed with nematocysts. Possible modes of infection of
the host are discussed.
(Institute of Cytology of the Russian
Academy of Sciences, 4 Tichoretsky Ave, 194064 St. Petersburg, Russia. E-mail: Eraikova@or1659.spb.edu)