DEVELOPMENT OF CYST-LIKE CELLS OF THE FLAGELLATE LEPTOMONAS ONCOPELTI IN THE MIDGUT OF THE HEMIPTERAN
ONCOPELTUS FASCIATUS
M. N. Malysheva,1 A. O. Frolov,1,* S. O. Skarlato 2
1 Zoological Institute RAS, and 1 Institute of Cytology RAS, St. Petersburg;
* e-mail: frolal@online.ru
The structure of cyst-like cells of Leptomonas oncopelti (Trypanosomatidae) found in the midgut of the bug
Oncopeltus fasciatus (Lygaeidae) was examined with light and electron microscopy. The formation of "cysts"
begins with an unequal division of active flagellates with promastigote configuration. Cytokinesis starts on the
lateral side of the flagellate, and then the cleavage furrow moves toward the apical end of the cell. The anterior
part of a smaller daughter cell, referred to as cell C1, remains associated with the flagellum of maternal
promastigote. C1 divides twice to give rise first to two equivalent cells (C2), and then to four morphologically
similar cells (C3). C2 join with each other, and afterwards C3 attach between themselves as well via short
cytoplasmic outgrowths, which appear instead flagella. In the point of outgrowth attachment of only one C2 and then
of only one C3 to maternal flagellum zonal desmosomes occur. C1-C3 of L. oncopelti are similar to so-called
straphangers (cyst-like parasites attached to the flagellum of maternal promastigote) known in some species of the
genera Leptomonas and Blastocrithidia. Basal bodies are present in C1 and C2 but not in C3. DNA fibrils
in the kinetoplast lack their common circular configuration, they progressively condense to form a disordered mass.
C3 chromatin becomes denser to acquire eventually a characteristic "labyrinthine structure" looking like a huge
bundle of whorled filaments 3-5 nm width. Inside this bundle there are channels of 10-12 nm in diameter filled with
karyoplasm. On becoming ovoid, C3 are separated from the maternal promastigote flagellum and differentiate into
mature "cysts". Straphangers C1-C3 and mature "cysts" lack any visible outer extracellular protective envelope (cyst
wall). Instead, these cells have a cortical complex made of a reinforced plasmatic membrane underlined by a layer of
a dense granular cytoplasm free of subpellicular microtubules. The mature "cyst" endoplasm shows a high electron
density, and because of this identification of the majority of cellular organelles is next to impossible.
Nevertheless, in both C3 and mature "cysts" some unusual membranes are seen composed of two electron lucent layers,
with a single electron dense layer in between.
Key words: flagellates, trypanosomatids, cyst-like cells
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