The ultrastructure of paraspermatogenesis is examined in the
littorinid
subfamily Littorininae, with special emphasis on Littoraria
(Palustorina)
articulata (PHILIPPI 1846). In particular the study focuses on the
fate of the nucleus and origin of the rod bodies during parasperm
development.
Parasperm of the Littorininae are rounded or oblong cells, which
undergo
an abortive meiosis and eliminate part of the nucleus but often retain
a nuclear remnant. The cytoplasm is filled with numerous spherical
vesicles
in all Littorininae, but in Littoraria (and in certain species
of
Nodilittorina, Tectarius and Cenchritis) dense
'rod-bodies'
also occur. Littoraria (Palustorina) are unique in possessing a
flagellum-like structure termed the 'pseudotrich', which lacks an
axoneme
but contains microtubules during its development. Paraspermatogonia
differ
from euspermatogonia in the structure of the nucleus and in the
extensive
rough endoplasmic reticulum (RER) and swollen cytoplasm. Two types of
secretions
develop in Littoraria: (1) numerous, spherical granules (composed of
putative
glycoprotein, also seen in other Littorininae) and (2) rhomboid
granules
(composition uncertain but reacting positively to RNA stains; these
granules
arising within RER cisternae close to the nucleus). As the rhomboid
granules
fuse to form the larger, rod-bodies (polygonal in cross section), the
RER
membrane enclosing the rod-bodies becomes confluent with the outer
nuclear
membrane, thereby forming a common compartment. Results of this study
clearly
show that the rod-bodies are secretions of the RER cisternae and not,
as
claimed in some light microscopic accounts, the product of fusion of
eusperm
nuclei which have entered the parasperm cytoplasm (either by active
eusperm
penetration or by phagocytosis). Developmental characteristics of
littorinid
parasperm show differences between species and may, in some cases,
provide
characters diagnostic of subgenera.