Introduction to Fungi, Third Edition

BASIDIOSPORE DEVELOPMENT
491
Fig18.3 Coprinus cinereus.Transmission electron micrographs
of sections illustrating development of basidiospores. (a) Stage
4 basidiospore expanding at the tip of its sterigma.The hilar
appendix body (HAB) is appressed to a wall thickening. (b) An
early stage 4 basidiospore with the conical hilar appendix body
in firm contact with the spore plasmalemma.For further
explanation see p. 490. Photomicrographs kindly provided by
D. J. McLaughlin.
of its initiation and, in most cases, persists
throughout the development of the basidiospore,
disappearing only shortly before spore discharge.
It is hemispherical or conical, lying immediately
within the plasma membrane of the apophysis
and closely appressed to the wall of the hilar
appendix (see Figs. 18.3a,b). This structure is
the hilar appendix body and has been reported
from several basidiomycetes, including Boletus
rubinellus, Coprinus cinereus (McLaughlin, 1973,
1977), Lactarius lignyotellus (Miller, 1988) and
Panellus stypticus (Lingle et al., 1992). It is probably
present in all ballistosporic species. Its function
is not understood. Possibly it is involved in the
softening of the closely adjoining wall layers
of the hilar appendix or in the extrusion of
material related to the expansion of Buller’s
drop (see Fig. 18.8). Another possibility is that it
blocks the movement of wall vesicles into the
adaxial side of the spore, so possibly contributing
to the asymmetric shape of the spore. During
further development, the wall of the hilar
appendix thickens considerably, and uneven
expansion of the wall of the developing spore
occurs. Expansion of the abaxial face is more
rapid, leaving the spore asymmetrically perched
on the sterigma, attached at the hilum and with
the remains of the apophysis forming the hilar
appendix (see Fig. 1.20). In Coprinus cinereus,
McLaughlin (1977) has distinguished four successive
stages of basidiospore expansion (see
Fig. 18.5a).
Stage 1, inception. This is characterized by
the spherical enlargement of the sterigma apex
to form a basidiospore primordium 0.6 0.8 mm
in diameter. The hilar appendix body is already
differentiated. The thin basidiospore wall is
three-layered at first. Microtubules are occasionally
present in the sterigma, being orientated
parallel to its long axis.
Stage 2, asymmetric growth. The basidiospore
initial grows asymmetrically on its abaxial side,
and the hilar appendix develops. The hilar
appendix body becomes conical and appressed
to the plasma membrane of the spore initial. The
hilar appendix is initiated adjacent to the hilar
appendix body. The basidiospore wall thickens,
being thickest at the apex of the spore, and is sixlayered.