Introduction to Fungi, Third Edition

300 PLECTOMYCETES
Fig11.12 Phialoconidium ontogeny in Aspergillus niger (modified from Subramanian,1971). (a) Young phialide. (b) Mitosis in phialide.
(c) Conidium initial with daughter nucleus. (d) Breakage of phialide wall and formation of a new wall layer surrounding the conidial
cytoplasm. (e,f) Mitosis in the phialide and extrusion of the second phialospore. Note that the phialide has not increased in length.
(g,h) Further mitosis in the phialide and formation of the third spore. Note the formation of an inner conidial wall within the outer,
and the isthmus connecting mature adjacent spores within the chain.
layer of the conidium within which an inner wall
develops later (Fig. 11.12h). Nuclear division
continues within the phialide, and cytoplasm
and a wall are laid down around a daughter
nucleus to form a second conidium which is
extruded from the broken tip of the phialide
(Figs. 11.12e,f). The second and all subsequent
conidia differ from the first in that they are not
enveloped by remnants of the broken phialide
wall. The cytoplasm of the second conidium is
initially continuous with that of the first by
means of a cylindrical isthmus. The formation of
an inner wall layer by the conidia severs this
cytoplasmic connection. The surviving empty
isthmus is sometimes termed the connective.
Each phialide can produce 100 spores or more,
so that the total crop from one conidiophore
may be more than 10 000 conidia (Adams et al.,
1998). The fine structural details of conidium
production in A. nidulans have been described
by Mims et al. (1988).
In the phialide, a transition from the polarized
growth pattern of the mycelial hypha, conidiophore
and phialide to yeast-like (isodiametric)
growth of the nascent conidium takes place.
Results obtained from experiments with the