Introduction to Fungi, Third Edition

324 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.6 Schematic diagram of ascus development in the two pseudohomothallic ascomycetes Neurosporatetrasperma and Podospora
anserina (based on Raju & Perkins,1994). Neurospora tetrasperma: (a,b) First meiotic nuclear division M I
.The mating type alleles A and
a are closely linked to the centromere and segregate at the first division. (c) During the second meiotic nuclear division M II
the
spindles of the dividing nuclei lie in tandem parallel to the long axis of the ascus. (d) Interphase of M II
; the nuclei are aligned in pairs.
(e) Telophase of a post-meiotic mitosis PMM I
; the spindles are paired and lie transversely to the long axis of the ascus. (f) Interphase
of PMM I
; the four pairs of nuclei are realigned more or less regularly to one side of the ascus prior to ascospore delimitation.
(g) Young ascospores are binucleate and heterokaryotic, containing both kinds of mating type allele. (h) Following a second
post-meiotic mitosis PMM II
, the ascospores contain four nuclei, two of each mating type. Podospora anserina: the alleles for mating
type are not tightly linked to the centromere and do not segregate at M I
(b) but at M II
(c). (d f) as for N. tetrasperma.(g)Following
ascospore delimitation, four binucleate, heterokaryotic ascospores are formed. (h) A second post-meiotic division has occurred.
Three of the four resulting nuclei, two of one mating type and one of the other, remain in the body of the ascospore whilst the
fourth nucleus migrates to the basal appendage and degenerates.
division (PMM) brings A and a nuclei close
together in four pairs, and walls develop
around the nuclei and cytoplasm so that the
binucleate ascospores each contain one A and
one a nucleus. In P. anserina, the same result is
achieved by different means. The mating type
idiomorphs (þ) and ( ) are sufficiently distant
from the centromere for a single cross-over to
occur between the centromere and the locus of
the idiomorphs. Second-division spindles do not
overlap and, by the mechanism outlined above,
nuclei of opposite mating types are brought close
together at the PMM stage and become enclosed
in walls as binucleate ascospores. A further
mitosis brings the number of nuclei in each
ascospore to four, and three nuclei remain in the
body of the ascospore whilst one migrates to the
tail (Raju & Perkins, 1994).