Introduction to Fungi, Third Edition

OPHIOSTOMATALES
365
Fig12.36 Ophiostoma piceae.
(a) Perithecium showing spore drop at tip
of neck. (b) Details of ostiole with ring of
setae. (c) Asci. (d) Synnematous
conidiophore. (e) Details of apex of
conidiophore.
similarity, the two genera are not closely related,
Ophiostoma being in a sister clade to the
Diaporthales (p. 373) whilst Ceratocystis is allied
to the Microascales (p. 368; Hausner et al., 1993;
Spatafora & Blackwell, 1994). The similarity in
perithecial morphology suggests parallel evolution
in adaptation to the insect dispersal of
ascospores (Malloch & Blackwell, 1993). There
are differences in anamorphs between the two
groups. In Ceratocystis sensu stricto the anamorphs
are phialidic, of the Thielaviopsis type (see
Figs. 8.8, 12.42e), whereas the anamorphs of
Ophiostoma are annellidic (de Hoog, 1974). Other
points of difference are listed on p. 369.
Most species of Ophiostoma are heterothallic
and the ascospores are of two mating types,
A and B. Although ascogonia have been described,
there are no antheridia. When isolates
of different mating types from the same ascocarp
of O. ulmi or O. piceae are inoculated near each
other in agar cultures, the two mycelia intermingle
and perithecia develop. However, when
cultures of different origin are opposed to each
other, a line of barrage analogous to that seen
in Podospora anserina may develop between the
approaching mycelia, a phenomenon associated
with vegetative incompatibility (Brasier, 1993).
Perithecium development in O. ulmi has
been described by Rosinski (1961) and the ultrastructure
of the ascogenous hyphae and ascosporogenesis
by Jeng and Hubbes (1980). The
multinucleate coiled ascogonium, differentiated
by its greater width from the hyphae which
subtend it, becomes surrounded by a mantle
of branched hyphae probably derived from the
ascogonial stalk. Ascogenous hyphae arise as
buds from the ascogonium which is positioned
near the base of the ascocarp. A central cavity
arises as the result of enlargement of the outermost
cells of the ascocarp, and the ascogenous