Introduction to Fungi, Third Edition

428 HYMENOASCOMYCETES: PEZIZALES (OPERCULATE DISCOMYCETES)
germinate soon after discharge to form a septate
mycelium with multinucleate segments. Frequent
anastomosis may result in the formation
of heterokaryons. Later in the season sclerotia
develop, and it is in this form that the fungus
survives the winter. Ascospores themselves do
not remain viable in the soil for very long
(Schmidt, 1983). The mycelium may also develop
‘muffs’ around the roots of various hosts, mostly
young trees, and within such muffs the mycelium
may penetrate as far as the phloem, an
association which is probably non-mycorrhizal
(Buscot & Roux, 1987; Buscot, 1989). However,
in association with spruce (Picea abies) rootlets
already infected with basidiomycetous mycorrhizal
fungi, the mycelium of Morchella is weakly
mycorrhizal, forming a Hartig net of intercellular
mycelium around one layer of cortical cells of
the host (Buscot & Kottke, 1990). Several morphologically
distinctive types of such secondary
ectomycorrhiza have been observed, often in
association with endobacteria which invade
the hyphae of M. elata making up the Hartig
net and also the host plant cells (Buscot, 1994).
Pure culture synthesis of sheathing mycorrhizae
between Morchella spp. and four species of
Pinaceae has been reported (Dahlstrom et al.,
2000).
The mating system of Morchella is not fully
understood, and it is as yet uncertain whether
it is homo- or heterothallic. A conidial state,
Costantinella cristata, has been reported to develop
following ascospore germination of M. esculenta.
The conidiogenous cells (phialides?) are formed
in verticils arising from lateral branches of the
erect conidiophores. They give rise to minute
spherical conidia which do not germinate readily
(Costantin, 1936). Possibly they function as spermatia.
When certain mycelial isolates derived
from single ascospores are confronted in pure
culture, a barrage phenomenon occurs (Hervey
et al., 1978), and this may be due to heterokaryon
incompatibility (Volk & Leonard, 1989). The
assumption that sexual reproduction occurs, in
the sense that different parental genomes are
involved in ascocarp formation, is supported
by electrophoretic data confirming that allelic
variation exists in natural populations. It is also
possible that the fertilization events leading to
the production of different asci within one fruit
body may involve several individuals (Gessner
et al., 1987).
The possibility of cultivating morel ascocarps
commercially is being explored and patents
have been taken out to protect the techniques
involved. They are based on the observations by
Ower (1982) that sclerotial development followed
by ascocarp differentiation can be encouraged
by growth of the mycelium on sterilized wheat
grain.