Introduction to Fungi, Third Edition

288 PLECTOMYCETES
Fig11.2 Ascosphaera apis. (a) Dead infected bee larvae. (b) Young sporocyst. (c) Several intact mature sporocysts. (d) Ruptured
sporocyst with released ascospore balls.
require up to 40% glucose in the medium. On
lower-strength agar media they may still grow but
often fail to produce ascospores. Ascosphaera apis is
a pathogenic species causing ‘chalk brood’ disease
of honey bees (Apis mellifera). Dead infected larvae
appear white and hard like chalk (Fig. 11.2a), and
black spore balls may break through the integuments
(Skou, 1972, 1975, 1988). The disease occurs
as an epidemic in some years and can seriously
weaken bee colonies, especially if accompanied by
pests such as Varroa mite infestations.
Some species are homothallic but A. apis is
heterothallic. Ascospores are produced in a
unique structure termed a sporocyst (Skou,
1982). The ‘female’ colony produces an ascogonium
terminating in a trichogyne, and plasmogamy
occurs between the trichogyne and an
undifferentiated hypha of the opposite mating
type. Following plasmogamy, the trichogyne
grows backwards into the ascogonium, the wall
of which swells greatly to form the sporocyst
(Fig. 11.2b; Spiltoir, 1955). When it is mature,
the sporocyst acquires a brown pigmentation
(Figs. 11.2c,d). Within the sporocyst, a system of
binucleate cells with croziers forms eight-spored
asci in clusters. The ascus walls are evanescent,
and the ascospores from the asci of any one
cluster stick together as spore balls which are
released when the sporocyst wall breaks
(Figs. 11.2c,d). The sporocyst is not homologous
with a cleistothecium because it arises
from a single cell which enlarges prior to formation
of the asci, whereas a cleistothecium is
multicellular and grows around the developing
asci.
In order to produce ascospores on infected
bee larvae, A. apis requires a slight reduction of
temperature (normally around 33 36°C in intact
hives) to about 30°C. Infections by A. apis are
usually most severe in cool weather, especially
in spring. Interestingly, bee colonies have been
found to respond to A. apis infections by
elevating their temperature, and this so-called
‘behavioural fever’ may retard the outbreak of
the disease (Starks et al., 2000). A further way for
bees to control the disease is hygiene, i.e. they
uncap brood cells and remove dead larvae before
A. apis can sporulate on them. Bees can be bred
for hygiene, and the basis of this is thought
to be an enhanced sensitivity to the odour of