Biology Reference
In-Depth Information
been instrumental in the identification of forms of fungi and in recogni-
tion of patterns of hybridization among them. Many of these fungi also
hold the potential to cause widespread mortality of host plants when a
virulent form arises or is introduced to a new location.
A detailed example of the role that hybridization can play in fungal
virulence is provided by the Dutch elm disease fungi, which are
ascomycetes of the genus
Ophiostoma
(Brasier 2001). Three forms of
Ophiostoma
that act as biological species and attack elms are known:
O.
ulmi
,
O. novo-ulmi
, and
O. himal-ulmi
.
O. novo-ulmi
also shows two subspe-
cific forms, known by the acronyms EAN and NAN.
O. ulmi
and
O. novo-
ulmi
have been involved in the pandemics of Dutch elm disease in west-
ern Eurasia and North America.
O. himal-ulmi
is, as yet, restricted to the
western Himalaya Mountains, where it appears to be a relatively nonvir-
ulent associate of native elm species.
The Dutch elm disease fungi have been transported intra- and inter-
continentally by transport of elm logs and timber. The vector in the
fungi's spread through elm populations is bark beetles of the genus
Scoly-
tus
, which occur throughout the world distribution of elms.
The first major outbreak of Dutch elm disease began in Europe in
about 1910, in North America about 1927, and in central Asia in the late
1930s. This outbreak was caused by the arrival in these areas of
O. ulmi
from some as yet unknown source. In the 1940s,
O. novo-ulmi
initiated a
second outbreak in two areas, eastern Europe, where the EAN form was
involved, and the Great Lakes region of North America, where the NAN
form was involved. In Europe, the EAN form spread both westward to
the Netherlands and eastward into central Asia.The North American out-
break spread continent-wide, and the NAN form was introduced to the
British Isles and western Europe in the 1970s and 1980s. NAN and EAN
forms of
O. novo-ulmi
now overlap in Italy and parts of central Europe to
the north. In the course of the spread of
O. novo-ulmi
,
O. ulmi
has likely
been competitively eliminated in much of the northern hemisphere
(Brasier and Buck 2001).
Although
O. ulmi
and
O. novo-ulmi
tend to behave as reproductively
isolated species, some interchange of genes, and also of viruses, has
occurred.The genetic interchanges seem to have involved the transfer of
vegetative incompatibility genes from
O. ulmi
to
O. novo-ulmi
.These genes
prevent the fusion of cells of different compatibility types and, most
importantly, the transfer of viruses between these types. The viruses in
question are capable of destroying the fungi.Thus, interspecific hybridiza-
tion has strengthened the overall virulence of
O. novo-ulmi
.
