Introduction to Fungi, Third Edition

536 HOMOBASIDIOMYCETES
Breeding of Agaricus bisporus
The development of new strains of A. bisporus
with superior qualities such as more efficient
substrate conversion rates, rapid fruiting, higher
yield, resistance to disease, ease of picking,
extended shelf-life, better appearance, flavour
or consistency is actively pursued by the mushroom
industry (see Raper, 1985; Sonnenberg,
2000). This task is difficult but worthwhile in
view of the high commercial value of the crop,
and various techniques are being used:
1. Selection of strains collected in the field
and arising during cultivation.
2. Hybridization. The small amount of variation
in the gene pool of commercial stocks of
A. bisporus creates little scope for recombination.
However, a search for new genetic resources
within wild Agaricus populations, the Agaricus
Resource Program (ARP), has made available
novel germ plasm which will prove useful in
breeding (Kerrigan, 1996). Of particular interest
is the discovery in the Sonoran desert (California)
of tetrasporic populations, named A. bisporus var.
burnettii, which are completely interfertile with
commercial lines (Kerrigan et al., 1994; Kerrigan,
1995). Agaricus bisporus var. eurotetrasporus,
another four-spored variety, has been discovered
in Europe. Whilst var. burnettii is amphithallic
and predominantly heterothallic, var. eurotetrasporus
is homothallic (Callac et al., 2003).
3. Protoplast fusion. The fusion of protoplasts
from different homokaryons is an alternative to
the conventional hybridization technique involving
the pairing of compatible homokaryotic
mycelia.
4. Transformation (genetic modification)
using Agrobacterium as a vector for the transforming
DNA is also possible, but genetically modified
food products are unpopular with consumers
and certain political parties.
Macrolepiota (30 spp.)
The best-known species of Macrolepiota are the
parasol M. procera (Fig. 19.14b) and the shaggy
parasol M. rhacodes, both of which grow in parks,
pastures and woodland. They are saprotrophs.
The pale brown fruit bodies are large and
generally considered good to eat, although
those of M. rhacodes may cause gastric upsets.
A feature of both species is that the ring
is detachable and is free to move up and
down the stem. Macrolepiota procera and
M. rhacodes have been separated from Lepiota,
and Vellinga et al. (2003) have proposed that they
should be separated from each other as well,
with M. procera showing affinities with
Leucoagaricus and Leucocoprinus, and M. rhacodes
with Agaricus.
There are concerns about the ability of
M. procera to concentrate mercury absorbed
from the soil into the basidiocarp tissues, with
values in the caps as high as 13 mgg 1 reported
from various sites in Poland. The mercury
content of the caps and stalks was generally
independent of the soil substrate concentration,
suggesting a remarkable ability of the M. procera
mycelium to bioconcentrate mercury (Gucia &
Falandysz, 2003).
Coprinus
Molecular evidence indicates a relationship
between lepiotoid fungi and two species of
Coprinus, namely C. comatus and C. sterquilinus
(Hopple & Vilgalys, 1999; Redhead et al., 2001).
Although it is now possible, with hindsight, to
recognize other features shared by these species
and other Agaricaceae, e.g. the shaggy surface of
the cap or the moveable annulus (Fig. 19.14c), we
will discuss them in the context of the many
biological features uniting them with their
former allies (see below).
19.4.2 Coprinaceae (now Psathyrellaceae)
Molecular studies have shown that the genus
Coprinus is not monophyletic (Hopple & Vilgalys,
1999; Redhead, 2001; Redhead et al., 2001),
implying that the coprinoid character of deliquescent
gills has evolved more than once.
This finding has caused a taxonomic nightmare
because one of the two species found to
be related more closely to Macrolepiota than
to Coprinus happened to be the type-species
C. comatus. Its move from the Coprinaceae to
the Agaricaceae meant that those numerous
species remaining in the Coprinaceae were
given new names, namely Coprinopsis (e.g. C.
atramentarius, C. cinereus, C. lagopus, C.