Abstract
The natural nutritional environments of most fungi are spatially non-uniform, yet the majority of studies of fungal growth
take no accourlt of this fact. An experimental system is described which permits the growth responses of eucarpic fungi to
heterogeneously distributed nutrient resources to be studied. The system comprises tesselations of agar tiles of contrasting
nutrient status separated by air gaps. Growth responses in such systems of Alterttm~in altcrrtara, MWW sp., Phma jh~ara.
Rhizncronia solurti and Tricku&rmu r,iuidr are described. Gen;xIIy. ihe growth of the fungi reflected the nutrient status of
the underlying substrate. There was evidence for growth in low-nutrient tiies heing greater when high-nutrient tiles were
included in the tessellation. Reproductive structures tended to be formed only in low nutrient tites with T/-icho&~-mu and
Rhizocronia and only high nutrient tiles with Ahaaria.
Growth responses of Rhizoctmia were strongly asymmetric in
nutritionally symmetric, but heterogeneous, tesselations. The consequences of the observations for fungal growth in
heterogeneous environments such as soil is discussed.
Kqwords:
Fungi: Growth
response; Heterogeneity:
Mycelia; Nutrition
1. Introduction
Fungi, in common with most terrestrial organisms, inhabit environments which are non-uniform in
space and time. Eucarpic fungi, with their indeterminate mycelia comprised of extending and intcrconnetting hyphae, are well suited to growth in such
heterogeneous environments. For example, hyphae
are capable of extending great distances reiative to
their diameter in order to locate new food bases and
* Carresponding
author. Tel:
0382 562731;
Fax: 0382 562426;
E-mail ccpkr(i_scri.sari.ac.uk.
OlbX-64Y6/Y~/$llY.Xl
SSDI
0
0168-64Y6(Y4)00(1YO-S
1995 Federation of European Microbiological
to assimilate nutrients there. Such foraging strategies
have been observed predominantly in woodland inhabiting basidiomycetes (e.g. [ 1,2]). Trichoder-ma
[*iride, a common soil fungus, has also been shown
to exhibit foraging strategies and distribute hyphal
mass within the myceliutn commensurate with the
local availability of nutrients (31.
These phenomena are important in relation to soil
fertility for several reasons. Fungi act as primary
decomposers of soil organic matter and thus mineralise nutrients essential for plant growth. The distribution of organic matter in soils is heterogeneous,
and an understanding of ‘ hotspot’ dynamics, or ihe
microbiology
of localised zones of microbial activity
Socictics.
All
rights rcxrvcd
centred around nutrient resources is an important
contemporary theme in soils research [4,51. Fungi
also influence transport processes in soil in that
nutrient elements may be dispersed within myceiia
from iocaiiy high concentrations in the vicinity of a
nutrient resource and thence through the soil, SO
short-circuiting slow diffusive processes through the
soil solution. Nutrient transiocation by fungi may
also occur between spatially remote sites along interconnecting hyphae [6]. This is certainly a feature of
mycorrhizal systems [7]. Hyphai networks influence
soil structure by binding aggregates together [8,0];
the spatial architecture of myceiia will thus affect the
scale and degree of such binding and hence the
genesis and stability of soil structural characteristics.
Thus, fungi can themselves create some of the structural hetcrogcneity in their environments.
Suprisingiy little experimental work has been reported which aims to elucidate the relationships
which occur between fungai myceiia and the spatial
distribution of their nutrient resources. Most studies
of fungai morphogenesis are based on observations
and measurements of colonies grown on or in ‘uniform’ media. in reality, only chemostats approximate
to truly uniform environments since here nutrients
are replenished continuously and rapid gas exchange
is assured; nevertheless, at small size scales, nonuniformity must occur even in well configured
chemostats. for exampic as boundary layers. Cultures
on agilr plates encounter initially near uniform nutritional conditions but the dcvcloping myceiia will
alter the distribution of nutrients through assimiiation, which may result in the development of staling
zones and associated patchy growth. Such hetcrogcncity is esscntialiy random and not conducive to
elucidation of its origins or conscyucnces. The approaches of Crawford cl crl. [S], where fungi were
grown on cci~uioso membranes, suffered from the
inability to ascertain the precise distribution of nutrients in the system. Approaches using soil and solidsubstrate nutrient depots (e.g. [IO]), are more suited
to the study of the surfxe behaviour of cord-forming
basidiomycetes, since the opacity of the soil permits
oniy surface-based responses to be visualised. Recently. Rayncr [ ii ,121 has described the application
of a chambered vessci to study fungal growth in
non-uniform media, but here physical barriers separate nutrient domains and there is a distinct ‘focus-
ing’ effect on the myceiia of small holes between
adjacent chambers. This pqer dessribes a simple
approach to generating heterogeneous media where
the distribution of nutrients can be controlled in the
absence of such barriers. The responses of a number
of common soil-inhabiting fungi to such conditions
are also reported.
The basic microcosm design used in these studies
consisted of square tiles of agar (IO X 10 X 3 mm)
arranged in a 3 X 3 array in Petri dishes. Heterogeneous systems were established by employing tiles
of different nutritional composition in a variety of
A
\
r/J
E
Fig. 1. Productir~n
of tcsscllalcd
iqqar tile microcosm
studies cd fungal rcsponscs IO hcfcrogcncously
rwwrwi.
for 11s~ in
distrihutcd
nutrient
tessellations. Nutrients were prevented from diffusing between adjacent tiles by placing them approximately 2 mm apart. The metkod requires the production of numerous precisely square agar tiles of identical size so that that they align in a parallel fashion,
the air gaps remain a constant width and that the
arrays are uniform behveen replicates. This was
achieved routinely using a cutting device consisting
of a bank of 5 single-edge razor blades, separated at
1 cm intervals by mounting pillars, mounted on
threaded brass rods. A series of 5 parallel cuts were
made simultaneously in an agar plate by pressing the
blades into the gel (Fig. la); a second series of cuts
made tangentially to the first thus produced 16 identically sized tiles (Fig. lb). These were excised with
a sterile microspatula and arranged in the required
pattern in a sterile plastic petri dish (Fig. ICI. The
central tile in the array was then inoculated with a
single freshly germinated colony. To prevent disruption of young colonies during transfer to the arrays,
spore suspensions of the fungus under test were
placed on cellulose membranes overlaying 2% tap
Summary
of cxpcrimcntal
water agar (Fig. Id). Young colonies were then
excised on small pieces of membrane and transferred
to the inocu%ation tile (Fig. le). Such small volumes
of agar as used here tend to dry out rapidly. Moist
sterile cotton wool was placed in a ring around the
border of the dish to prevent desiccation, and dishes
were incubated at 15 or 20°C in plastic bins lined
with damp tissue.
The basic experimental design offers unlimited
possibilities in terms of the number of spatial patterns which may be investigated. In these studies, a
basic set of 6 symmetrical tesselations were used,
with tiles of relatively high nutritional value combined with tiles of considerably ilower nutrient status.
Control patterns consisted of all tiles being of the
same nutrient status (i.e. analogous to ‘uniform’
media). The tesselations adopted are summarised in
Table 1. Media composition was as follows (all per I
of medium): tap water agar; 15 g standard agar
designs UWJ in thl: studies rcpr>rtcd hcrc
Fungal species
‘Low’ nutrient status
Trichoderma virile 472’
Tap water agar
Mucor Sp.
Tesselation of gel
used for all above
species:
’ Numbers
rcfcr to culture
collection
‘High’ nutrient status
Nutrient agar
Nutrient agar
AIternariu dternut~ I7
Rhiroctoniu soluni 468
q
Tap water agarose
Nutrient agar
Tap water agarose
Cmpek-Dox agar
Tap water agarose
Czapek-5ox agar
Tap water agarose
Potato dextrose agari
Czapek-Dox agar
tiles
rcfcrencc
of the Scottish Crop Research Institute. Dundce.
272
K.
Ritz/ FEW
Microbiology
(Merck, pH 7.2): tap water agarose; 15 g molecular
biology grade agarose (Merck, pH 6.7): nutrient
agar; 28 g nutrient agar (Oxoid, pH 7.2): Czapek-Dox
agar; 30 g sucrose, 15 g agar, 2 g NaNO,, 1 g
K,HPO,, OS g KCl, 0.5 g MgS0,.7HO,,
0.01 g
FeSO,.7H,O,
pH 6.8: potato dextrose agar; 22 g
dehydrated potato powder, 20 g glucose, IS g agar,
pH 5. Fungal cultures were obtained from the stock
collection at the Scottish Crop Research Institute.
Their origin was from arable soils supporting potato
crops, and they were selected to provide a range of
species known to have saprophytic phases in soil+
Colonies were grown in the microcosms for 2 to 6
weeks, and periodic observations made on their development using a stereo microscope. Three replicate
microcosms of each design were used. Parameters
were scored visually on a zero:low:medium:high
scale. These included extent of colony (i.e. location
of colony margin), colony density (i.e. mass of hy-
Ecology
16 f/995)
269-280
phae per unit volume), degree of pigmentation, and
development and extent of reproductive structures.
3. IResuits
3. I. Trichoderma r-bide
Trichodema, in common with most of the species
used in these studies, grew readily across the air gaps
between adjacent tiles (e.g. Fig. 2a). Colonisation of
tiles was also achieved by some hyphae growing
down the edge of tiles, across the base of the petri
dish, and thence to the adjacent tiles. The spatial
arrangement of the tiles resulted in a typical colonisation sequence of central tile. followed by lateral
tiles, then radial tiles. The latter, however, were
often colonised by pioneer hyphae which had grown
across the corner of the central tile, but also by
Fig. 2. Growth of ~Trickoclcrrncr riridc in tcssclla~cd agar tile microcosm. (a) hyphac bridging air gap between tiles of nutrient agar
(tcssellaticln I). Dark ibid i!Llmirratic?n.‘3‘.:!c bar = I mm. (bl single hypha [arrowcd] bridging air gap bctwcen tilt of tap water agar (<‘:$I)
and nutrient a&l:ar(lcftl. Dark hyphae arc on surfac\: of agar. light hyphal: arc cmbcddcd in pcl. Dark field illumination, scale bar = 0.5 mm.
hyphae bridging from adjacent Merally
positioned)
tiles. Myphal proliferation was extremely rapid once
high nutrient tiles were encountered.
Growth respouses of Trickohma
were generally
SyiT4iTICika1,
figures to be drawn by averaging
responses across the replicates. During the early
stages of the experiment. colonies of Trishodetwta
extended considerably faster on low nutrient tiles
than on high nutrient tiles. Thus after 7 days, hyphae
had extended across all low nutrient tiles, whilst
colonisation
of high nutrient tiles was incomplete;
this occurred in all tessekions
(Fig. 3b). Colony
density reflected the nutrient sitatus of the tiles. wirh
enabling
summary
mycelia being considerably more dense where nutrier~ts were
high (Fig.
36).
After
11 days, co]any
density remained depressed on low nutrient tiles in
arrangements where lo-w ilu;ricnt tiles predominated
(Fig. 3e-I and 3e-III), but was elevated in other
cases, where at least half the tiles were of high
nutrient status, i.e. tesselations V and VI. Patterns of
sporulation varied among the different tesselations.
Where all tiies were of low nutrient status. sporulation was uniform across all tiles (Fig. 3f-I), whilst in
tessellation 2, sporulation only occurred on the central rile (Fig. ,Sf-II). In the hctcrogeneous tesseiations, spore production tended to be more prolific on
a) Tesselation
pattern of blocks
b) Colony extent
(7 days)
c) Colony extent
(14 days)
dj: Colonif deiisii-y
(7 days)
e) Colony density
(14 days)
f): Sponrlation
frequency
\’I44 $$‘C\
J-I
high
medium
Fig. 3. Growth
rcsponscof
Thhkrnzu
indicates mean magnitude of paramckr.
riridc to spatial tcssclatiorts of tilts
scored qualitatively
of agar of diffcrcnt
for three rcplicatcs (cxccpt for
nutrient status. Density
rclw;I).
of shading of hl~k
low nutrient tiles. Spore densities also tended to be
greater near the edge of individual tiles than in the
central zones.
Growth responses of this species were symmetrical and related directly to the underlying nutrition of
the tiles as they were colonised, i.e. low colony
density on low nutrient tiles and extensive hyphal
proliferation on high nutrient tiles. Colony expansion
rates were the same on either type of tile. Colony
density was somewhat greater in low nutrient tiles in
tesselations V and Vi than in the other tesselations.
Sporulation occurred predominantly on high nutrient
tiles.
This species grew symmetrically with respect to
the tessellation patterns and radial growth was not
influenced by the nutrient composition. Colony density directly reflected the nutrient status of the tiles,
being very dense in high nutrient tiies and consider-
Fig. -t. Growth
rcspcmsca of Phom
ilCCOrdillg to pittcrns
ably more spar-e in low nutrient tiles, including in
tesselations V dnd VI. Brown pigmentation developed exclusively and consistently in the high nutrient
tiles (Fig. 4). Sporulation was very slight and only
occurred on high nutrient tiles. Occasionally some of
the tiles made contact after a few days in this
experiment. When this occurred, the growth responses of the fungus were consistent with the nutrient status of the tiles resulting from such connection.
For example. in one replicate of tessellation VI, the
central tile joined to the lateral tiles (Fig. d-VI)
which effectively made it a high nutrient status tile;
dense hyphal growth and pigmentation duly develaped.
Growth on low nutrient tiles by Muror- was extremely sparse: in tessellation 1, surface growth did
not extend beyond the central tile and only a few
sporangia were produced. In contrast, where high
nutrient tiles comprised potato dextrose agar (PDA),
growth was extremely rapid compared to both the
growth on low-nutrient tiles and the other species
/;N twltr to spatial tcssclaticms of tilts
BS dcnotcd in Tiihlc
I.
Diiimctcr
of dish = 0 cm.
nf agar of diffcrcnt
nutrient
status.
Agnr tilts
arc arrangcd
used in these experiments:
all such tiles occurred
Hyphal
density
species
produced
giophores,
few
hyphae
tiles.
Spores
tiles
tion
thus
5).
liberated
which
Such
I. Growth
With
based
the
same
Branching
contrasted
with
there
prolific
was
higher-order
those
6f-i
formed
(Fig. fif-III
in low nutrient
two tilts
was
within
as in
of mycclia
ivw
nutrient
of
of decreasing
after 7
tiles
were
PDA
based
T&c possibility
tween adjacent
colonised
uniformly
not be mapped
exclusively
rciatively
slow
Fig.
5. Growth
(lowcrl
of nutrients
be-
across the Petri dish
by the experimental
design
tiles
n~~tworks. cspccially
when dense, may act as wicks”,
permitting
transfer
in that
secondary
and
diameter
(Fig.
along
Rhknctorliu
were
tiles were
charac-
authors
by capillary
their external
occurred
hyphol
which
responses
developed,
of the tiles,
suggest
in the microcosms
action
and
[ 131. However,
surface
of the growth
status
suggest
l
nutrient
diffusion
Some
and persisreflecting
that
the
if such
here, it was only
slight.
not
and thus average responses could
nutrienl
IFig.
coloniscd
of Mucor
and potato dcxtrax
water agarosc (right)
tended
in just one or
used in these studies.
asymmetric;
as for the other
low
completely
of
Sclerotia
of some transfer
tiles by diffusion
base was not precluded
transfer
strongly
to VI).
tiles, and often
uniin the
on CD tiles
underlying
responses
were
extensively
was also
the coherence
by being
patterns
any one microcosm.
tcnce of the patterns
Growth
was
in the tessela-
severely
SI.
terised
where
and II), but formed
other tessclations
variable
status of
of sclerotia
by hctcrogeneity
no sclerotia
(Fig.
were
the nutrient
The formation
influenced
to form
to give
CCD) tiles
development
branches
form
hc-e).
with
and absent in tessela-
manner
on
strongly
of pigmentation
clearly
colonised
microcosms.
patterns
by
neighbour-
germinated
aimd patterns
the tiles (Fig.
tions;
adjacent
all such tiles were coloniscd
CD
in
systems.
onto
germination
tiles.
This
sporan-
occurring
density
and not correlated
Relatively
between
subsequently
on Czapek-Dox
sparse, although
of
tiles.
on PDA tiles.
predominantly
mycelia
(Fig.
colonised
on PDA
(up to 7 mm)
developed
on low nutrient
days.
great
of the sporangiophores
rise to satellite
curtailed
colonisation
4 days of inoculation.
stalked
colonisation
the coltapsc
the
long
most extensively
with
ing
within
was very
*bridging’
tiles,
complete
tiles
species.
hb), although
after
Growth
on
1) was
All
all the tiles were
capable
(tessellation
4.3 days growth.
Hyphal
sp. in tcsscl!atcd agar ti!c miCrM<fim.
ngar (upper). Uark t~cld ~ilumin;ltion,
and Czapub-Des
qar
(left).
Bright
spccics
studied
of limited
exclusively
(;I) sptxuq.$ophorcs
low nutrient
bridging
here. apart from
growth
tiles.
scale bar = I mm.
Triclrud~~r~~tlr colonised
air gap IIC~WWII
ldging
wale
bar = 1 mm. (h) sporangiophorc
.
field illumination.
Iklrrcc~. were
in the systems comprising
of lap w~cr ilg;IrOhC
gap hctwccn tilt of tap
tilts
a) Tesselation
pattern of
b) Colony extent
(17 days)
c) Colony density
(17 days)
d) Colony density
(43 days)
e) Pigmentation
(43 days)
f) Frequency of
sclerotia
(43 days)
zero
Fig. h. Typical growth rcsponws of Kltimctortict sohi
%
low
medium
high
to spatial tcssclations of tiles of agar of diffcrcnt nutrient status. Density of shading
of block indic;ltcs magnitude uf pnramctcr. swred qualitatively as dcnacd in the k-y. Thcsc data rcprcscnt one rcplicatc microcosm; due to
the asymmetry of the growth rcsponsos. sp;ltial ;lvcragitl~ ~1sin Fig. 2 is not appropriate.
than high nutrient tiles, which is
a
foraging
relating
to
an
indicative
of
exploration/exploitation
strategy depending upon the
relative abundance of nutrients [2,14]. This also implies that the extension rate of Trichodemta may be
faster in low nutrient domains whilst the rate of
biomass accumulation is fldaste~in nuirittlll rich LC)II~S.
The colonies of all other species expanded at essensuch
tiles
faster
tially
the same rate on tiles
of contrasting
nutrient
branch
frequency) on high nutrient tiles is commensurate
with exploitative growth and a high fractal dimenstatus.
The
higher
density
of
hyphae
(i.e.
sion [ 141 but it was not possible to measure this
parameter rigorously in this experimental system.
The radially symmetric arrangement of the nutrient tiles was generally reflected by symmetrical
growth and development of all species studied apart
from Rhizoctonia. Such regular growth is a direct
cxirapolation of the radially symmetrica! (circa!;rs)
shape that colonies generally form on uniform konventional) agar plates. The introduction of even small
amounts of heterogeneity in the patterns caused major asymmetry to develop in the Rhizoctonia
colonies. This was most notable with the formation
of sclerotia. Whilst sclerotia did not form in tesselations I or 91, the introduction of a single contrasting
tiie (i.e. tesselations 1x1or IV) resulted in sclerorial
deposition in some tiles, predominantly of low nutrient status. A hierarchy also developed between these
tiles with one or two containing a grcster frequency
of sclerotia. This implies that sclerotia, once initiated, become strong sinks for resources within the
mycelium, and once dominant remain so at the expense of other spatial zones within the colony. This
has been clearly demonstrated previously using isotopes of C and P [15]. Such sink relationships may
also develop within vegetative sectors of Rhizoctorrihz mycelia which would account for its tendency to
grow asymmetricaiiy in the heterogeneous systems.
Contrasting patterns in the production of reproductive structures were observed between the different species, i.e. Aknaaria mainly on high-nutrient
tiles, MMCO~exclusively so, and Rltizoctmia
on
low-nutrient tiles, as with Trichodermn in the heterogeneous tesselations. These responses may reflect the
extent to which translocation can occur within the
colonies; reproductive structures are demanding in
terms of nutrient resources and consequently can
only be formed in Iow nutrient domains if substrate
can be imported from elsewhere in the mycelium.
Schiitte [16] postulated that fungi fall into two distinct groups, the ‘ translocators’ and ‘non-translocators’, on the basis of whether species could colonise
‘deficient’ Ii.c. low nutrient status) media in the
presence of nutrient rich media. Thrower and
Thrower also distinguished species on this basis but
were unable to correlate such colonising ability to
t3e translocation of “C and “P, since all species
they studied carried isotopes to their apices [ 15,171.
Subsequently, Howard suggested that the ability to
grow on low nutrient media was related simply to
the “ability of the mycelium to support the gro!vth
of the fungi” [ 181. It is now clear that many soil
fungi are capable of growth under strictly oligatrophic conditions [19,20]. As such, growth on low
nutrient domains might indeed be merely a direct
rcflcction of oligotrophic ability. However, formation of relatively more mycelium in low nutrient
domains in the presence of high nutrient domains is
indicative of transport of resources. And formation
of spores or sclerotia provides a stronger indication
for this, especially since considerably fewer such
structures are formed, if at Al, in the ‘contrd’ tessellalion of exclusively Iow nutrient tiles (e.g. K/I;=:><~nr~iu). Whether such transport occurs by active
translocaaion, or passive diffusion inside or outside
of hyphae [‘Pl], is extremely difficult to demonstrate
conclusively (see also [Z!]).
Why reproductive structures should be formed
preferentially in particular nutrient zones is unclear.
Formation in nutrient poor domains may be due to
the fungus ‘detecting’ such a poor environment and
laying down reproductive structures as a survivai
mechanism. This may only occur if the fungus has a
dispci:,ition (or ability?) for translocation or transport
to such structures. In tessellation I for Trichodernza
and all tesselations for Aitemcrria, sporulation only
occurred when the physical boundaries of rhe agar
tiles had been reached. an event which represents the
onset of circumstances no longer favourable for
growth and thus conducive to the formation of survival structures. Qnc possible selective advantage of
sporulating away from a nutrient rich resource relates to the likelihood of spores being consumed and
destroyed by other organisms or microbes. Nutrient
resources will become heavily coltonised by bacteria
as well as fungi and such sites of activity lead to the
invasion by predators and grazers. If spores are
deposited away from such sites there is a lower
likeiihood of spore consumption
by secondary
colonisers.
Although based on growth in *.ritro, the observations made in these studies have a number of possible implications for the ways fu;igi may grow in
heterogeneous environments such as soils, and the
consequences of such growth for the functioning of
soil systems. Firstly, hyphal proliferation appears to
be limited mainly to the spatial zones where nutrient
resources are (relatively)
high. However, in some
species, such resources also supplement growth or
formation of reproductive structures in nutrient poor
zones, pcl niitLii@ ‘Ci;raging’ growth to new domains.
Such growth requires the relocation of resources
within mycelia. which consequently results in the
transport of substances away from the nutrient-ricn
domains, and hence dispersal of elements which, in
the short-term, are fungally bound but ultimately will
be released to soil nutrient pools. In terms Of nutrient
dispersal, these studies suggest two patterns are pOssible: (i) symmetrical, which would serve a~ a ‘diffusive’ mechanism, spreading nutrients uniformly
through zones of soil supporting the mycelium, and
(ii) asymmetrical which would serve to relocate nutrients
in localised zones spatially remote from the
original resource. Such symmetric or asymmetric
growth would influence structural aggregation of
soils by hyphae in different ways, the former possibly operating across greater size scales than the
latter.
These experiments demonstrate the strong ability
of eucarpic t’ungi to grow through air, and to bridge
gaps between semi-solid domains. Such phenomena
arc of consequence for the colonisation of soils,
since effective growth through soil will often necessitate bridging pores between aggregates. Little is
known about factors which influence the ability of
vegetative hyphae to extend aerially, although factors
likely to be influential include intra-hyphal pressure,
the mechanical properties of the wall (including
presence of septae), hyphal mass and the angle of
growth. In these experiments, many hyphae grew
readily across 2 mm gaps apparently perpendicular
to the vertical fr-lcesof the agar blocks. Sporangiophores, which can attain lengths of several mm (e.g.
M~cor in these studies) are commonly regarded as
specialised structures to locate spore clusters into air
currents, but more consideration should be given to
the potential that vegetative hyphae have in growing
through free space. Likewise. the poteutial for collapsed sporangiophores to bridge air gaps is a previously undocumented consequence of a possible function in some species.
Whilst the microcosm design here is suitable for
observations on growth in nutritionally heterogeneous environments, it suffers from the difficulty of
permitting any quantitative assessment of growth.
The same problem applies to the ‘Repli-plate’ approach [ 121. Much valuable information could be
gained about mycelial development if it were possible to visualise hyphal distribution with the degree of
precision obtained by Crawford et al. [3], in systems
where the dic;tribution of nutrients could be controlled as precisely as in the microcosms described
here.
Aclmowledgements
1 thank Sandra Millar for technical assistance, and
John Crawford, Alan Rayner and Brian Sleeman for
useful discussions. This work was financially supported by the Scottish Qffice Agriculture and Fisheries Department.
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I,J ’frtri g~~rc*t;~lly
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