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WHOLE SPORES
COLOR:
Red-brown (20-80-60-0)
to dark red-brown (20-80-100-10).
SHAPE: Subglobose
to oblong, with the latter more common
(many
spores form within roots).
SIZE DISTRIBUTION:
160-320 µm, mean = 240 µm (n = 100).
SPORE WALL:
Two layers (L1 and L2) that
are adherent that in juvenile spores are of equal thickness, with the laminate
layer thickening as the spore wall is differentiated.
L1: An outer permanent rigid layer with a smooth surface, dark
red-brown (20-60-100-10), 0.8-1.2 µm thick. Tightly adherent to L2.
L2: A layer consisting of dark orange (20-80-80-0) to red-brown
(20-80-100-0) sublayers (or laminae) that increase in number with thickness;
3.2-7.4 m (mean of 6.8 m) thick in mature spores.
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GERMINAL WALLS: Three flexible hyaline inner walls (gw1, gw2, and gw3), all of which tend to be adherent to each other (see photos above and closer details in the sequence below).
GW1: Two adherent
layers (L1 and L2) are formed. L1 is less than 0.5 µm thick; L2 is slightly
thicker (0.5-0.8 µm). Both layers are so thin that when they are adherent, they
appear as one layer. This layer often is hard to detect because it often is
tightly adherent to IW2 or breaks up when a spore is broken.
GW2: Two adherent layers (L1 and L2) are formed. L1 is less
than 0.5 µm thick; L2 is 1.2-3.2 µm thick and becomes slightly discolored (0-0-20-0)
in Melzer's reagent.
GW3: Two adherent layers (L1 and L2) are formed. L1 is 0.8-2.4
µm thick; L2 is 0.9-1.6 µm thick and stains pinkish (0-40-20-0) to pinkish purple
(0-60-30-10).
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Left to right: (i) detail of the three germinal walls, (ii) position of shield between gw2 and gw3, (iii) remnant scar of germ tube pushing through gw1 and gw2 (and the spore wall) |
WIDTH OF SPOROGENOUS CELL:
38-50 µm (mean = 45 µm).
SPOROGENOUS CELL WALL:
Two layers (L1 and L2) probably are present (continuous with the two
layers of the spore wall), but only L2 is readily discernible at the level of
the compound microscope.
L2: Yellow-brown in color (0-10-60-0), 2.4-4.8 µm thick near
the spore and then thinning to 1.0-1.5 µm beyond the sporogenous cell.
OCCLUSION:
Closure by a plug concolorous with L2 of the spore wall.
COLOR:
Yellow brown (0-10-100-10)
SHAPE: Oblong,
with length approximately 1.5 times that of the width. Position of the shield
is on the innermost germinal wall (gw3). Margins of the shield are deeply invaginated
(2-30 µm deep), with some bifurcating at the tips. The outer boundary of the
shield is very robust (shields rarely break apart in broken spores), 2.5-3 µm
thick.
SIZE:
Generally 45-90 x 95-210 µm.
AUXILIARY
CELLSAggregate (2-15) cells borne on coiled brown (20-40-80-0) hyphae 5-7 µm in diameter; thin-walled (< 1 µm thick), with wall pale yellow (0-0-5-0) in transmitted light; each cell with tuberculate surface, with swellings 1.8-3.2 µm wide and 8.6-12 µm high.
Intraradical arbuscules and hyphae consistently stain darkly in roots treated with trypan blue. Arbuscules with thick trunk and often short coils (6-8 µm in diameter) and many finely tipped branches. Hyphae often with knobs or projections, 5-10 µm in diameter, usually densely coiled near and around entry points but also widely distributed throughout the root cortex. Extraradical hyphae consisting of very fine hyphae (< 2 µm in diameter) and coarser dark brown (20-40-100-0) hyphae 3-8 µm in diameter (see hyphae associated with auxiliary cell cluster in photo above).
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Some spores are formed
within the root cortex, and may account in part for the large proportion of
oblong spores. Since many soil-borne spores are of similar shape, this trait
also appears to be heritable (under genetic control). Spores in dried pot culture
inoculum are not as long-lasting as their lighter colored relatives, rarely
extending longer than 12 months. In our experience, many spores collected from
field soils contain internal parasites, even though they appear healthy when
examined intact under a dissecting microscope.
This species was described before Scutellospora was separated from
Gigaspora. Koske and Walker (1984) described spores as having an inner
thin "unit wall" (probably gw1) and an inner "laminated wall"
(which actually consists of gw2 and gw3 when tightly adherent as exemplified
in the photo sequence above). The difficulties in interpretation of germinal
wall structure and organization was understandable at the time this species
was first described, since little was known of kinds and properties of these
walls or of the immutable position of the germination shield on the innermost
germinal wall (regardless of how many of these inner walls are present). The
two innermost germinal walls are easiest to distinguish, despite their tight
adherence, because they separate where a germination shield forms (see photos
above).
REFERENCES
Koske, R. E. and C. Walker. 1984. Gigaspora erythropa, a new species forming arbuscular mycorrhizae. Mycologia 76:250-255.