Biology Reference
In-Depth Information
Fig. 8.4
Schematic dorsal
views of two pollinators
showing placement of pollen.
(
a
) A large bee, such as a
Bombus
queen; (
b
) A small
bee, such as a
Bombus
worker or
Megachile
.
Position of pollen placement
for
Arethusa bulbosa
(a),
Calopogon tuberosus
(c), and
Pogonia ophioglossoides
(p)
can be removed only as the insect is leaving the flower and it rarely, if ever, visits
the same flower more than once, self-fertilization is usually avoided (Thien and
Marcks
1972
). On subsequent floral visits, pollen may be transferred when the tho-
rax contacts the stigma; the head and abdomen of the insect usually fail to contact
either the anther case or the stigma.
The process is similar in
Pogonia
. The pollinator again lands on the fleshy, ultra-
violet absorbing hairs of the lip (Fig.
8.1a
) and crawls toward its base, inserting its
head deep into the flower (Guignard
1879
; Thien and Marcks
1972
). However, the
flower of
Pogonia
is so constructed that pollen grains from the anther are attached
to the head, rather than the thorax or abdomen, of the withdrawing bee (Fig.
8.4a, b
).
On subsequent floral visits, the head may deposit pollen on the stigma. The insect's
thorax and abdomen usually fail to contact either the anther or the stigma.
The geographic ranges and flowering periods of all three species overlap in
northern regions, and all have been reported to share a number of insect visitors, yet
natural hybrids are rare (Thien and Marcks
1972
; Pinkepank
1993
). The differences
in the positioning of pollinia on the body of the pollinator have often been consid-
ered to provide an explanation for intraspecific fidelity in pollen transfer, acting as
a mechanical isolating mechanism and presumably permitting, in some cases, the
simultaneous placement of pollen from two or even all three orchids on different
parts of a common pollinator's body (Thien and Marcks
1972
).
However, the intergeneric crossing experiments outlined above clearly imply
that differences in the positioning of pollinia are redundant in the reproductive iso-
lation of contemporary populations of
Pogonia
from
Arethusa
and
C. tuberosus.
The effect of pollinia placement on other factors, such as the avoidance of stigma
contamination by foreign pollen, might yet prove to be important, however. Such
contamination can significantly reduce male and female fitness by limiting the
available space and relative numbers of compatible pollen grains on the stigma and
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