Tribe Neottieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 68

is dorsal, two-chambered, and contains two pairs of soft, mealy pollinia comprised
of pollen tetrads. Rostellar glue is located at the top of the rostellum and covered by
a fragile membrane, but caudicles are absent and the pollinia are probably not attached
to the rostellar glue prior to the intervention of a pollinator (Dressler 1993 ).
Compatibility and Breeding System
Based on hand pollinations, Wilson ( 2009 ) found that 59% ( n = 101) of self-pollinated
flowers, 72% ( n = 85) of outcrossed flowers (using donor pollen from plants at least
10 m distant), and 63% ( n = 67) of flowers receiving pollen from other flowers in the
same raceme (geitonogamy) set fruit. Contrary to Brunton ( 1986 ) and Thornhill
( 1996 ), tests for autogamy and agamospermy were also positive. Thirty-eight percent
( n = 679) of unmanipulated, bagged flowers and 11% ( n = 62) of emasculated, bagged
flowers set fruit, although some level of experimental error was suspected in the latter
case (Wilson 2009 ).
The establishment of self-compatibility is of interest as this orchid is known to
form vigorous clonal colonies (Allen 1982 ; Mantas 1993 ; Coleman 1995 ; Thornhill
1996 ). Thornhill [personal communication in Wilson ( 2009 )], in a study of 12 popu-
lations in California, Oregon, Idaho, and New Mexico, concluded that dispersal
routinely involves a single colonization event followed by asexual reproduction and
selfing. Subsequent drift can lead to high levels of differentiation among colonies,
suggesting a very low level of pollen or seed exchange among populations. However,
despite low levels of intrapopulation heterozygosity, extensive examination of
enzyme loci indicates that colonies are not entirely clonal and that the occasional
influx of individuals from other populations may contribute to differentiation from
the original founder.
Pollinators and Pollination Mechanism
E. gigantea is pollinated by syrphid flies, but provides no compensation in the way
of nectar or other reward (e.g., Wiefelsputz 1970 ; Correll 1978 ; Ross 1988 ; Wilson
2009 ). Long-distance attraction is visual and, according to Ross ( 1988 ), is based on
the orchid's yellow lip, a color particularly visible to insects. At close range, the flies
are apparently induced to enter the flowers by deceit. The floral fragrance mimics the
smell of honeydew, the excess sweet sap excreted by aphids during feeding. Adult
syrphids eat honeydew, and the females lay their eggs among masses of aphids, the
normal food supply for syrphid larvae. Coleman ( 2002 ) suggested that the warty
calli on the lip might be mimetic, the syrphids mistaking them for aphids.
Wilson ( 2009 ), in a study conducted over 2 years at three elevations in the Rocky
Mountains of Colorado, identified six species of syrphid flies in four genera as pol-
len vectors: Dasysyrphus creper (Snow), Eupeodes americanus , E. luniger
(Wiedemann), E. volucris Sacken, Platycheirus immarginatus (Zetterstedt), and