Subtribe Spiranthinae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 29

also the predominant mode of reproduction for S. cernua near the edge of its range
in eastern Nebraska. However, experimental treatments testing for geitonogamy and
xenogamy produced a significantly higher proportion of nonpolyembryonic seeds
than tests for agamospermy (Schmidt and Antlfinger 1992 ). They considered that
agamospermy was most likely facultative on the assumption that some of the addi-
tional nonpolyembryonic seeds were produced sexually. Based on seed morphology
in natural populations, the maximum estimates of sexual seed production per cap-
sule ranged from about 20 to 34% in successive years. This would provide sufficient
genetic diversity to account for the amount of allozyme variation observed in this
population (Schmidt and Antlfinger 1992 ).
S. parksii Correll, described as a facultatively agamospermous tetraploid species
endemic to postoak savannas in East Central Texas (Catling and McIntosh 1979 ;
Catling 1990 ; Sheviak and Brown 2002 ), is not distinct from S. cernua (Dueck
2008 ; Dueck and Cameron 2008 ).
Fruiting Success and Limiting Factors
Schmidt ( 1987 ) suggested that the high proportion of agamospermic seeds produced
in eastern Nebraska populations of S. cernua might be due to pollinator limitation
(discussed below under sexual populations) or to the precocious development of
adventitious embryos prior to anthesis. Indeed, Sheviak ( 1982 ) reported that cap-
sules developed prior to anthesis in some plants of this species, and as noted earlier,
Catling ( 1982 ) described the initiation of adventitious embryos while the flower was
still in the bud stage. Precocious agamospermic reproduction might increase the
proportion of asexual embryos by reducing the resources available for the later
development of meiotic embryos (e.g., Nogler 1984 ). If so, any factors that suppress
or delay the initiation of asexual reproduction might be expected to permit the matu-
ration of more megagametophytes and hence to increase the level of sexual repro-
duction (Catling 1982 ), but such factors have yet to be identified (Schmidt 1987 ).
Schmidt ( 1987 ) and Schmidt and Antlfinger ( 1992 ) observed significant yearly
fluctuations in the proportion of polyembryonic seeds in unmanipulated plants of
S. cernua at their study site. Although such fluctuations might be attributed to dif-
ferences in pollinator availability or to genetic diversity among the plants observed,
they might also reflect environmental influences other than those affecting pollina-
tor abundance. Clausen ( 1954 ) and Marshall and Brown ( 1981 ) considered the level
of agamospermy to mirror an interplay of environmental and genetic factors, and
Knox ( 1967 ), for example, demonstrated that differences in photoperiod at different
latitudes correlated with the level of agamospermy in grasses. However, a more
complete understanding of interrelationships between asexual and sexual seed pro-
duction in S. cernua must await the results of additional genetic and developmental
studies.
Facultative agamospermy is more common and versatile in angiosperms than
obligate agamospermy and does not represent the evolutionary blind alley often