Lm. O'Connell et Cg. Eckert, Differentiation in sexuality among populations of Antennaria parlinii (asteraceae), INT J PL SC, 160(3), 1999, pp. 567-575
It is widely thought that many apomictic angiosperms exhibit variation in s
exual versus apomictic seed production, yet there has been little investiga
tion of the extent to which apomixis varies among or within natural populat
ions. In dioecious Antennaria parlinii, previous reports of wide variation
in both population sex ratios and the capacity of individual plants for aut
onomous seed production indicate that sexuality varies extensively both amo
ng and within populations. We used a combination of population surveys and
pollination experiments to test this hypothesis. A survey of seed productio
n in 15 populations containing only females and nine populations containing
both sexes revealed that the absence of males was not associated with redu
ced seed set in female populations (mean = 0.74) compared with mixed-sex po
pulations (0.69). In mixed-sex populations, seed production appeared strong
ly dependent on pollination. Seed set of females in mixed-sex populations c
orrelated positively with both the total number of inflorescences in a popu
lation, (r = 0.68) and the frequency of male inflorescences (r = 0.80), whe
reas seed set was independent of inflorescence number in female Populations
. In addition, the seed set of individual plants within two mixed-sex popul
ations correlated negatively with distance to the nearest male inflorescenc
e. Excluding pollinators from inflorescences almost entirely prevented seed
set in two mixed-sex populations but had no effect on seed production in t
wo female populations. In a pollinator-free greenhouse, plants from nine mi
xed-sex populations only set seed when hand pollinated, whereas pollination
had no effect on seed production in plants from 13 female populations. Tak
en together, these results indicate that populations of 4. parlinii are eit
her predominantly sexual or predominantly apomictic. Partial apomixis and/o
r polymorphism for sexuality within populations appear to be uncommon in th
is species. Strong differentiation for sexuality is in accord n with theore
tical models that predict disruptive selection for apomixis when pollinatio
n is not required for seed maturation.