Differentiation in sexuality among populations of Antennaria parlinii (asteraceae)

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.