Gynodioecy is a dimorphic breeding system in which male-sterile individuals (i.e., females) coexist with hermaphroditic individuals in populations. Previous studies of two species of Lobelia in North America have documented gynodioecy in parts of their ranges and here, we document gynodioecy for a single population of L. siphilitica and in two populations of L. spicata from western Massachusetts. Our objectives were to (1) determine sex ratios in natural populations of these species, (2) use controlled pollinations to investigate the capacity and extent of self fertilization in hermaphrodites and (3) compare the relative fertility and magnitude of sexual dimorphism in floral morphology between females and hermaphrodites. The frequency of females was only 3% in L. siphilitica, whereas both populations of L. spicata contained 12% females. Hermaphrodites in both species were self-compatible and estimates of mating system parameters confirmed mixed mating in L. spicata. Females of L. spicata had higher fruit and seed production compared to hermaphrodites and there was significant floral size dimorphism. In contrast, there was no sexual dimorphism and no differences in fruit and seed set between females and hermaphrodites in L. siphilitica. We also used herbarium records to document that flowering of these two congeners is well separated temporally in the Northeast and that northeastern collections of L. spicata, but not L. siphilitica, flower significantly later in the season compared to herbarium specimens collected from outside this region. Further, herbarium collections were used to test previous hypotheses concerning the validity of infrageneric classifications in Lobelia spicata; specifically, we confirmed that L. spicata var. campanulata is a male-sterile form of this species.
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Vol. 134 • No. 3