Prairie environments are becoming increasingly fragmented, and thus the remaining native species are frequently in smaller populations. Theoretical models predict that small populations are strongly influenced by random processes such as genetic drift and not natural selection. This change in population dynamics is expected to result in lowered reproductive success which can contribute to local extinction of the species. We have examined the impact of habitat fragmentation on the sex ratio (female-only vs. male/female individuals) of a prairie plant (Lobelia spicata, a gynodioecous perennial) in fourteen prairie preserves of different sizes to determine whether the ratio of flower/sex types in the smaller prairies is skewed in comparison to larger preserves due to genetic drift. We have examined reproductive success of these plants to determine whether there is a relationship between seed set and the ratio of flower types. Furthermore, we have examined the sex ratio of the offspring from four of the populations to determine the extent that male function is restored in offspring of female plants or lost in the offspring of hermaphroditic plants. We predict that due to genetic drift in the smaller fragments, populations that have a female biased sex ratio will have limited restoration of male function from female plants’ offspring due to a lack of allelic diversity at the restorer loci compared to larger female biased population. In the small hermaphroditic biased populations, the loss of male function from the hermaphrodites would be rare compared to the larger hermaphroditic biased population. So far the results from the prairie preserves indicated that the sex ratio ranges from 2%-55% females plants in the populations. There is no apparent pattern with either population or fragment size. These results will be discussed in terms of the evolution of the gynodioecous breeding system in small isolated populations.

Key words: breeding system evolution, gynodioecy, habitat fragmentation, Lobelia spicata, prairies