EFFECT OF CROSS PROXIMITY ON PROGENY FITNESS IN A RARE AND A COMMON SPECIES OF EUPATORIUM (ASTERACEAE)

Offspring from matings between near neighbors may exhibit lower fitnes s relative to offspring front more distant matings due to spatial stru cturing of populations resulting from limited dispersal of pollen and seed. This response. which can be interpreted as inbreeding depression , is studied in the rare species, Eupatorium resinosum, and a closely related congener, E. perfoliatum, through the use of hand pollinations representing three distance classes (near-within a population, far-wi thin a population, and between populations) and an assay of the offspr ing in an experimental plot, Early traits such as seed mass and first- year stem length were not significantly affected by the cross type, al though they were affected by maternal parentage. Size and reproduction in the second field season increased with increased pollen donor dist ance from the maternal plant. Cross type was significant for many trai ts in the second field season, indicating inbreeding depression in cro sses of neighbors (for E. resinosum) and hybrid vigor between populati ons (both species). This suggests that the rare species, E. resinosum, had a more spatially structured population than E. perfoliatum. The i mplications of these results for conservation of rare species indicate that protection of habitats sufficient for large populations is neces sary to maintain genetic diversity since each population likely consis ts of many smaller subpopulations.