Outbreeding depression varies among cohorts of Ipomopsis aggregata plantedin nature

Outbreeding depression in progeny fitness may arise from disruption of loca l adaptation, disruption of allelic coadaptation, or a combination of these "environmental" and ''physiological'' mechanisms. Thus the minimum spatial scale over which outbreeding depression arises should depend on the spatia l scale of gene dispersal and (with an environmental mechanism) of change i n selection regimes. We previously reported substantial outbreeding depress ion in lifetime fitness of progeny resulting from crosses among parents sep arated by 100 m in natural populations of the herbaceous plant Ipomopsis ag gregata. In this paper we explore the effect of crossing distance on fitnes s in two additional experiments begun in 1987 and 1990. We planted seed pro geny derived from partial diallel crossing designs in randomized blocks in maternal environments and scored emergence of seedlings, survival, and even tual flowering of individuals over the subsequent six to eight years. Neste d within each diallel design were crossing distances of 1 m, 10 m, and 100 m. Compared to 1-m and 10-m progeny, 100-m progeny of the 1987 diallel suff ered a significant reduction in seedling emergence, and both 1-m and 100-m progeny that survived to flower achieved lower lambda-values on average tha n 10-m progeny. Total outbreeding depression suffered by 100-m relative to 10-m progeny was approximately 10%, compared to approximately 30% in our ea rlier study of I. aggregata. Progeny of 10-m crosses also outperformed 1-m and 100-m progeny of the 1990 diallel by approximately 5%, but no differenc e among crossing distance treatments was significant. Thus, the magnitude o f outbreeding depression in 100-m crosses varied among experiments. This is not surprising given likely spatial and temporal variation in gene flow an d selection regimes, different population histories, and different parental and progeny environments. Characterizing outbreeding depression on the sho rtest spatial scales over which it is expressed, as well as its variation a nd causes, is worthwhile because it promises to shed light on the earliest stages of angiosperm speciation.