Post-zygotic reproductive isolation among populations of Iris atropurpurea: The effect of spatial distance among crosses and the role of inbreeding and outbreeding depression in determining niche width

Question: What is the role of inbreeding and outbreeding depression in creating spatial patterns of reproductive isolation among populations within a species? Hypothesis: A combination of inbreeding and outbreeding effects create an optimal crossing distance at which reproductive isolation is minimal. Organism: Iris atropurpurea Dinsm., an endangered and endemic Israeli plant, with a fragmented distribution throughout the coastal plain. Field sites: Two I. atropurpurea populations, one in the Shafdan dunes and one in the Netanya Iris Reserve, about 19 km south and north of Tel Aviv respectively, on the coastal plain in Israel. Methods: We performed artificial cross-pollination within and between populations of I. atropurpurea at various distances and measured seed germination and seedling survivorship. Results: Theoretical considerations led us to expect that inbreeding depression acts mostly at the small scale, and that higher offspring fitness is revealed at distances <10 km. Results of the experiment show that reproductive isolation acts differently in consequent stages of the hybrid life history. The pattern of total reproductive isolation among populations along a geographical axis showed different patterns in the two natural populations; whereas in the Netanya population no pattern appeared, in Shafdan we found a pattern of intermediate distance where reproductive isolation is the highest, and at short and long distances reproductive isolation is relaxed.