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

Abstract

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 distriution throughout the coastal plain. Field sites: Two 'I. atropurpurea' populations, one in the Shafdan dunes, and one in the Netanya Iris Reserve, both ca. 19 km S or N (respectively) of Tel Aviv, 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 from the experiment showed that reproductive isolation acts differently in consequent stages of the hybrid life history. Pattern of total reproductive isolation among populations along a geographical axis showed different patterns in the two natural populations: while in the Netanya population no pattern appeared, in Shafdan we found a pattern of intermediate distance where reproductive isolation is the highest, and in short and long distances reproductive isolation relaxed