CROSSING-DISTANCE EFFECTS IN DELPHINIUM NELSONII: OUTBREEDING AND INBREEDING DEPRESSION IN PROGENY FITNESS

Depending on its genetic causes, outbreeding depression in quantitative characters may occur first in the free-living F₁ generation produced by a wide cross. In 1981-1985, we generated F₁ progenies by hand-pollinating larkspurs (Delphinium nelsonii) with pollen from 1-m, 3-m, 10-m, or 30-m distances. From the spatial genetic structure indicated by previous electrophoretic and reciprocal transplantation studies, we estimate that these crosses range from being inbred (f ≈ 0.06) to outbred. We planted 594 seeds from 66 maternal sibships under natural conditions. As of 1992, there was strong evidence for both inbreeding depression and outbreeding depression. Progeny from intermediate crossing distances grew approximately twice as large as more inbred or outbred progeny in the first 5 yr after planting (P = 0.013, repeated measures ANOVA), and survived almost 1 yr longer on average (contrast of 3-m and 10-m treatments versus 1 m and 30 m; P = 0.028, ANOVA). Twenty maternal sibships produced flowering individuals; only four and two of these represented 1-m and 30-m crossing distances, respectively (P = 0.021, G-test). The cumulative fitness of intermediate distance sibships averaged about twice that of 1-m sibships, and five to eight times that of 30-m sibships (P = 0.017, ANOVA). Thus, even though progeny of 1-m crosses were inbred to a degree only about one-eighth that of selling, inbreeding depression approximated 50%, and outbreeding depression equaled or exceeded 50% for all fitness components.