Effects of pollen contamination and non-random mating on inbreeding and outbreeding depression in a seedling seed orchard of Eucalyptus urophylla

Abstract

Microsatellite loci were used to assess pollen contamination and mating patterns in an Eucalyptus urophylla seedling seed orchard (SO2) along with inbreeding and outbreeding depression in a second generation progeny test (PT2) established with SO2 seeds. We measured survival, diameter at breast height (DBH), and height (H) in SO2, PT2, and two other stands (SO1, PT) located at a distance of at least 210 m from SO2. Pollen contamination was substantial (11.9%), with the largest proportion coming from the closer SO1 stand (6.1%) than PT (4.7%) and 1.1% from unknown sources. The pollen dispersal pattern was isolation by distance but reached long distances (600 m). PT2 offspring were mainly the result of outcrossing (92.6%), but mating in SO2 was not random due to self-fertilization (7.4%), mating among relatives (6.6%), correlated mating (13.7%), and high male fertility success of trees with large DBH. Thus, families are a mixture of different levels of relatedness and some inbreeding (12.7%). Inbreeding depression was variable among traits; it was higher for selfing (39.6–49.5%) than mating among relatives (11.6–20.9%) and higher for mating between full-sibs (4.4%) than half-sibs (2.2%). Outbreeding depression was higher for pollen immigration from less improved sources (33.9–41.2%). Non-random mating resulted in a coancestry (0.152) higher than expected for half-sib families and an overestimation of heritability by 17.8%. Our results indicate that isolation distances greater than 600 m are necessary to avoid pollen contamination in Eucalyptus seed orchards. Furthermore, pollen immigration results in outbreeding depression, and selfing results in higher levels of inbreeding depression than mating among related trees.