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Using native trees from near the northern and southern extremities of the relatively continuous eastern distribution of  Eucalyptus globulus  in Tasmania, we compared the progenies derived from natural open-pollination (OP) with those generated from within-region and long-distance outcrossing. Controlled outcrossing amongst eight parents - with four parents from each of the northern and southern regions - was undertaken using a diallel mating scheme. The progeny were planted in two field trials located within the species native range in southern Tasmania, and their survival and diameter growth were monitored over a 13-year-period. The survival and growth performances of all controlled cross types exceeded those of the OP progenies, consistent with inbreeding depression due to a combination of selfing and bi-parental inbreeding. The poorer survival of the northern regional (♀ N ♂ N ) outcrosses compared with the local southern regional outcrosses (♀ S ♂ S ) indicated differential selection against the former. Despite this mal-adaptation of the non-local ♀ N ♂ N  crosses at both southern sites, the survival of the inter-regional hybrids (♀ N ♂ S  and ♀ S ♂ N ) was never significantly different from that of the local ♀ S ♂ S  crosses. Significant site-dependent heterosis was detected for the growth of the surviving long-distance hybrids. This was expressed as mid-parent heterosis, particularly at the more northern planting site. Heterosis increased with age, while the difference between the regional ♀ N ♂ N  and ♀ S ♂ S  crosses remained insignificant at any age at either site. Nevertheless, the results for growth suggest that the fitness of individuals derived from long-distance crossing may be better at the more northern of the planting sites. Our results demonstrate the potential for early-age assessments of pollen dispersal to underestimate realised gene flow, with local inbreeding under natural open-pollination resulting in selection favouring the products of longer-distance pollinations. Indeed, heterosis derived from long-distance pollinations may be sufficient to counter local mal-adaptation, at least in the first generation.

Heterosis May Result in Selection Favouring the Products of Long-Distance Pollen Dispersal in Eucalyptus