title: Heterosis may result in selection favouring the products of long-distance pollen dispersal in Eucalyptus.
abstract: 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.
authors.affiliation: Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Lisboa, Portugal.
authors.forename: João
authors.initials: J
authors.lastname: Costa E Silva
authors.affiliation: School of Biological Sciences and National Centre for Future Forest Industries, University of Tasmania, Hobart, Tasmania, Australia.
authors.forename: Brad M
authors.initials: BM
authors.lastname: Potts
authors.affiliation: School of Biological Sciences and National Centre for Future Forest Industries, University of Tasmania, Hobart, Tasmania, Australia.
authors.forename: Gustavo A
authors.initials: GA
authors.lastname: Lopez
mesh_terms: D003433:Crosses, Genetic; D005052:Eucalyptus / Q000235:genetics* / Q000254:growth & development / Q000502:physiology*; D005843:Geography; D006823:Hybrid Vigor / Q000235:genetics*; D011058:Pollen / Q000502:physiology*; D011336:Probability; D058614:Seed Dispersal / Q000235:genetics*; D012641:Selection, Genetic; D013648:Tasmania; D014197:Trees / Q000235:genetics / Q000254:growth & development
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