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.

Managing Australia’s eucalypt gene pools: assessing the risk of exotic gene flow

“…This pattern observed for interspecific hybridisation contrasts with the positive heterosis observed for inter-provenance crosses within E. globulus (Volker et al 2008; Costa e Silva et al 2014).…”

“…Assisted migration involves moving species beyond their current range, or moving populations within a species range (sometimes referred to as 'assisted gene flow ' -Aitken & Bemmels 2016). Debate about the need for assisted migration and assisted gene flow in forest trees is ongoing, because it is possible that naturally occurring long-distance gene flow may be enough to facilitate rapid natural adaptation to climate change (Kremer et al 2012; Costa e Silva et al 2014). Certainly high levels of such gene flow have been noted among remnant eucalypt populations in highly fragmented landscapes in Western Australia (Sampson & Byrne 2008;Byrne et al 2008).…”

Inbreeding depression and differential maladaptation shape the fitness trajectory of two co-occurring Eucalyptus species

“…F < 0) often observed in wild eucalypt populations despite the excess of homozygosity in OP seed (see Potts and Wiltshire 1997). However, in the case of E. globulus, there is evidence of heterosis in inter-population crosses (Volker et al 2008;Costa e Silva et al 2014), suggesting that some mild build-up of ID within wild populations are likely due to bi-parental inbreeding (Hardner and Potts 1997;Mimura et al 2009), although the rare survival of selfs cannot be dismissed. The long-term monitoring of the field trial revealed a dynamic interplay between ID for survival (ID surv ) and ID for growth (height and DBH) of survivors (ID growth ).…”

Population Divergence along a Genetic Line of Least Resistance in the Tree Species Eucalyptus globulus

“…However, the changes caused by migration were found to be slow and to increase over time, and tended to be reduced with increased correlational selection and/or mutation correlation [30]. In the present case, the contribution of gene flow cannot be completely dismissed, considering: (i) E. globulus to be pollinated by animals, including a key bird pollinator which annually migrates between the Mainland and Island populations [67,99]; and (ii) the demonstrated potential for post-dispersal selection favoring the products of long-distance dispersal due to a release from inbreeding depression [100]. While the effective pollen dispersal curve has been shown to be fat-tailed, the average dispersal distance has been estimated as only between 69 m and 833 m, depending on the studied population [74].…”