F1 hybrid inviability in Eucalyptus: the case of E. ovata × E. globulus

Abstract

The impact of inbreeding and hybridization on fitness was compared in the two co‐occurring forest tree species, Eucalyptus ovata and E. globulus, aimed at explaining the rarity of their hybrids in nature. The success of selfing, open‐pollination and outcrossing of both species and interspecific hybridization was monitored from seed‐set to 10‐year’s growth in a field trial. There was a unilateral barrier to hybridization with seed‐set obtained only with E. ovata females. The F1 hybrids exhibited reduced viability compared to intraspecific cross‐types at virtually all stages of the life cycle and are clearly at a selective disadvantage compared with their open‐pollinated E. ovata half‐sibs with which they would directly compete in nature. Eucalyptus ovata and E. globulus overlap in their flowering time but the F1 hybrids flowered later with virtually no overlap with either species. The asynchronous flowering and reduced reproductive fitness of F1 hybrids would markedly limit the opportunity for advanced generation hybridization. Inbreeding similarly had a deleterious effect on the fitness of both species, and the F1 hybrids were most competitive with the E. ovata selfs. It is argued that changes in inbreeding levels of parental populations may be a key factor affecting the relative fitness of hybrids and their potential to impact on the pure species gene pool. Reduced fitness of the pure species through inbreeding may result in hybridization having its greatest evolutionary impact in small founder or relict populations.

Interspecific Hybridization within Eucalyptus (Myrtaceae): Subgenus Symphyomyrtus, Sections Bisectae and Adnataria

“…The phylogenetic relationships between the species affected hybridization success, with intraseries crosses producing more hybrid seedlings than interseries and intersectional crosses. Lopez et al (2000) found that in E. ovata # E. globulus hybrids, there was a significant reduction in hybrid fitness relative to the parents, with not only seedling deformity but also progressive mortality in young trees over time. This means that even though hybrids have been formed here and can be identified at early stages for further development, there is still no guarantee of their longterm survival, let alone horticultural merit.…”

“…The long time to flowering can also make the morphological assessment of hybridity in offspring difficult or, at best, a long-term prospect, especially where there are longer-term problems with hybrid fitness and survival (Lopez et al 2000) or where the ratio of male to female parental dominance alternates within the first few years (Bouvet andVigneron 1995, 1996), although predictive parental contribution based on multivariate morphological assessments is possible for some features (Baril et al 1997a(Baril et al , 1997b. The current study found that at least for the seedling stage, the hybrid offspring with E. gillii as the mother were either intermediate or tended to resemble the mother (crosses with E. orbifolia and E. transcontinentalis), whereas nearly all the female E. socialis crosses showed apparent early maternal dominance.…”

Gene flow between introduced and native Eucalyptus species: exotic hybrids are establishing in the wild

“…Research has shown that both first-and advanced-generation hybrid breakdowns are common in Eucalyptus (Potts and Dungey 2003; and can be effective barriers to inter-specific gene flow (Potts and Wiltshire 1997). For example, F 1 hybrids between E. ovata and E. globulus, a species closely related to E. nitens, showed poor survival by flowering age, compared with the pure species (Lopez et al 2000b). Furthermore, while these two species overlap in their flowering time, the flowering of their F 1 hybrid does not overlap with either species (Lopez et al 2000a).…”

Effects of inbreeding on population mean performance and observational variances in Eucalyptus globulus

“…Inbreeding depression is commonly observed in fitness related traits associated with survival, reproduction and growth, and is believed to arise through two main non-mutually exclusive mechanisms: firstly dominance, whereby inbreeding unmasks deleterious recessive or partially recessive alleles; and secondly, whereby inbreeding increases homozygosity at loci exhibiting overdominance (Charlesworth and Charlesworth, 1987;Lande and Schemske, 1985). From an evolutionary and conservation perspective, inbreeding depression is important as it will initially lower population fitness, resulting in a phase where small populations of normally outbred species are more susceptible to ecological or genetic displacement (Ellstrand and Elam, 1993;Lopez et al, 2000). However, such selection may eventually result in the purging of deleterious alleles from populations surviving bottlenecks (Klekowski, 1988).…”

“…This is certainly the case in the predominantly Australian genus Eucalyptus where, across the 23 species tested, outcrossing rates in the wild average 74% (Byrne, 2008;Potts and Wiltshire, 1997). The few eucalypt species studied to date have all shown significant inbreeding depression on traits related to fitness, not only following controlled selfing (Griffin and Cotterill, 1988;Hardner and Potts, 1995;Hardner and Tibbits, 1998;Lopez et al, 2000), but also in offspring derived from open-pollinated seed collected from the wild (see previous citations; Hodge et al, 1996;Volker, 2002). However, the limited number of founder parents used in these studies would limit their power for evaluating the effects of inbreeding, particularly on observational variance components.…”