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Expect the unexpected in Corymbia hybrids

Myralyn Abasoloa, David Leeb and Mervyn Shepherda
aSouthern Cross University, Lismore, NSW 2480
bUniversity of the Sunshine Coast, Maroochydore, QLD 4558
Hybrid experiment Gympie QLD

Myralyn Abasolo measures Corymbia hybrid seedlings.

Hybridisation is one mechanism by which plants can alter their adaptive range, influencing their ability to colonise and survive in novel environments.

One observation from a recently completed study of glasshouse-grown seedlings comparing Corymbia torelliana hybrids with their parental taxa showed that F1 hybrids transgressed the range of the parental taxa for specific leaf weight (SLW - the ratio of leaf dry weight to leaf area).  SLW may be indicative of adaptive potential because plants with higher SLW tend to have relatively thick, tough leaves which may provide them with greater resistance to insect attack and/or drought.

The adaptive range of Corymbia torelliana hybrids is of interest to land managers in southern Queensland and northern NSW where C. torelliana or its hybrids are planted as locally exotic amenity trees and are also being trialled for plantation forestry. Corymbia torelliana has recognised weedy attributes, spontaneously hybridises with native spotted gums (Genus Corymbia Section Maculatae) and produces fertile, vigorous offspring.

The study found that when C. torelliana was crossed with a spotted gum taxon that had a similar SLW (C. henryi), the F1 had a significantly lower SLW than both parental taxa. When C. torelliana was crossed with individuals from a spotted gum taxon (C. citriodora variegata, or CCV) that had lower SLW than C. torelliana, the F1 was, on average, not significantly different from the C. torelliana parent but was higher than the CCV parental average. When this F1 taxon was backcrossed to C. torelliana, the progeny - on average - had an SLW that was significantly lower than that of C. torelliana but not significantly different from CCV.  This suggests that in many crosses the behaviour of some hybrids may deviate from the general expectation of intermediate inheritance for characters of a quantitative nature.  In this example, a trait (SLW) that has potentially important fitness implications under drought conditions deviates in a negative direction from the increase that might normally be expected from the phenotypes of two parental taxa.

The study of SLW was part of a larger common-garden study of thirty leaf and stem traits of Corymbia hybrids. There were three hybrids in the experiment.  The maternal parent was always Corymbia torelliana (subgenus Blakella, section Blakella; Parra-O et al. 2009); the pollen parent was one of the following three spotted gum species (subgenus Blakella, section Maculatae; listed in order of increasing genetic divergence from C. torelliana): C.citriodora citriodora, C.citriodora variegata and C. henryi.   The hybrids of the more genetically divergent parents (e.g.,  C. torelliana x C. henryi hybrids) were more easily distinguished from their parent taxa on the basis of morphology than hybrids of genetically more similar parents (C. torelliana x C. c. citriodora  hybrids) because they had more distinguishing features. For example, the C. torelliana x C. henryi hybrids differed from their parents in twelve traits, whereas C. torelliana x C.c. variegata hybrids differed from both parents in only  five traits (see related article in Biobuzz issue fourteen).

These findings and other results are discussed further in an article that was submitted recently for publication.

Biobuzz issue fifteen, December 2011