Gynodioecy and Male Sterility in Eucalyptus leucoxylon F. Muell. (Myrtaceae)

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Abstract
The breeding system of Eucalyptus leucoxylon was investigated using crosses, microscopy, and isozyme analysis. In the study population, 57% of trees were male sterile because microspores were produced by meiosis and formed exines; however, they contained little starch, lacked intines, and had collapsed by anthesis, surrounded by incompletely degenerated tapetum. Hermaphrodite trees produced bisexual flowers, which were protandrous. Anthers dehisced in the bud and 93% of pollen was deposited on the upper style and lower stigma as the filaments extended; only 7% remained on the anthers. Hermaphrodite flowers set low levels of autogamous and assisted selfed seed, compared with open pollinations. Counts of pollen tubes in open-pollinated pistils of each morph showed that female flowers received less pollen than hermaphrodite flowers, but despite this, there was no difference in open-pollinated seed set between the two morphs. Multilocus estimates of outcrossing rates (t) from male-sterile trees showed values approaching complete outcrossing (0.96), while hermaphrodite trees showed significant levels of inbred seed in open-pollinated seed crops (0.83) but still maintained a higher outcrossing rate than reported for most other eucalypts. This study has shown that E. leucoxylon maintains high levels of outbreeding, despite the deposition of self pollen on the stigma in the secondary pollen presentation mechanism; it may achieve this through protandry, pollen removal, male sterility, and late-acting self-incompatibility.

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