Outcrossing in interspecific hybrids between 'Eucalyptus spathulata' and 'E. platypus'

Abstract

Outcrossing was investigated in interspecific hybrids between self-fertile 'Eucalyptus platypus' Blakely and partially self-sterile 'E. spathulata' Hook., which shows both pre- and post-zygotic timing. Four hybrid trees were used for the study, two with 'E. spathulata' and two with 'E. platypus' as female parent. Each hybrid had a similar number of locules to each other and to the 'E. platypus' parent, and an intermediate number of ovules per flower compared with the parent species. Controlled hand-pollinations were carried out, in which both self- and cross-pollen from the other hybrid tree with the same female parent species was applied to flowers on each of the four trees, and observations were made 10 days, 4 weeks and 8 weeks after pollination and at seed maturity. In all hybrids, mean seeds per capsule was consistently higher following cross-pollination than following self-pollination. All hybrids showed a reduction in pollen tube number between the top and base of the style when examined by fluorescence microscopy. One tree had significantly fewer cross- than self-pollen tubes at the base of the style, but a similar number of ovules was penetrated by pollen tubes following both treatments. In the other three, there was no difference between cross- and self-pollination in pollen tubes in the style. In three of the four trees there was no difference in ovule penetration following self- or cross-pollination, but in the other, more crossed than selfed ovules were penetrated. Light-microscopy observation of ovules indicated that ovule abortion following fertilisation accounted for the reduced numbers of seeds following self-pollination and to a lesser extent following cross-pollination. All four hybrid trees, irrespective of female parent, were partially self-sterile and resembled the partially self-sterile 'E. spathulata' rather than the self-fertile 'E. platypus'. While the timing of outcrossing control of 'E. spathulata' was both pre- and post-zygotic, only one hybrid was similar, with the other three showing post-zygotic control.