Unilateral cross-incompatibility in Eucalyptus: the case of hybridisation between E. globulus and E. nitens

The growth of <i>E. globulus</i> and <i>E. nitens</i> pollen tubes in styles of <i>E. globulus</i> was examined in order to elucidate the site of the unilateral barrier to hybridisation. Pollen tubes of <i>E. nitens</i> failed to grow the full length of the larger <i>E. globulus</i> style. <i>E. globulus</i> pollen tubes grew an average of 1.4 mm per day for the first 4 days, compared with 0.8 mm per day for pollen tubes of <i>E. nitens</i>. From days 4 to 14, the growth of <i>E. nitens</i> pollen tubes slowed to an average of 0.2 mm per day and virtually no growth occurred after day 14. In contrast, <i>E. globulus</i> pollen tubes grew through the style and into the ovary between days 5 and 14. By day 28, at about the time of style abscission, <i>E. nitens</i> tubes had grown only 6 mm, well short of the full length of the <i>E. globulus</i> style (9-10 mm). A similar difference in growth was obtained <i>in vitro</i> where <i>E. nitens</i> pollen tubes were significantly shorter than those of <i>E. globulus</i>. A comparison also including <i>E. ovata</i>, <i>E. urnigera</i> and <i>E. gunnii</i> indicated a significant correlation between style length and <i>in vitro</i> pollen tube length. It is argued that the unilateral cross-incompatibility between <i>E. globulus</i> and <i>E. nitens</i> is due to a structural barrier arising from an inherent limit to pollen tube growth which is associated with pistil size.