Characteristics of the staminate flower and pollen from autotetraploid Betula platyphylla

Comparison of the staminate flower and pollen of Betula platyphylla between autotetraploid and diploid individuals may reveal the reason for cross sterility when an autotetraploid male parent is crossed with a diploid female parent. Staminate flowers and pollen from seven autotetraploid individual trees were examined. The morphologies of the staminate flower and pollen and pollen germination of these autotetraploid individuals were compared with diploid trees. The structure of the staminate flower did not differ between autotetraploids and diploids, whereas the size of the staminate flower was larger in the autotetraploids compared with the diploids. The mean diameter and pore depth of the pollen were significantly larger in autotetraploid individuals compared with diploids, and several abnormalities in pollen morphology were found commonly amongst pollen from the autotetraploid trees. The germination rate of pollen from autotetraploids was significantly inferior to that of diploids and there was a significant negative correlation between germination rate and the percentage of abnormal pollen. These results demonstrate that the autotetraploid should not be chosen as the male parent to obtain hybrid triploid progenies because the pollen is abnormal and results in low fertility despite its large size. The results provide important information for selecting and distributing parent strains with the aim of building seed orchards of triploid birch trees.