Induction of unreduced megaspores with high temperature during megasporogenesis in Populus

Triploid breeding is one of the most powerful approaches for improvement of the genus Populus L. Pollination with artificial unreduced (2n) pollen was inefficiency, owing to weak competition of 2n pollen. To induce 2n megaspores and improve the efficiency of triploid production, female buds of Populus pseudo-simonii x Populus nigra 'Zheyin3#' were exposed to high temperature during megasporogenesis. A relationship between megasporogenesis and morphological changes of female buds was established to guide the high-temperature treatments. In the progeny, 146 triploids were obtained and the highest efficiency of triploid production was 66.7%. Both 41A degrees C and 44A degrees C were suitable for megaspore chromosome doubling. Cytological analysis showed that meiotic stages from pachytene to diplotene may be the optimal period for megaspore chromosome doubling at high temperature in the 'Zheyin3#.' Because megasporocytes both in the first meiotic division and the second division were treated, the first meiotic division restitution typed and the second meiotic division restitution typed 2n megaspores could be obtained. Our findings indicated that hybridization with high-temperature-induced 2n megaspores is an ideal approach for triploid production. Offspring with different heterozygosity are valuable for genetic research and breeding programs of Populus.