Selfing of a single monoecious Populus tremula tree produces viable males, females and "supermales"

Nearly all species of the genus Populus are dioecious, i.e., form male and female flowers on separate individuals. However, bisexual poplar can occasionally be found under natural conditions or during the breeding process. According to the XX/XY model of sex determination, “supermale” (YY) individuals should only occur in the selfed progeny of a genetically male (XY) tree and not in the one of a genetically female (XX) parent. We performed detailed molecular genetic analyses of a bisexualPopulus tremula XY-individual (ZP8) and its selfed progeny. The ZP8 tree was confirmed as P. tremula by chloroplast and nuclear markers, and the progeny was genotyped by microsatellite markers to verify self-pollination. In the selfed progeny but also in about 420 different poplar individuals from several breeding programmes and the wild, the X and Y chromosomes were tracked using previously reported and newly developed markers. From two self-pollination experiments of the bisexual ZP8 tree, 39 S1- individuals were obtained. Application of X- and Y-specific molecular markers revealed 15 XY, 17 YY, and 7 XX individuals. Additional investigations of about 300 different poplar clones employed in several breeding programmes as well as about 120 individuals from wild populations revealed only three trees with two Y chromosomes. Indications for inbreeding depression in the selfed progeny were obtained during germination and glasshouse cultivation, but are independent from the sex chromosome combination. Even though YY-poplar individuals are rarely found in natural populations or breeding collections, they can arise and can be as viable as regular XY- and XX-plants.