Modern Phytomorphology 6: 67–68, 2014 © Te Author(s), 2014 Avena fatua L. (common wild oat) and A. sterilis L. (sterile oat) are common weeds with a wide range of distribution (Baum 1977; Baum et al. 1972). Both hexaploid species have AACCDD genomes (2n = 42) (Rajhathy & Morrison 1959). Tey belong to the Avena section, in which natural hybrids occur. Natural intercrossing between A. fatua and A. sterilis as well as between these species and cultivated oats (A. sativa L. and A. byzantina K. Koch) was observed (Florell 1931). For both species and artifcial hybrids from the cross between them (obtained from National Small Grains Collection, Idaho, USA), a comprehensive analysis of the endosperm was performed. Te studies were conducted at mature and nuclear developmental stages of the tissue. Aleurone layers from the ripe caryopses were manually isolated. In addition, transverse sections of the kernels were made. Developmental relationships between the aleurone layer and starchy endosperm in A. fatua, A. sterilis and their hybrid were researched. It was noted that these tissues are strongly modifed in the hybrid caryopses. Starchy cells occurring in the aleurone layer as well as aleurone cells appearing between starchy cells, an intrusive growth of aleurone cells into the starchy endosperm, large aleurone cells and cells with mutated globoids were the most frequent developmental disorders. Several cases of somatic crossing-over in the hybrid aleurone layer have been found. Some abnormalities were also observed in parental species, but their frequency was signifcantly lower than in a hybrid progeny. Transsections of hybrid kernels revealed multicellular aleurone layer, especially in the crease area. Some examples of endospermal domains of various structure were also reported. Tese domains can be units of the free growth in the space released by apoptotic nuclei. Parental genomes play a diferent role in apoptosis of dysfunctional nuclei and cells in hybrid endosperm (T omaszewska & Kosina 2013; Kosina & Tomaszewska 2013). Tese data provide evidence of correctness of the model of domainal hybrid endosperm development presented by Kosina et al. (2013). Te multilayered aleurone and the domainal arrangement of endosperm were not found in parental forms. Te study conducted at the nuclear stadium of endosperm showed numerous cytogenetic disorders, such as: bridges in anaphases and telophases, elimination of chromosomes in anaphases, as well as merging of nuclei. Te increased micronucleus frequency (sometimes 2 or 3 in one cell), which indicates the elimination of fragments or whole MICROSTRUCTURL AND CYTOGENETIC DISORDERS IN ENDOSPERM OF AN AVENA FATUA × A. STERILIS HYBRID Paulina Tomaszewska * & Romuald Kosina ** Abstract. A signifcant diference for cytogenetic and developmental stability of endosperm was discovered between two wild oat species and their hybrid progeny. Te development of hybrid endosperm was regulated by apoptosis and proceeded according to a domainal patern. Many cytogenetic anomalies were also noted, and their appearance was atributed to the activity of transposons or somatic crossing-over. Te normal development of endosperm should be considered as an important factor determining viability of seedlings and their intrapopulational competitiveness. Key words: Avena fatua, Avena sterilis, wild oats, hybrid progeny, endosperm, cytogenetics, development Plant Speciation Group, Institute of Experimental Biology, University of Wroclaw, Kanonia 6/8, 50-328 Wroclaw, Poland; * ptomaszewska@interia.eu, ** kosina@biol.uni.wroc.pl