Structure, DNA content and DNA methylation of synergids during ovule development in Malus domestica Borkh

DNA content and DNA methylation of synergids of Malus domestica Borkh have been evaluated in two different stages before fertilization. A monoclonal antibody against 5-methylcytidine has been used to evidence the level and distribution of DNA methylation. In the first stage, both synergids were alike in structure, with a large chalazal vacuole, a large nucleus and nucleolus and a well developed filiform apparatus. One of the two cells was localized closer the mycropilar region of embryo sac. At this stage both cells showed a 2C DNA content and a similar level and pattern of DNA methylation. The two synergids in Malus had different fates. In the second stage, near the time of fertilization, the mycropilar synergid showed signs of degeneration: a reduced size, a compact chromatin and a more darkly stained cytoplasm. DNA methylation of this synergid was twice as much as the other cell and a different distribution of DNA methylation was also evidenced between the two cells, while their DNA content was unaltered. These observations are discussed in relation to the hypothesis of a different gene expression in the two synergids, thus supporting the assumption of a preprogramming for the degeneration of one of the two cells.