Polytene chromosomes are interphase chromosomes consist of homologous chromatids, which are amplified through consecutive cycles of endoreduplication and conjugate together. Polytene chromosomes are formed in the nuclei of cells that require intense synthesis of substances in highly specialized tissues.
Antipodal cells of embryo sac of wheat is an example of plant cells with polytene chromosomes. Antipodal complex is located in the ovule between the nucellus and endosperm. Antipodal cells form a complex consisting of three cell layers – basal, which contacts with the conductive tissues, middle and apical, which contacts with the endosperm. After double fertilization antipodal cells produce substances necessary for the emerging endosperm coenocyte. Antipodal cells belong to the tissues that perform trophic and barrier functions between the maternal organism and the tissues formed after fertilization. So they play key role in development of endosperm and moreover are crucial for formation of endosperm in cultivated cereals, which are food for all the humanity.
The aim of our work was to study the morphology of the nuclei and giant polytene chromosomes of antipodal cells. The work was performed on total specimens of embryo sacs isolated from fixed wheat ovules. We used methods of light microscopy and transmission electron microscopy.
We measured DNA content in the nuclei of the antipodal cells and found that nuclear DNA content varies in the cells of the complex. The basal layer cells contain less DNA than apical layer cells. During the differentiation DNA content of nuclei increases.
Structure of antipodal polytene chromosome differ from structure of animal polytene chromosomes. Animal polytene chromosomes have disks and interdisks as a result of conjugation of chromatids along the entire length of the polytene chromosome. In regions of polytene chromosomes with the intensive synthesis of RNA, decondensation of chromatin occurs and puffs are formed. On the contrary antipodal cells are characterized by giant chromosomes in the form of bundles of threads and conjugation is found only in centromere regions. At the initial stages of the differentiation nuclei of antipodals have approximately the same size and nonsegregated polytene chromosomes. Then the nuclei of middle and apical layer cells increase in size, they have isolated individual chromosomes.
To conclude, the antipodal cells of the embryo sac in cereals are a unique object for solving the fundamental problems of cell biology and a convenient model for studying the structure of the nuclei and the stages of reforming polytene chromosomes during ontogenesis.
Tatiana Doronina is a 2nd year PhD student in Lomonosov Moscow State University, Biology Faculty in Russia at the Department of Cell Biology and Histology. The field of interest is plant cell biology, polytene chromosomes and programmed cell death.
• Doronina, T. V., Chaban, I. A., & Lazareva, E. M. 2019) Structural and Functional Features of the Wheat Embryo Sac’s Antipodal Cells during Differentiation. Russian Journal of Developmental Biology, 50(4), 194-208.
• Doronina, T. V., Chaban, I. A., & Lazareva, E. M. 2019. Structural reorganization of nuclei of wheat antipodal cells during programmed cell death. Biopolymers & Cell, 35(3), 206.