Polyploidization of endosperm chalazal haustorium of Rhinanthus serotinus [Scrophulariaceae]

The paper reports a study of karyological differentiation of the chalazal endosperm haustorium of Rhinanthus serotinus (Schönheit) Oborny. Polyploidization began soon after the formation of the haustorium cell. The fully differentiated chalazal haustorium is a large elongated cell containing two huge nuclei. The nuclei are situated halfway along the length of the haustorium cell. Measurements of their nuclear DNA content revealed its degree of ploidy, which could attain a maximum 768C. Nuclei with higher (192C-384C) and the highest (768C) levels of ploidy were found in mature chalazal haustorium cells (from 100- to 500-celled endosperm proper). During polyploidization, the volume of the nuclei increased, and changes in the chromatin structure of polyploid nuclei were noted. With increasing levels of ploidy, polytene chromosomes were observed in haustorium nuclei. The rhythmical increase of DNA content and changes in nuclei size and chromatin structure point to endoreduplication as the mechanism of polyploidization of the haustorium cell.