Quantitative analysis of aposporous parthenogenesis in Poa pratensis genotypes

Several embryological and cytological techniques were developed to determine the degree of aposporous parthenogenesis vs. sexuality in different genotypes of the facultative apomictic grass Poa pratensis L. Fixed embryo sacs were mechanically isolated and individually examined with phase-contrast and interference-contrast light microscopes. The degree of apospory was calculated as the ratio of embryo sacs with an embryo but without endosperm and the total number of embryo sacs studied per plant. The degree of apospory varied between 9 and 76% in nine plants. Photocytometry and flow cytometry were used to determine ploidy levels in embryo and endosperm of individual embryo sacs in order to determine their origin, asposporous vs. sexual. Results based on the calculated degree of apospory agreed with the degree of apomixis deduced from homogeneity scores in field progeny tests. Callose deposition on the megaspore mother cell during megasporogenesis was studied in the same set of genotypes and the percentage of abnormal, micropylar callose deposition closely agreed with results both of the embryo sac analysis and the progeny tests. Combined use of these techniques yields quantitative data on the aposporous tendency of individual genotypes of Poa pratensis.