Anomalous pollen grain development after nondifferentiating microspore division in Eremurus

Anomalous pollen grain development in Eremurus is caused by an anomalous position of the mitotic spindle and microspore protoplast division into two cells different than in differentiating division. The nuclei of the abnormal gametophytes are always spherical and that of the smaller of the two cells, notwithstanding the shape and position of the latter, has a more compact structure resembling rather that of the generative cell nucleus. Binucleate abnormal gametophytes always have equal nuclei. The wall separating the cells in abnormal pollen grains at first contains callose and, after disappearance of the latter, probably pectins and cellulose. Abnormal pollen grains contain less cytoplasmic RNA than normal ones arid most of them degenerate. If their viability is preserved they do not form' normal pollen tubes in vitro. The frequency of anomalous microspore division is higher in E. robustus (max ca. 50%) than in E. himalaicus (max. ca. 30%) and shows considerable seasonal variations. The results obtained suggest that disturbances in microspore development in Eremurus have a genetic background, but are stimulated by temperature variations in the period preceding mitosis in the microspore