Angiosperm species that produce sperm cell pairs or generative cells with polarized distribution of DNA-containing organelles

Abstract

We examined the distribution of DNA-containing organelles (plastids or mitochondria) in pollen from 115 angiosperm species (104 genera, 56 families) to determine whether there was polarized distribution of sperm cells (SC) and generative cells (GC). Using an improved method of fixation and squashing, we found polarization in the GC or SC pairs in mature pollen (obtained from the flower at anthesis) from five species (Abelia spathulata, Campsis grandiflora, Erythrina variegata, Limonium sinuatum, and Tecoma capensis). Including Plumbago, polarization is now known to occur in six genera representing four unrelated families. The results show that the production of pairs of dimorphic SC occurs in species other than those in the genus Plumbago, in which this phenomenon has already been well characterized. These results suggest that differentiation of SC and preferential double fertilization are not restricted to Plumbago and may occur in a variety of angiosperms. In species with significant polarization in GC or SC pairs, polarization of GC occurs at an especially early stage when GC are freed and become spherical, or just after microspore mitosis. This suggests that the distribution of DNA-containing organelles is strictly controlled during the early development of the GC.