A pentaploid endosperm and a Penaea-type embryo sac are likely synapomorphies of Azorella (Apiaceae, Azorelloideae)

Abstract

About 80% of angiosperms form a monosporic Polygonum-type embryo sac, whereas in the remaining species, eleven other types of embryo sac are found. Evidence as to the type of embryo sac is lacking for many plant species, and the role of higher-ploidy endosperm is unknown. In contrast to the rest of the Apiaceae, where a Polygonum-type embryo sac (3n endosperm) has been reported, the few species of the Azorelloideae studied to form a Drusa-type embryo sac (3n endosperm) or a Penaea-type embryo sac (5n endosperm). This variation within Azorelloideae makes this subfamily, and its genus Azorella in particular, a good candidate for studying the evolutionary importance of the embryo sac and endosperm in diversification. We studied the variation in the type of embryo sac and the ploidy level of the endosperm in Andean-Patagonian Azorella and closely related Pozoa on a sample of 101 individuals from 31 populations of 21 species. We employed flow cytometric seed screening and calibrated the results of ploidy level estimation against embryological observations. In addition, we examined the genome size variation of the species sampled. All species of Azorella formed Penaea-type embryo sacs and a pentaploid endosperm, whereas one species of Pozoa formed triploid and the other tetraploid endosperms. Variations in the type of embryo sac and endosperm ploidy have probably shaped the evolution of the different lineages of Azorelloideae in the southern Andes. A Penaea-type embryo sac, which represents a likely synapomorphy of Azorella, is a feature of underestimated significance in the evolution of angiosperms.