Pollen wall ultrastructure of the genus Adansonia L. species

Abstract

The genus Adansonia (baobabs, Malvaceae) includes nine tropical species grouped in the Brevitubae, Longitubae and Adansonia sections. The ultrastructure of pollen from eight species of baobabs was studied using transmission electron microscopy. The pollen grains correspond to 3-colporate and 4-colporate types. The ectexine is formed by a perforate tectum with isolated spinules. An important distinguishing feature between the Brevitubae section and the Longitubae and Adansonia sections was observed in the infratectum. The apertural region was similar in all the species studied here. However, the most unusual structure was observed in the apertural margins where the ectexine consisted of a thick lamellate annulus and the endexine had a cracked appearance; the intine was composed of a well-developed oncus under the endopore with unusual fibrillar structures and the outer layer had a remarkable structure consisting of columns. Despite the few differences observed in the structure of the pollen wall within the genus, an original arrangement of the structure found in the aperture of the Adansonia pollen grains studied provides additional information about the new types of apertural structures. This type of sporoderm adds to our knowledge of the diversity of angiosperm pollen. Moreover, this apertural structure is probably an adaptation that occurs during the formation of the pollen tube and of harmomegathy.