Anther Wall Formation in Aeonium balsamiferum and A. ciliatum (Crassulaceae)

Similarities (features of the family Crassulaceae) and differences in anther development and structure were observed between Aeonium balsamiferum and A. ciliatum. Microsporangia are rounded in A. ciliatum and elongate oval in A. balsamiferum. The septum between microsporangia is consequently longer in A. ciliatum and extremely short in A. balsamiferum. In the latter, pollen can spill out through both the stomium and the apical pore (the phenomenon first discovered in the family Crassulaceae), and both modes of theca opening occur simultaneously, suggesting a greater specialization for pollination for the A. balsamiferum anther. The outer tapetum is single-layered in both species, but its cells are mononuclear in A. ciliatum and binuclear in A. balsamiferum. The inner tapetum is usually single-layered and irregularly two-layered in rare cases; its cells increase in size by a factor of 2 in A. ciliatum and 3-4 in A. balsamiferum (resembling papilloid cells in structure in the latter). Similarities in anther structure were observed between the genera Aeonium and Sedum. In both genera, the anther was isobilateral (on a transverse section) and had an epiconnective, a four-beamed connective, and an annular fibrous layer on the inner side of the connective in the lower region, where there is no fusion of the anther and filament; pollen grains were of the same structural type: tricolporate with a striated surface. The findings were consistent with the cladistic reconstructions that put species of the polyphyletic genus Sedum in the tribes Aeonieae, Semperviveae, and Sedeae. Aeonium balsamiferum and A. ciliatum fall into the Aeonium clade and occupy an intermediate position between the Telephium clade (Sedum kamtschaticum) and the Acre clade (S. palmeri).