Pollen Morphology of Zelkova sicula (Ulmaceae), a recently discovered relic species of European tertiary flora: its description, chromosomal relevance, and paleobotanical significance

Pollen morphological characteristics of Zelkova sicula Di Pasquale, Garfi et Quézel and Z. carpinifolia (Pallas) Koch (Ulmaceae) are described. Z. sicula is a shrub species which was recently discovered on southeastern Sicily. Z. carpinifolia is the Zelkova species to which some European Tertiary to Quaternary leaf fossils have been attributed in recent studies. The new results were compared with the pollen morphological characteristics of some other Ulmaceae species such as Z. abelicea (Lamark) Boissier, Z. serrata (Thunberg) Makino, Ulmus glabra Hudson, U. laevis Pallas, U. minor Miller, and U. procera Salisbury. It was observed that Z. sicula usually possesses 5 to 6 pores, whereas the other Zelkova species normally have fewer than 5 pores. Higher frequencies of tetrads, a Y-mark on the proximal face, and a smoother exine sculpture (compared with other Zelkova species) were noted for pollen of Z. sicula. The equatorial diameter of Z. sicula was larger than every other Ulmaceae species, with a smaller PE ratio. Some of these differences can be explained by the triploid nature of Z. sicula, which is quite exceptional for Ulmaceae. Given these remarkable characteristics, it should be possible to recognise Z. sicula pollen grains in fossil pollen assemblages. It is thus anticipated that a more comprehensive pollen analytical study will lead to a revision of the Mediterranean phytogeographical history of the genus Zelkova.