Pollen morphology of the genus Omphalodes Mill. (Cynoglosseae, Boraginaceae)

Abstract

To better understand the taxonomy and phylogeny of the genus Omphalodes Mill. (Boraginaceae), the pollen morphology of 23 species encompassing all major geographical and morphological groups was studied from acetolysed pollen grains using light and scanning electron microscopy. Three pollen types were distinguishable through apertural morphology and sculpture. A dichotomous key to the pollen types and pollen descriptions are provided. Despite being fairly homogenous, palynological data provided useful information to understand the relationships of Omphalodes, considering the uselessness of most floral and fruit characters, karyological homogeneity, and striking disjunct geographical distribution. At the suprageneric level, pollen morphology supports traditional tribal classification of Omphalodes within Cynoglosseae, in contrast to published molecular phylogenetic analyses. At the infrageneric level, pollen morphology agrees with phylogenetic analyses suggesting that Omphalodes could be a polyphyletic group. Palynological data would support the segregation of a monophyletic group comprising the Japanese species, corroborating recent phylogenetic results, and define two different pollen types for the Omphalodes s.str. group. Biogeographical considerations are also discussed.