Embryology of Harrisonia (Cneoroideae, Rutaceae): A comparison with related genera and families

Abstract

Harrisonia, a genus of three or more species distributed from tropical Africa and Southeast Asia to northern Australia, has long been considered a member of Simaroubaceae. Recent molecular analyses, however, have shown that the genus is assigned to subfamily Cneoroideae of Rutaceae, in which Cneoroideae (eight genera) are sister to the remainder of the family ('core Rutaceae'). Here, we report embryological features of Harrisonia based on published and newly obtained data and provide morphological corroboration for the molecular affinities of the genus and Cneoroideae. We compared its embryological features with those of seven other genera of Cneoroideae and with those of the core Rutaceae and related families (Meliaceae, Simaroubaceae and Sapindaceae). Comparisons showed that Harrisonia fits within Cneoroideae through possessing campylotropous seeds with a multi-cell-layered endotesta. Embryological evidence, like molecular evidence, further shows that, in Cneoroideae, Harrisonia probably has affinities to a group of Bottegoa, Cedrelopsis, Cneorum and Ptaeroxylon by sharing a micropyle formed by the inner integument alone and solitary oil cells in the testa. Except for Harrisonia, Cneoroideae are still poorly understood embryologically and further investigations are needed.