The occurrence of a micropylar exudate in Musa and Enset (Musaceae)

Abstract

Wild bananas are diploid while most cultivated varieties are triploid. Triploidy leads to sterility that has a number of causes, but in Musa a micropylar exudate is not listed amongst them. We examined ovules of seeded diploid Musa acuminata (AA) and edible triploids (AAA) from the same cytogenetic groups (AAB, ABB) and the related genus Ensete sp. to determine whether there was any association between the presence of a micropylar exudate and sterility. Pre-anthetic, anthetic and post-anthetic specimens were embedded in methacrylate, sectioned and examined under the bright-field microscope. The micropyles of seeded diploid and seedless edible triploid cultivars of M. acuminata. contained an exudate. In contrast to the locular fluid, the exudate persisted after fixation, thus it was a viscous mucilaginous material. It was strongly PAS-positive and counter stained purple with Toluidine blue; it contained insoluble polysaccharides and proteins and appeared to arise from the nucellar cap. The exudate was not observed in ovules of pre-anthetic M. acuminata or post-anthetic Ensete, both of which are diploid. However, the exudate in post-anthetic fertilised and unfertilised diploid M. acuminata ovules was dissimilar. In the fertilised ovules the exudate was redder and less purple, indicating it contained more polysaccharides and less protein. Both pre-anthetic and post-anthetic ovules of the edible cultivars (A and B genome) exhibited a micropylar exudate. Early in ovule ontogeny the exudate was variable in presence. There appeared to be a determinate age when the triploid ovule begins to produce an exudate, between archesporia and megaspore. The production of an exudate may depend on the presence of a functioning megagametophyte or embryo sac. Embryo sac competency could be the cue to turn off pollen tube inhibition. The exudate may be linked to an ovarian self-incompatibility system.