Characterization of mode of reproduction in Commiphora wightii [(Arnot) Bhandari] reveals novel pollen–pistil interaction and occurrence of obligate sexual female plants

Abstract

Guggul [Commiphora wightii (Arnot) Bhandari], a polygamous woody tree valued for its medicinal oleoresin gum rich in guggulsterone, is reported to reproduce via sporophytic apomixis. Details about its natural diversity, and mode and extent of sexual reproduction are, however, scanty. Therefore, a comprehensive investigation of guggul reproduction was made employing histology, controlled pollination, flow cytometry and RAPD analyses of progeny to assess the occurrence and extent of sexual reproduction. We report the discovery of obligate sexual female plants of guggul through these studies. Also, we document a unique pollen–pistil incompatibility that prevents all but one pollen tube growth into the style to effect fertilization. Consequently, obligate sexual female plants produced single-seeded fruit although each flower contains four ovules. In apomictic plants bearing more than one seed per fruit, at most only one seed was of sexual origin. Further, flow cytometric analysis conclusively demonstrated that endosperm development occurs either autonomously or following triple fusion. Autonomous endosperm development was invariably associated with endoreduplication, a unique feature of apomixis in guggul. Despite predominance of apomixis, a low frequency of sexual reproduction was found to persist in apomictic plants yielding new genetic variation. RAPD analysis clearly distinguished accessions and was useful in identifying sexual progenies. The implications of the novel pollen–pistil interaction on establishment and spread of apomixis in guggul are discussed. The study has not only revealed novel features of guggul reproduction but also opened new opportunities for molecular genetic analysis of sporophytic apomixis and breeding improvement of guggul.