Novelties in the embryology of Parodia (Cactaceae) and its potential application to the genus taxonomy.

Abstract

Background and aims: The problem related to the circumscription of Parodia originates in the classification of Acanthocephala, Eriocephala, and Wigginsia as subgenera of Notocactus sensu lato, and continues after the merger of Notocactus sensu lato with Parodia sensu stricto, which originated Parodia sensu lato. Embryological data was not considered in the decisions leading to the circumscription of Parodia sensu lato, but recent studies have demonstrated its potential applicability in discussions involving the genus. Therefore, this study aimed to describe embryological data related to ovule development and the processes of megasporogenesis and megagametogenesis, and to apply them to existing circumscriptions.

Methods: Floral buds at different stages of development and pre-anthesis flowers from six Parodia species were analysed. For sample processing, standard techniques used in anatomical studies were employed. The material was analysed and photographed under polarised light, epifluorescence microscopy, and light microscopy.

Key results: The analyses revealed the absence of a developmental pattern during ovule formation in Parodia, with species exhibiting triad or tetrad of megaspores, apomixis, mitotic division of archespores, and different mechanisms of cell selection during ovule development. The species also exhibited an atypical pattern of callose deposition during megasporogenesis, which could be considered exclusive to Cactaceae. Furthermore, one critically endangered species showed ovule malformation.

Conclusions: The observation of exclusive features in these species, allowed the application of the data to one of the less inclusive circumscriptions, in which Acanthocephala, Brasiliparodia, Eriocephala, Notocactus, Parodia, and Wigginsia are considered distinct but related genera. In addition, the embryological data also demonstrated potential use as a tool in understanding the reasons that may cause the population decline of critically endangered species.