Updating embryonic ontogenesis in Araucaria angustifolia: from Burlingame (1915) to the present.

ABSTRACT

This study addresses gaps in our understanding of pre-fertilization and archegonia development and reinterprets embryonic ontogenesis from Burlingame (Bot Gaz 591-39, 1915) to the present based on timescale and structural features allowing us to determine functionally and developmentally accurate terminology for all these stages in A. angustifolia. Different from previous reports, only after pollination, pre-fertilization tissue development occurs (0-13 months after pollination (MAP)) and gives rise to a mature megagametophyte. During all this period, pollen is in a dormant state at the microphyla, and pollen tube germination in nucellus tissue is only observed at the stage of archegonia formation (13 MAP) and not at the free nuclei stage as reported before. For the first time, 14 months after pollination, a fertilization window was indicated, and at 15 MAP, the polyzygotic polyembryony from different archegonia was also seen. After that, subordinated proembryo regression occurs and at least three embryonic developmental stages of dominant embryo were characterized proembryogenic, early embryogenic, and late embryogenic (15-23 MAP). Along these stages, histochemical and ultrastructural analyses suggest the occurrence of cell death in suspensor and in cap cells of dominant embryo that was not previously reported. The differentiation of meristems, procambium, pith, and cortex tissues in late embryogenic stage was detailed. The morphohistological characterization of pre-fertilization and embryonic stages, together with the timescale of megastrobili development, warranted a referential map of female reproductive structure in this species.