Shedding light on the embryo:endosperm balance diversity that pseudogamy can achieve in the polyploid apomictic Psidium cattleyanum (Myrtaceae, Myrteae)

Abstract

Apomixis is defined as the asexual mode of reproduction through seeds. Nearly all gametophytic apomictic species are polyploid. Psidium cattleyanum, a polyploid species with various described ploidy levels, exhibits apomixis and requires effective pollination for fruiting (pseudogamy). The aim of this study was to determine the frequency of apomixis and sexuality in P. cattleyanum among heptaploid and octoploid accessions, and to analyze the maternal and paternal contribution to endosperm formation. Flow cytometry was employed on mature seeds obtained from hand-pollinated crosses between two octoploid and two heptaploid accessions to determine the ploidy levels of embryo and endosperm. Within the 492 analyzed seeds the ploidy balance of embryo:endosperm ranged from the expected 2:5 (64.2%, pseudogamous apomixis) and 2:3 (4.7%, sexuality involving reduced gametophytes), to unexpected values, including 1:3 (0.4%),), 2:6 (2.4%), 2:7 (0.2%), 3:5 (8.8%), 2:3:5 (18.5%) and 2:4:6 (0.8%), associated with polyhaploid parthenogenesis, pseudogamous apomixis involving reduced and unreduced gametophytes, and female gametophytes with two or three polar nuclei in the central cell. The interpretations and implications of these results for apomictic polyploid species and their consequences for wild populations are discussed.