The transition from somatic to germline identity shows conserved and specialized features during angiosperm evolution

How and why specific plant cells adopt germline identity during ovule development has proved challenging to address, and the pathways that are active in the ovules of basal/early-divergent angiosperms possessing a multilayered nucellus are still unclear. Here, we compare megasporogenesis between two early-divergent angiosperms (Annona cherimola and Persea americana) and the evolutionarily derived Arabidopsis thaliana, studying the three-dimensional spatial position of the megaspore mother cell (MMC), the compositional details of the MMC wall and the location of PIN1 expression. Specific wall polymers distinguished the central position of the MMC and its meiotic products from surrounding tissues in early-divergent angiosperms, whereas, in A. thaliana, only callose (in mature MMCs) and arabinogalactan proteins (AGPs) (in megaspores) distinguished the germline. However, PIN1 expression, which regulates polar auxin transport, was observed around the MMC in the single-layer nucellus of A. thaliana and in the multilayered nucellus of A. cherimola, or close to the MMC in P. americana. The data reveal a similar microenvironment in relation to auxin during megasporogenesis in all three species. However, the different wall polymers that mark MMC fate in early-divergent angiosperms may reflect a specific response to mechanical stress during differentiation, or the specific recruitment of polymers to sustain MMC growth.