Pollen embryogenesis: atavism or totipotency?

Summary

The origins of pollen embryogenesis are still in doubt. Totipotency of plant cells has traditionally been put forward as an explanation for this phenomenon but we have found this interpretation to involve some shortcomings. The pollen grain is a highly differentiated structure which should have a very reduced capability of regenerating a whole plant, whereas in some species the induction of androgenesis appears to occur with greater facility than somatic embryogenesis. Furthermore, some microspores seem to have a tendency to morphogenesis and organogenesis; spontaneous androgenesis occurs naturally in various species and many examples also occur of pollen dimorphism. Totipotency would seem to be insufficient to explain androgenesis and we propose that its origin might be found in the phenomenon of atavism. According to studies published on ancestral precursors of pollen, these structures appear to have had high proliferation capacity. The ability to form a multicellular structure from a single haploid cell is shared by the meiocytes of ancestral algae, of the first land plants, and of present-day ferns, which are evolutionarily related to pollen. Atavism is only expressed under certain circumstances, as indeed is androgenesis, normally as a consequence of an environmental stress. Our conclusion is that there is evidence enough to suggest that androgenesis may well be the expression of archaic genes of meiocytes with morphogenic capacity which were naturally expressed in the ancestors of flowering plants.