POLLEN EMBRYOGENESIS - ATAVISM OR TOTIPOTENCY

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 wh ich should have a very reduced capability of regenerating a whole plan t, whereas in some species the induction of androgenesis appears to oc cur with greater facility than somatic embryogenesis. Furthermore, som e microspores seem to have a tendency to morphogenesis and organogenes is; spontaneous androgenesis occurs naturally in various species and m any examples also occur of pollen dimorphism. Totipotency would seem t o be insufficient to explain androgenesis and we propose that its orig in might be found in the phenomenon of atavism. According to studies p ublished on ancestral precursors of pollen, these structures appear to have had high proliferation capacity. The ability to form a multicell ular structure from a single haploid cell is shared by the meiocytes o f 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. Out conclusion is that there is evidence enough to suggest that androgenesis may well be the expres sion of archaic genes of meiocytes with morphogenic capacity which wer e naturally expressed in the ancestors of flowering plants.