POLLEN SELECTION - PAST, PRESENT AND FUTURE

A series of studies, recently reviewed, has established that approxima tely 60% of the structural genes which are expressed in the sporophyti c portion of the angiosperm life cycle are also expressed and exposed to selection in the pollen. Given the haploidy and large population si zes of pollen grains, a substantial portion of the sporophytic genome could thus be periodically exposed to a bacterial type of mass screeni ng. This extraordinary possibility is often subject to some skepticism which may, of course, be justified. However, recent attempts to apply models appear to be inappropriate in this context, in part because th ese attempts overlook an important source of genetic variation, and al so because they assume fixed values for selection and fitness. More re cently, studies of pollen/pollen interactions have suggested that what Linskens termed the ''programic phase'' may represent an arena for im portant, and largely unexplored phenomena, some of which are discussed here.