PHYLOGENETIC SIGNIFICANCE OF POLLEN NUCLEAR NUMBER IN THE EUPHORBIACEAE

The systematic importance of the nuclear number of mature pollen grains in angiosperms was first clearly articulated by Schiirhoff (1924, 1926), who noted that the taxa in most families have either binucleate (II) or trinucleate (III) pollen. Schiirhoff not only characterized the families of angiosperms according to pollen nuclear number, but proposed that taxa with III pollen are phylogenetically advanced compared to related taxa with II pollen. Rather daringly-in view of the limited data available to him-Schiirhoff proposed the hypothesis that III pollen has arisen independently many times during the evolution of the angiosperms, and that reversals from III to II pollen have never occurred. Schnarf (1931), in his survey of comparative cytology in angiosperms, accepted the proposal that III pollen is phylogenetically advanced, but left open the possibility that reversals to II pollen might occur. Although Poddubnaya-Arnoldi (1936) questioned the systematic value of pollen nuclear number because it was known to vary within families and because she was able to manipulate it experimentally in a few species, Schnarf (1937) reaffirmed its systematic importance, and emphasized the need for investigation of those taxa within which the number varies. Any uncertainty with regard to the phylogenetic significance of pollen nuclear number in angiosperms has now been dispelled by extensive researches of Brewbaker (1967). His detailed survey of over 1900 species in 265 families, based on original investigations of many more taxa than were reported on by Schiirhoff (1926) or Schnarf (1939), shows that the majority (179) of angiosperm families have II pollen, 54 have III pollen, and in only 32 families does both II and III pollen occur. Brewbaker's results have confirmed in a striking fashion the speculations of Schiirhoff, and his conclusions about phylogenetic trends in the angiosperm male gametophyte may be appropriately epitomized as the "Schiirhoff-Brewbaker Law": (1) II pollen is primitive in the angiosperms as a whole and within individual taxa; (2) III pollen is derived from II pollen in all instances; and (3) the shift from II to III is irreversible. The large family Euphorbiaceae is of interest not only because it includes both genera with II and genera with III pollen, but also because Euphorbia is one of only five angiosperm genera in which both II and III pollen have been recorded (Brewbaker, 1967). In his pioneering investigation of the Geraniales, Schiirhoff (1924) had characterized the Euphorbiaceae as III, even though one of the three genera he studied (Mercurialis) has II pollen. Brewbaker, on the basis of observations on 35 species in 17 genera, has found II pollen in 13 genera, III pollen in 3 genera, and both II and III pollen in 16 species of Euphorbia (including Poinsettia). He has concluded that the distribution of II and III pollen within the Euphorbiaceae is "evidently random. " In an earlier study (1957), Brewbaker noted that in plants with self-incompatible breeding systems II pollen is associated with gametophytic control of incompatibility, and III pollen with sporophytic control. Our investigation of pollen nuclear number in the Euphorbiaceae has been undertaken in order to (1) determine whether pollen nuclear number is in fact