The systematic importance of the nuclear number of mature pollen grains in angiosperms was first clearly articulated by Schiirhoff (1924Schiirhoff ( , 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 EVOLUTION 27:524-531. September 1973 52'~p ollen, 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-inco...
White or ladino clover (Trifolium repens L.) is an important forage or legume worldwide. However, lack of drought tolerance and disease resistance limit its usefulness. Although other Trifolium species express these traits, routine crossing to develop interspecific hybrids and introduce desirable traits has achieved limited success. The objective of this work was to culture embryos and ovules to rescue hybrids and regenerate plants in new interspecific hybrid combinations. A media regime is described that is generally useful with Trifolium hybrid embryos at the torpedo stage or later. Novel interspecific combinations include T. ambiguum Bieb. with T. montanum L. and T. occidental Coombe; T. isthmocarpum Brot. with T. repens and T. nigrescens Viv.; a trihybrid, designated RUO, of T. repens, T. uniflorum L., and T. occidentale with hexaploid T. ambiguum. The RUO‐T. ambiguum hybrid was produced in several genotypic combinations, one of which bloomed and yielded viable pollen. Interspecific hybridization in Trifolium offers a route to enhancement of forage germplasm by introducing traits that increase longevity while maintaining the superior forage quality of T. repens.
A single rhizome explant of the Venus fly-trap has the potential to produce 14 or more rooted plantlets in 40 to 60 days when cultured on a medium containing half strength Murashige and Skoog salts, organic components, naphthaleneacetic acid (NAA) at 1.9 mg/liter and 6-benzylamino purine (BA) at 0.2 mg/liter. Cultures were grown in 16 hour cycles of Cool White fluorescent light at 23° to 26°C. Explants derived from either lateral buds or adventitious buds from leaf cuttings have equal potential for rapid multiplication. This same medium produced optimum plantlet size and quality. Supplementing the basal medium with 0.3 or 1.0 mg/liter of GA3 decreased the number of explants and increased the size of plantlets prior to acclimatization. Media containing higher and lower salt concentrations and higher and lower IAA, NAA, 2,4-dichlorophenoxyacetic acid (2,4-D), BA, or 6(γ,γ, dimethylallylamino)-purine (2ip), produced fewer plantlets while increasing deleterious effects. The rapid plantlet multiplication procedure described will increase commercial availability of the plants while decreasing collection pressures on wild germplasm pools.
Microsporogenesis was studied in the male-female sterile F2 segregants of a safflower cross involving 'US-10' and '57-147' cuitivars and was compared with normal meiosis in the F1 hybrids. In the microspores of the sterile plants contraction of chromosomes during prophase was irregular and patchy and they did not stain well with acetocarmine. At MI, most of the 24 chromosomes tended to stick to each other at random resulting in the formation of several chromosome bridges during AI. Multipolar meiotic divisions were observed. The second division was rarely seen. Apparently, meiosis ends with a multipolar AI separation and with the formation of 3- to 9-celled "quartets" which later develop nonfunctional pollen grains. The onset of meiosis in sterile plants was delayed and microsporogenesis subsequently progressed at a much slower rate than in fertile F1 hybrids. Three interacting nuclear genes appear to cause male-female sterility and affect microsporogenesis by interfering with some component essential for normal meiosis.
The systematic importance of the nuclear number of mature pollen grains in angiosperms was first clearly articulated by Schiirhoff (1924Schiirhoff ( , 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) 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 b...
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