Gabriella Bergamini Mulcahy

The influence of gametophytic competition on sporophytic quality in Dianthus chinensis.

SummaryPollinations were made on either the tip or the basal portions of the stigmatic surface in Dianthus chinensis. These two treatments provided, respectively, either good or modest opportunity for pollen tube competition. The pollen used came from a single clone. Technical and statistical methods were used to reduce greatly the influence of variation in seed weight. Seeds resulting from the two contrasting treatments were planted, and it was found that there were statistically significant differences in germination time and seedling weight between treatments. These results suggest that the quality of the F1 generation can be significantly modified by competition between pollen tubes from a single plant.

Pollen selection — past, present and future

A series of studies, recently reviewed, has established that approximately 60% of the structural genes which are expressed in the sporophytic portion of the angiosperm life cycle are also expressed and exposed to selection in the pollen. Given the haploidy and large population sizes of pollen grains, a substantial portion of the sporophytic genome could thus be periodically exposed to a bacterial type of mass screening. 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 these attempts overlook an important source of genetic variation, and also because they assume fixed values for selection and fitness. More recently, studies of pollen/pollen interactions have suggested that what Linskens termed the “programic phase” may represent an arena for important, and largely unexplored phenomena, some of which are discussed here.

Gametophytic Self-Incompatibility Reexamined

The conventional hypothesis of gametophytic self-incompatibility in the angiosperms involves one to four multiallelic incompatibility loci and the positive inhibition of incompatible pollen tubes. However, this concept does not accommodate recent experimental data indicating that there may be many loci. An alternative hypothesis which incorporates many loci and complementary pollen-style interactions suggests that there may be no S gene, as previously thought, and that gametophytic self-incompatibility is perhaps merely one aspect of extensive pollen-style interactions.

The effect of supplemented media on the growth in vitro of bi- and trinucleate pollen

Abstract Casein hydrolysate (CH) was added to pollen cultures to study the metabolic differences between binucleate pollen from Petunia hybrida Hort. Vilm.-Andr., and trinucleate pollen from Zea mays L., Silene dioica (L) Clairv., and Brassica cam pestris L. The time at which the CH supplement becomes effective supports the hypothesis that, while binucleate pollen starts its growth autotrophically, and switches later to heterotrophic nutrition, trinucleate pollen grows heterotrophically from the start.

Pollen-Pistil Interaction

The need, site, and eventually, mode of pollen-pistil interaction; in other words—the extent of compatibility—is the focus of my work. In the case of the angiosperm flower, stigma, style and ovary are the sites where interaction occurs. In binucleate species, no active interaction seems to occur at the stigma site. Konar and Linskens (1966) suggested the stigma acts just as a site for germination of pollen without being involved in its nutrition. Let’s skip the style for now and consider the ovary.

The Heterosis Model: A Progress Report

The classical model of gametophytic self-incompatibility is based on a single multi-allelic locus, the S-locus. However, exceptions may involve as many as 4 different loci. According to this interpretation, the S-locus (or loci) control the active inhibition of incompatible pollen types. Although widely accepted, this model leaves several observations unexplained, three of which will be considered in the present paper.

Tests of the heterosis model

SummaryAn alternative to the classical model of self-incompatibility indicates that genetic control of self-incompatibility could involve more than one locus and function through passive failure of incompatible pollen tubes rather than through their active inhibition. Both aspects of the alternative model have been tested and, in each case, the data support the classical single-locus oppositional interpretation rather than the alternative. On the basis of these data, and others now available, we conclude that, in the Solanaceae, and presumably also in some others, self-incompatibility is better explained by the classical interpretation. Several points, however, remain to be resolved.

Use of Vital Dyes in Conjunction with the Semivitro Technic for the Detection of Pollen Tube Sperm Nuclei

The technique we describe here is a modification of that used by Hough et al. (1985), combined with “semivitro” pollen tube observations. With the semivitro technique, pollen tubes grow from the cut ends of pollinated styles (Brewbaker and Majumder 1961). Pollen of Nicotiana alata was presoaked for 15 min in simplified medium (Brewbaker and Kwack 1963) (10% sucrose, 300 ppm Ca(NO3)2, 100 ppm H3BO3 with the addition of 0.5 mg/ml of Hoechst 33258 stain from Serva Biochemicals, Heidelberg, Control H, purchased June 1983). (For germination of Nicotiana alata pollen in vitro, we use this same solution, except with 12% sucrose). After this prestaining, the pollen suspension was centrifuged for 5 min at 1200 × g, the pellet resuspended in control Brewbaker medium (i.e., no stain), recentrifuged and used to pollinate detached pistils. The pistils were then incubated at 25 C in a water-saturated atmosphere for 20 hr. At this time, the styles were cut just ahead of the front of the growing pollen tubes (Mulcahy and...