David L. Mulcahy

The Rise of the Angiosperms: A Genecological Factor

In the primitive angiosperms, closed carpels are believed to have evolved as protection for ovules, which would otherwise be injured by animal pollinators. The hypothesis is presented that, whatever the origin and other functions of angiosperms, insect pollination and closed carpels may, in combination, reduce the influence of random variation on pollen tube competition, thus enhancing the ability of natural selection to act on the gametophytic phase of the life cycle. The microgametophytic phase represented by vast numbers of haploid individuals can then serve, by insect pollination and closed carpels, as a screen against any genome not functioning with a high degree of metabolic vigor. Poorly balanced genomes could thus be eliminated at relatively little cost. Insect-pollinated angiosperms would therefore benefit from positive aspects of sexual recombination. Such a system may have allowed the angiosperms to undergo their rise to dominance.

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.

Genetics of Angiosperm Pollen

Publisher Summary The angiosperm life cycle consists of two alternating phases—a diploid, morphologically elaborate and conspicuous sporophyte, and a much reduced, haploid gametophyte. At least four phenomena in the ontogeny of pollen impinge on the genetic determination of pollen quality: Cytoplasmic interconnections, which establish developmental synchrony among meiocytes, callose envelopes, which isolate microspores from each other, tapetal deposits, which coat the pollen exine, and sporophytic inputs, which remain in the meiocyte cytoplasm. Some pollen characteristics are determined exclusively by the sporophyte, as is the case with the exine. Others, such as Ga factors, gametophytic self-incompatibility, and the presence of specific enzymes (alcohol dehydrogenase, P-galactosidase, are under gametophytic control. Pollen competition may be significant phenomenon in the evolution of crop species and also in the reduction of genetic load. Pollen competition may play a significant role in the adaptive capacities of the angiosperms, as this group possesses three characteristics, which enhance pollen tube competition. These are: Heavy stigmatic loads of pollen, the simultaneous arrival of many pollen grains on the stigma, and the requirement for extensive pollen tube growth. Finally, direct and indirect in vivo interactions between pollen grains and particularly pollen-pistil interactions need evaluation.

Pollen Tube Growth Rates in Zea mays: Implications for Genetic Improvement of Crops

Speed of pollen tube growth is positively correlated with the quality of the resultant sporophytic generation. Therefore, gametophytic competition may be an important adaptive mechanism. Furthermore, pollen tube growth rates may be used to predict the quality of F1 crosses in crop species.

A Correlation between Gametophytic and Sporophytic Characteristics in Zea mays L.

If a mixture of types of corn pollen, identified by genetic markers, is applied to the silks of other inbred lines, the rate of pollen tube growth often varies with type of pollen. This gametophytic differential is correlated with a sporophytic differential—relatively heavier seeds in seed mixtures result from fertilization by gametes from faster growing tubes. The increased seed weight is due to greater competitive ability of the zygotes thus formed.

The use of random amplified polymorphic DNAs to fingerprint apple genotypes

Abstract Twenty-five accessions of apple, representing eight cultivars (‘Golden Delicious’, ‘Delicious’, ‘Gala’, ‘Jonagold’, ‘Jonathan’, ‘Florina’, ‘Fior di Cassia’, and ‘Imperatore Dallago’) have been characterized with Random Amplified Polymorphic DNAs (RAPD). The reliability of the method was tested by analyzing separate scions of the same clone and also by comparing different accessions of the same cultivar. Using two separate ten bp primers, it was possible to obtain a distinctive fingerprint for each of the cultivars. The method is simple, rapid and should provide a useful system for documenting the identity of clonal material.

DNA PROBES FOR THE Y-CHROMOSOME OF SILENE-LATIFOLIA, A DIOECIOUS ANGIOSPERM

SummaryIn order to obtain markers for the Y chromosome ofSilene latifolia, we pooled equal weights of leaf tissue from 18 female siblings into one sample and repeated the process with 18 male siblings. Pooling was intended to provide a common genetic background for each sample, leaving the absence or presence of the Y chromosome as the primary difference between the two samples. DNA was extracted from each sample and subjected to polymerase chain reaction (PCR) amplification with arbitrary 10 bp primers. Four of 60 primers used gave an amplification with the male DNA not found among those from the female DNA. Each of these was subsequently shown to provide a reliable marker for the Y chromosome.

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.

Genetic dissection of pollen competitive ability in maize

Pollen competition and variability in pollen fitness can produce non-random fertilization with respect to pollen genotypes, and, owing to the large extent of genetic overlap between the gametophytic and sporophytic phases of the life-cycle, can affect the latter. Differences in pollen fitness are due to many factors, of which pollen grain germinability and pollen tube growth rate are the main components. The identification and chromosomal localization of the genes that mainly affect pollen fitness variability were carried out by RFLP analysis, applied to a recombinant inbred population that had been characterized for about 200 restriction loci. Germination ability and pollen tube growth rate were evaluated by means of the pollen mixture technique. Both traits revealed a large variability and high heritability (0.71±0.05 for tube growth rate and 0.77±0.04 for grain germinability). Analysis of the association between the expression of the characters and the allelic composition at each of the restriction loci revealed a significant regression on 29 loci in the case of pollen tube growth rate and on 26 in the case of pollen grain germinability. However, considering only uncorrelated loci, in order to avoid false assignments, the minimum number of quantitative trait loci (QTL)s with major effects was five for the tube growth rate and six for grain germinability. The amount of genetic variability of the characters explained by the molecular markers was 0.89 (tube growth rate) and 0.79 (grain germinability), signifying that almost all the genetic variability for these traits is due to QTL located in the chromosomal regions indicated by the analysis. Most of the QTL identified relate to either one trait or the other, which suggests that they are genetically controlled by specific sets of genes.