Gametophytic Self-Incompatibility Reexamined

Gametophytic vs. sporophytic control of pollen aperture number: a generational conflict

In flowering plants, the haploid phase is reduced to the pollen grain and embryo sac. These reproductive tissues (gametophytes) are actually distinct individuals that have a different genome from the plant (sporophyte), and are more or less independent. The morphology of pollen grains, particularly the openings permitting pollen tube germination (apertures), is crucial for determining the outcome of pollen competition. Many species of flowering plants simultaneously produce pollen grains with different aperture numbers in a single individual (heteromorphism). In this paper, we show that the heteromorphic pollen aperture pattern depends on the genetic control of pollen morphogenesis. This points out a conflict of interest between genes expressed in the sporophyte and genes expressed in the gametophyte. More generally, such a conflict should exist whenever heteromorphism is an ESS resulting from a bet-hedging strategy. For pollen aperture, heteromorphism has been observed in about 40% of angiosperm species, suggesting that conflicting situations are the rule. In this context, the sporo-gametophytic conflict could be one of the factors that led to the reduction of the haploid phase in plants.

Gametophytic self-incompatibility is controlled by a single major locus on chromosome 1 in Lycopersicon peruvianum

By using a number of previously mapped enzyme-coding genes as genetic markers, it has been possible to scan the genome of Lycopersicon peruvianum for gene(s) controlling the gametophytic self-incompatibility reaction. Regardless of genetic background or level of inbreeding, only a single locus (S), mapping to chromosome 1, was found to control the self-incompatibility reaction. Despite the widespread occurrence of this form of self-incompatibility in higher plants, to the best of our knowledge, the locus underlying the response has not been confirmed previously through genetic mapping, and the results cast doubts on hypotheses requiring multifactoral or dynamic control of gametophytic self-incompatibility.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Quantitatively determined self-incompatibility : 1. Theoretical considerations

It has been reported that incomplete self-incompatibility could be determined in Borago officinalis by many genes. Simple ten-gene models for such enforced cross-fertilization have been developed and their properties examined by computer simulation. Mutation rates necessary to maintain a given level of variability in small populations are high, as already determined theoretically for oligogenic self-incompatibility systems. However, the extent of ineffective pollination is very much greater in the ten-gene system. This finding may be verifiable in borage if it is indeed self-incompatible.

On the evolution of genetic incompatibility systems. III. Introduction of weak gametophytic self-incompatibility under partial inbreeding

I explore the proposition that genetic incompatibility systems serve as a means for parents to evaluate and discriminate among their own offspring. Conditions for the initial increase of gametophytic self-incompatibility in a self-compatible population undergoing selfing, sibmating, and random outcrossing are reported. The adaptive value of reducing the concordance between offspring and maternal genotypes depends upon the relative changes in the numbers of offspring derived by the three modes, parent-offspring relatedness, and the magnitude of distortion of transmission ratios through pollen. Recessivity of stylar expression and low rates of receipt of pollen from related individuals facilitate the evolution of self-incompatibility. Viewed as a means of preferential maternal investment in offspring of high quality, self-incompatibility may be regarded as serving a function in common with diverse phenomena, including sexual selection, brood reduction, and other forms of prezygotic and postzygotic incompatibility. Associations between incompatibility loci and loci expressing inbreeding depression are expected to improve the reliability of the level of concordance at incompatibility loci as a measure of genomic homozygosity and offspring quality.

POLLINATION DYNAMICS IN EPILOBIUM CANUM (ONAGRACEAE): CONSEQUENCES FOR GAMETOPHYTIC SELECTION

Several hand-pollination experiments have shown that pollen tube competition leads to gametophytic selection, and thus affects genotypic and phenotypic characteristics of the next generation. This study is one of the first to quantify natural levels of pollen tube competition. In a population of Epilobium canum, I measured both the amount of pollen deposited on stigmas and the timing of deposition. Approximately 20 tetrads were required for full seed set within fruits. Hummingbirds deposited >20 tetrads at 50-70% of the flowers examined, often in a single load. When pollen arrived in 2 loads, a portion of the ovules within each ovary was probably sired by competing pollen from the second load. Competition may be relatively weak unless at least 30 tetrads compete for ovules. About 20% of the flowers received >30 tetrads in the first load, and 13% acquired > 30 tetrads in 2 loads. The frequency and intensity of pollen tube competition varied among plants. In some styles, 80% of the pollen tubes were excluded from access to ovules, but in others no competition occurred. Further studies of pollination rates and progeny fitness are needed before we can assess the role of pollen tube competition in natural populations. Potential effects of gametophytic selection are discussed.

Breeding systems and population structure in Limnanthes

The breeding systems of seven Limanthes (Limanthaceae) populations, including one "inbreeding" and three "outbreeding" taxa, were quantified using a multilocus outcrossing rate estimator (tm) and autofertility estimates. Along with the assays of heterozygosity levels, these data were used to separate components of "effective" outcrossing in terms of Wright's equilibrium inbreeding coefficient (Fe) and adult (FA) and zygotic (FZ) fixation indices. The patchy distribution of alleles as a potential source of "substructure inbreeding" was tested from the allelic frequencies mapped along a linear transect. Evidence for consanguineous matings in restricted neighborhoods and for selection at two different life cycle stages, and the efficiency of the protandrous breeding system were noted and discussed. Multilocus estimates of outcrossing are useful for their greater precision and unbiased nature while single locus estimates can help in detecting the effects of selection and population substructure. The data generally support the "heterozygosity paradox" noted by Brown (1979) but further suggest that the paradox may often result from a lack of precision of outcrossing estimates and from overlooking the stages of the life cycle being sampled.

"Egg transformation" induced by irradiated pollen in Nicotiana: a re-examination

We have attempted to confirm previous reports of "egg transformation" in seven Nicotiana species, including genetic stocks obtained from the original experimenter. The methods employed in the original experiments were duplicated as closely as possible. In total, 1,622 pollinations were made involving irradiated pollen and mixtures of irradiated and self pollen. Consequently, 995 seedlings from 9,052 seeds were screened for 1,594 potentially detectable transformation events. A very low frequency of unexpected progeny resulted, but these results were not repeatable and appear to have arisen by mechanisms other than transformation. These results are strongly at odds with previous claims, when 50% of offspring were found to be transformed. We conclude that the previous observations of high frequency egg transformation are not reproducible. However, due to the ambiguous nature of the markers employed, it is very difficult to prove that such transformation events do not occur as very rare events.

Distorted segregation and linkage of alcohol dehydrogenase genes in Camellia japonica L. (Theaceae)

Alcohol dehydrogenase isozymes in Camellia japonica are encoded by two genes, Adh-1 and Adh-2. Both loci are expressed in seeds, and their products randomly associate into intragenic and intergenic dimers. Electrophoresis of leaf extracts reveals only the products of Adh-2. Formal genetic analysis indicated that the two Adh loci are tightly linked (combined estimate of r = 0.004). Most segregations fit expected Mendelian ratios, but in some families distorted segregation was observed at Adh-1, Adh-2, or both loci. The deficient progeny class varied across families, and in two apparent back-crosses three rather than two phenotypic classes were recovered. The mechanism underlying these distortions is not known, but evidence is presented that suggests that the phenomenon is genic or segmental in nature. Plausible hypotheses include linkage of the Adh structural genes with a gametophytic self-incompatibility locus, translocation heterozygosity involving the segment bearing Adh-1 and Adh-2, or a combination of these two mechanisms.