Loss Of Self-incompatibility Research Articles

One-locus sporophytic S-gene system with traces of gameto-phytic pollen control in Cerastium arvense ssp. stricturn (Caryophyllaceae)*

Lundqvist, A. 1990. One-locus sporophytic S-gene system with traces of gametophytic pollen control in Cerastium arvense ssp. strictum (Caryophyllaceae). —Hereditas 113: 203–215. Lund, Sweden. ISSN 0018–0661. Received August 6, 1990. Accepted September 21, 1990 Cerastium arvense ssp. strictum (2n=36), obtained from the Swiss alps, was investigated by fluorescence microscopy of pollen tube growth in 52 population plants, 77 F1-plants in four F1-families, and 51 backcross plants from two backcrosses F1 x parent. All plants were self-incompatible with pollen tubes arrested on the stigmatic surface. One disomic S-locus with sporophytic pollen control and independent action of the S-alleles was indicated by the four groups segregating in all four F1-families. The data are interpreted to show that any of the two S-substances produced in the pollen and style is sufficient alone to cause incompatibility, and that a fraction of the pollen grains react as if one of the two imprints were absent. It is suggested that, by occasional successive cell wall formation at microsporogenesis, S-gene transcription, even if initiated before S-gene segregation in the first anaphase of the pollen meiocyte, may be insufficient to prevent the S-gene of the dyad cell to put its imprint alone on the two microspores formed in the second meiotic division. This gametophytic imprint on the pollen grain may form a necessary substitute to a system of dominance relationships in the pollen, cross-compatibility being promoted without loss of self-incompatibility. Estimates of the number of S-alleles in the sample of population plants ranged from 7 to 19; the weighted mean was 10. — The presence of an S-gene lacking dominance interaction supports the opinion that the orders Caryophyllales and Ranunculales form a phyletic lineage.

Increase in mate availability without loss of self-incompatibility in the invasive species Senecio inaequidens (Asteraceae)

Increase in mate availability without loss of self-incompatibility in the invasive species Senecio inaequidens (Asteraceae)

ASSOCIATION OF PLOIDY AND SEXUAL SYSTEM IN LYCIUM CALIFORNICUM (SOLANACEAE)

In North American Lycium (Solanaceae), the evolution of gender dimorphism has been proposed as a means of restoring outcrossing after polyploidization causes the loss of self-incompatibility. Previous studies of this process in Lycium focused on comparisons between species that differ in ploidy. We examined intraspecific variation in floral morphology and DNA content in populations of L. californicum to determine correlations between sexual system and cytotype. We also used nuclear ITS and GBSSI sequence data to determine whether diploid and polyploid forms represent the same phylogenetic species, and the phylogeographic relationships among populations and ploidy levels. Within populations, no variation in ploidy was found, although among populations there was a perfect correspondence between sexual system and cytotype. Diploid populations were all hermaphroditic, whereas tetraploid populations were all gender dimorphic. There was no clear geographic pattern to the occurrence of diploid and tetraploid forms. Phylogenetic analysis confirms that L. californicum, regardless of ploidy, forms a monophyletic group within the genus Lycium. Sequences from diploid and polyploid individuals did not form reciprocally monophyletic clades, indicating either multiple gains of polyploidy, ongoing gene flow between cytotypes, or lack of lineage sorting since the evolution of polyploidy. The correspondence between ploidy and sex expression is consistent with the hypothesis that polyploidization triggers the evolution of gender dimorphism in this and other Lycium species.

Linkage and physical distances between the S-haplotype S-RNase and SFB genes in sweet cherry

Gametophytic self-incompatibility (GSI) in sweet cherry (Prunus avium, Rosaceae) is controlled by the multi-allelic S-locus. This complex locus contains two linked genes, an S-RNase that controls pistil specificity, and an F-box gene named SFB (S-haplotype-specific F-box protein gene) that is the putative determinant of pollen specificity in Prunus species. The S-locus is considered to be a region of repressed recombination, as recombination has not been observed. Recombination between the pistil-S and pollen-S determinant genes would result in non-functional S-haplotypes and loss of self-incompatibility. With the recent identification of multiple SFB alleles in sweet cherry, along with the existing S-RNase alleles, it is now possible to estimate the linkage distance and quantify the physical distance between the two GSI specificity genes in multiple sweet cherry S-haplotypes. A recombinational analysis between S-RNase and SFB was performed for four sweet cherry S-haplotypes (S2, S3, S4, S6) using F1 progeny from reciprocal crosses between ‘Emperor Francis’ (S3S4) and ‘NY 54’ (S2S6). For SFB genotyping, allele-specific primer sets were designed from the sequence of the SFB coding region. The S-RNase and SFB genotypes of 511 progeny, representing the outcomes of 1,022 meioses, were determined. All four S-haplotypes individually segregated according to the expected Mendelian ratio of 1:1. The S-RNase and SFB loci were completely linked as no recombinant individuals were identified, thus maintaining the co-evolved allele specificities for the pistil and putative pollen determinant. The physical distance between S-RNase and SFB in six sweet cherry S-haplotypes (S1–S6) was determined using PCR with genomic clones as the template. The relative order and transcriptional orientation of S-RNase and SFB were conserved across the six S-haplotypes. However, the physical distance between these two genes varied widely, ranging from 380 bp to approximately 40 kb. This study represents the first large-scale recombinational analysis of the S-locus region in the Rosaceae, serving as the starting point for future comparative analyses of physical distances, linkage distances, and sequence diversity among Prunus S-haplotypes.

ASSOCIATION OF PLOIDY AND SEXUAL SYSTEM IN LYCIUM CALIFORNICUM (SOLANACEAE)

In North American Lycium (Solanaceae), the evolution of gender dimorphism has been proposed as a means of restoring outcrossing after polyploidization causes the loss of self-incompatibility. Previous studies of this process in Lycium focused on comparisons between species that differ in ploidy. We examined intraspecific variation in floral morphology and DNA content in populations of L. californicum to determine correlations between sexual system and cytotype. We also used nuclear ITS and GBSSI sequence data to determine whether diploid and polyploid forms represent the same phylogenetic species, and the phylogeographic relationships among populations and ploidy levels. Within populations, no variation in ploidy was found, although among populations there was a perfect correspondence between sexual system and cytotype. Diploid populations were all hermaphroditic, whereas tetraploid populations were all gender dimorphic. There was no clear geographic pattern to the occurrence of diploid and tetraploid forms. Phylogenetic analysis confirms that L. californicum, regardless of ploidy, forms a monophyletic group within the genus Lycium. Sequences from diploid and polyploid individuals did not form reciprocally monophyletic clades, indicating either multiple gains of polyploidy, ongoing gene flow between cytotypes, or lack of lineage sorting since the evolution of polyploidy. The correspondence between ploidy and sex expression is consistent with the hypothesis that polyploidization triggers the evolution of gender dimorphism in this and other Lycium species.

Molecular mechanisms underlying the breakdown of gametophytic self-incompatibility.

The breakdown of self-incompatibility has occurred repeatedly throughout the evolution of flowering plants and has profound impacts on the genetic structure of populations. Recent advances in understanding of the molecular basis of self-incompatibility have provided insights into the mechanisms of its loss in natural populations, especially in the tomato family, the Solanaceae. In the Solanaceae, the gene that controls self-incompatibility in the style codes for a ribonuclease that causes the degradation of RNA in pollen tubes bearing an allele at the S-locus that matches either of the two alleles held by the maternal plant. The pollen component of the S-locus has yet to be identified. Loss of self-incompatibility can be attributed to three types of causes: duplication of the S-locus, mutations that cause loss of S-RNase activity, and mutations that do not cause loss of S-RNase activity. Duplication of the S-locus has been well studied in radiation-induced mutants but may be a relatively rare cause of the breakdown of self-incompatibility in nature. Point mutations within the S-locus that disrupt the production of S-RNase have been documented in natural populations. There are also a number of mutants in which S-RNase production is unimpaired, yet self-incompatibility is disrupted. The identity and function of these mutations is not well understood. Careful work on a handful of model organisms will enable population biologists to better understand the breakdown of self-incompatibility in nature.

Main role of self-pollination rate on reproductive allocations in pseudogamous apomicts

Most apomicts are hermaphroditic and pseudogamous (pollination is necessary to trigger parthenogenesis). In these plants, fitness depends on the number of progeny obtained by maternal reproduction. We determined the evolutionary stable strategy for male and female sex allocation. We show that the efficiency of pollination determines male and female resource allocations. Predictions are made of these allocations, of pollen/ovule ratio and of seed-set. We show that self-compatibility in apomicts is necessary for the maintenance of an apomictic population, and thus can account for the association between the loss of self-incompatibility and pseudogamous apomixis. In contrast to sexuals, male investment in pseudogamous apomicts increases with the rate of self-pollination.

Some Functions of Dioecy in Seed Plants

Comments are made on recent publications that emphasize that dioecy in seed plants can be more than just an outcrossing mechanism. It is accepted that "sexual selection" may play an important role in the evolution of dioecy. Circumstantial evidence is presented that when habitat conditions favor outcrossing easily evolved dioecy may be selected. Nevertheless, the disputed suggestion made by me a quarter of a century ago that dioecy appears to have arisen more frequently in hermaphrodite or monoecious taxa that do not have the genetical resources to produce a functional self-incompatibility system is defended, with some indication that this relationship should be examined at the genus level rather than at the level of the family as originally proposed. Disadvantages (to the plants) of dioecy compared to a self-incompatibility system, particularly a monomorphic multiple-allele one, are considered in this light. Distyly is the only incompatibility breeding system that may be expected to give rise sometimes to dioecy (for the flowers are already morphologically halfway to dioecious) although the classic example of distyly, the large genus Primula, shows no sign of dioecy, and some postulated cases of this transition (Bauhinia and Morinda) need to be reexamined. Examples of the apparent evolution of gynodioecy or dioecy after the loss of self-incompatibility are also considered (Armeria, Fragaria, Rubus, and possibly Solanum). It is suggested that all of the "outcrossing" and "sexual selection" hypotheses that have been put forward may have a basis in fact and may even be operative simultaneously.

Inheritance of Stylar Morphology and Loss of Self-incompatibility in the Progenies of Induced Autotetraploid Buckwheat

This paper deals with agronomic tiraits, chromosomal behavior, loss of incompatibility of stylar variants and inheritance of heterostyly in the progenies of colchicine-induced autotetraploid. Because of a small amount of aneuploids and low frequency of multivalent formation in M1, autotetraploids resulted neither terrible reduction of pollen fertility nor decline of seed set. Grain yield, therefore, increased rather than the original diploid strain and some native varieties. Two types of homostyled variants, namely one reduced style length in pin (PH) and the other increased style length in thrum (TH), were found in the progenies. Of these types, PH plants lost self-incompatibility and, therefore, set selfed seeds. Genetic analysis of heterostyly and incompatibility was attempted in connection with cytological evidences of autotetraploid.