Genomic organization of the S core region and the S flanking regions of a class-II S haplotype in Brassica rapa

Comprehensive computational analysis of the SRK-SP11 molecular interaction underlying self-incompatibility in Brassicaceae using improved structure prediction for cysteine-rich proteins

Plants employ self-incompatibility (SI) to promote cross-fertilization. In Brassicaceae, this process is regulated by the formation of a complex between the pistil determinant S receptor kinase (SRK) and the pollen determinant S-locus protein 11 (SP11, also known as S-locus cysteine-rich protein, SCR). In our previous study, we used the crystal structures of two eSRK-SP11 complexes in Brassica rapa S8 and S9 haplotypes and nine computationally predicted complex models to demonstrate that only the SRK ectodomain (eSRK) and SP11 pairs derived from the same S haplotype exhibit high binding free energy. However, predicting the eSRK-SP11 complex structures for the other 100 + S haplotypes and genera remains difficult because of SP11 polymorphism in sequence and structure. Although protein structure prediction using AlphaFold2 exhibits considerably high accuracy for most protein monomers and complexes, 46% of the predicted SP11 structures that we tested showed < 75 mean per-residue confidence score (pLDDT). Here, we demonstrate that the use of curated multiple sequence alignment (MSA) for cysteine-rich proteins significantly improved model accuracy for SP11 and eSRK-SP11 complexes. Additionally, we calculated the binding free energies of the predicted eSRK-SP11 complexes using molecular dynamics (MD) simulations and observed that some Arabidopsis haplotypes formed a binding mode that was critically different from that of B. rapa S8 and S9. Thus, our computational results provide insights into the haplotype-specific eSRK-SP11 binding modes in Brassicaceae at the residue level. The predicted models are freely available at Zenodo, https://doi.org/10.5281/zenodo.8047768.

S haplotype collection in Brassicaceae crops-an updated list of S haplotypes

Self-incompatibility is the system that inhibits pollen germination and pollen tube growth by self-pollen. This trait is important for the breeding of Brassica and Raphanus species. In these species, self-incompatibility is governed by the S locus, which contains three linked genes (a set called the S haplotype), i.e., S-locus receptor kinase, S-locus cysteine-rich protein/S-locus protein 11, and S-locus glycoprotein. A large number of S haplotypes have been identified in Brassica oleracea, B. rapa, and Raphanus sativus to date, and the nucleotide sequences of their many alleles have also been registered. In this state, it is important to avoid confusion between S haplotypes, i.e., an identical S haplotype with different names and a different S haplotype with an identical S haplotype number. To mitigate this issue, we herein constructed a list of S haplotypes that are easily accessible to the latest nucleotide sequences of S-haplotype genes, together with revisions to and an update of S haplotype information. Furthermore, the histories of the S-haplotype collection in the three species are reviewed, the importance of the collection of S haplotypes as a genetic resource is discussed, and the management of information on S haplotypes is proposed.

Classification and Identification of S Haplotypes in Radish Based on SRK Kinase Domain Sequence Analysis

Radish is a typical self-incompatible crop. The rapid and accurate identification of S haplotypes can circumvent the blindness of the hybrid combination process, which is critical in radish heterosis utilization and the breeding of new varieties. In this study, based on the gene sequence which encodes the S-locus receptor kinase (SRK) of radish, and the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis, the S haplotypes were identified among 79 cultivated radish genotypes. The PCR results indicated that 79 radish genotypes could be divided into 48 Class I, 13 Class II, and 17 Class I/II S haplotypes. Sequence alignment confirmed that the Class I materials contained 19 S haplotypes, of which three haplotypes (‘NAU-S53’, ‘NAU-S54’ and ‘NAU-S55’) were identified for the first time in radish. After digestion using the Hinf I restriction endonuclease, the SRK domain of DNA fragments of different genotypes showed high polymorphism. Homozygous materials S haplotypes could be quickly distinguished by the differences in the digested bands. Molecular identification of the S haplotype was highly consistent with the field pollination and pollen tube germination results. These results would provide an important approach for the rapid identification of radish S haplotypes and the efficient utilization of self-incompatibility in heterosis breeding.

Genetic and Molecular Characterization of a Self-Compatible Brassica rapa Line Possessing a New Class II S Haplotype

Most flowering plants have evolved a self-incompatibility (SI) system to maintain genetic diversity by preventing self-pollination. The Brassica species possesses sporophytic self-incompatibility (SSI), which is controlled by the pollen- and stigma-determinant factors SP11/SCR and SRK. However, the mysterious molecular mechanism of SI remains largely unknown. Here, a new class II S haplotype, named BrS-325, was identified in a pak choi line '325', which was responsible for the completely self-compatible phenotype. To obtain the entire S locus sequences, a complete pak choi genome was gained through Nanopore sequencing and de novo assembly, which provided a good reference genome for breeding and molecular research in B. rapa. S locus comparative analysis showed that the closest relatives to BrS-325 was BrS-60, and high sequence polymorphism existed in the S locus. Meanwhile, two duplicated SRKs (BrSRK-325a and BrSRK-325b) were distributed in the BrS-325 locus with opposite transcription directions. BrSRK-325b and BrSCR-325 were expressed normally at the transcriptional level. The multiple sequence alignment of SCRs and SRKs in class II S haplotypes showed that a number of amino acid variations were present in the contact regions (CR II and CR III) of BrSCR-325 and the hypervariable regions (HV I and HV II) of BrSRK-325s, which may influence the binding and interaction between the ligand and the receptor. Thus, these results suggested that amino acid variations in contact sites may lead to the SI destruction of a new class II S haplotype BrS-325 in B. rapa. The complete SC phenotype of '325' showed the potential for practical breeding application value in B. rapa.

Characterization of a Common S Haplotype BnS-6 in the Self-Incompatibility of Brassica napus

Self-incompatibility (SI) is a pollen-stigma recognition system controlled by a single and highly polymorphic genetic locus known as the S-locus. The S-locus exists in all Brassica napus (B. napus, AACC), but natural B. napus accessions are self-compatible. About 100 and 50 S haplotypes exist in Brassica rapa (AA) and Brassica oleracea (CC), respectively. However, S haplotypes have not been detected in B. napus populations. In this study, we detected the S haplotype distribution in B. napus and ascertained the function of a common S haplotype BnS-6 through genetic transformation. BnS-1/BnS-6 and BnS-7/BnS-6 were the main S haplotypes in 523 B. napus cultivars and inbred lines. The expression of SRK in different S haplotypes was normal (the expression of SCR in the A subgenome affected the SI phenotype) while the expression of BnSCR-6 in the C subgenome had no correlation with the SI phenotype in B. napus. The BnSCR-6 protein in BnSCR-6 overexpressed lines was functional, but the self-compatibility of overexpressed lines did not change. The low expression of BnSCR-6 could be a reason for the inactivation of BnS-6 in the SI response of B. napus. This study lays a foundation for research on the self-compatibility mechanism and the SI-related breeding in B. napus.

Mechanism of self/nonself-discrimination in Brassica self-incompatibility

Self-incompatibility (SI) is a breeding system that promotes cross-fertilization. In Brassica, pollen rejection is induced by a haplotype-specific interaction between pistil determinant SRK (S receptor kinase) and pollen determinant SP11 (S-locus Protein 11, also named SCR) from the S-locus. Although the structure of the B. rapa S₉-SRK ectodomain (eSRK) and S₉-SP11 complex has been determined, it remains unclear how SRK discriminates self- and nonself-SP11. Here, we uncover the detailed mechanism of self/nonself-discrimination in Brassica SI by determining the S₈-eSRK-S₈-SP11 crystal structure and performing molecular dynamics (MD) simulations. Comprehensive binding analysis of eSRK and SP11 structures reveals that the binding free energies are most stable for cognate eSRK-SP11 combinations. Residue-based contribution analysis suggests that the modes of eSRK-SP11 interactions differ between intra- and inter-subgroup (a group of phylogenetically neighboring haplotypes) combinations. Our data establish a model of self/nonself-discrimination in Brassica SI.

A complex dominance hierarchy is controlled by polymorphism of small RNAs and their targets

In diploid organisms, phenotypic traits are often biased by effects known as Mendelian dominant-recessive interactions between inherited alleles. Phenotypic expression of SP11 alleles, which encodes the male determinants of self-incompatibility in Brassica rapa, is governed by a complex dominance hierarchy^1-3. Here, we show that a single polymorphic 24 nucleotide small RNA, named SP11 methylation inducer 2 (Smi2), controls the linear dominance hierarchy of the four SP11 alleles (S₄₄ > S₆₀ > S₄₀ > S₂₉). In all dominant-recessive interactions, small RNA variants derived from the linked region of dominant SP11 alleles exhibited high sequence similarity to the promoter regions of recessive SP11 alleles and acted in trans to epigenetically silence their expression. Together with our previous study⁴, we propose a new model: sequence similarity between polymorphic small RNAs and their target regulates mono-allelic gene expression, which explains the entire five-phased linear dominance hierarchy of the SP11 phenotypic expression in Brassica.

Self-incompatibility in Brassicaceae crops: lessons for interspecific incompatibility

Most wild plants and some crops of the Brassicaceae express self-incompatibility, which is a mechanism that allows stigmas to recognize and discriminate against "self" pollen, thus preventing self-fertilization and inbreeding. Self-incompatibility in this family is controlled by a single S locus containing two multiallelic genes that encode the stigma-expressed S-locus receptor kinase and its pollen coat-localized ligand, the S-locus cysteine-rich protein. Physical interaction between receptor and ligand encoded in the same S locus activates the receptor and triggers a signaling cascade that results in inhibition of "self" pollen. Sequence information for many S-locus haplotypes in Brassica species has spurred studies of dominance relationships between S haplotypes and of S-locus structure, as well as the development of methods for S genotyping. Furthermore, molecular genetic studies have begun to identify genes that encode putative components of the self-incompatibility signaling pathway. In parallel, standard genetic analysis and QTL analysis of the poorly understood interspecific incompatibility phenomenon have been initiated to identify genes responsible for the inhibition of pollen from other species by the stigma. Herewith, we review recent studies of self-incompatibility and interspecific incompatibility, and we propose a model in which a universal pollen-inhibition pathway is shared by these two incompatibility systems.

Trait transitions in explicit ecological and genomic contexts: plant mating systems as case studies

Plants are astonishingly diverse in how they reproduce sexually, and the study of plant mating systems provides some of the most compelling cases of parallel and independent evolutionary transitions. In this chapter, we review how the massive amount of genomic data being produced is allowing long-standing predictions from ecological and evolutionary theory to be put to test. After a review of theoretical predictions about the importance of considering the genomic architecture of the mating system, we focus on a set of recent discoveries on how the mating system is controlled in a variety of model and non-model species. In parallel, genomic approaches have revealed the complex interaction between the evolution of genes controlling mating systems and genome evolution, both genome-wide and in the mating system control region. In several cases, major transitions in the mating system can be clearly associated with important ecological changes, hence illuminating an important interplay between ecological and genomic approaches. We also list a number of major unsolved questions that remain for the field, and highlight foreseeable conceptual developments that are likely to play a major role in our understanding of how plant mating systems evolve in Nature.

Self-(in)compatibility inheritance and allele-specific marker development in yellow mustard (Sinapis alba)

Yellow mustard (Sinapis alba) has a sporophytic self-incompatibility reproduction system. Genetically stable self-incompatible (SI) and self-compatible (SC) inbred lines have recently been developed in this crop. Understanding the S haplotype of different inbred lines and the inheritance of the self-(in)compatibility (SI/SC) trait is very important for breeding purposes. In this study, we used the S-locus gene-specific primers in Brassica rapa and Brassica oleracea to clone yellow mustard S-locus genes of SI lines Y514 and Y1130 and SC lines Y1499 and Y1501. The PCR amplification results and DNA sequences of the S-locus genes revealed that Y514 carried the class I S haplotype, while Y1130, Y1499, and Y1501 had the class II S haplotype. The results of our genetic studies indicated that self-incompatibility was dominant over self-compatibility and controlled by a one-gene locus in the two crosses of Y514 × Y1499 and Y1130 × Y1501. Of the five S-locus gene polymorphic primer pairs, Sal-SLGI and Sal-SRKI each generated one dominant marker for the SI phenotype of Y514; Sal-SLGII and Sal-SRKII produced dominant marker(s) for the SC phenotype of Y1501 and Y1499; Sal-SP11II generated one dominant marker for Y1130. These markers co-segregated with the SI/SC phenotype in the F₂ populations of the two crosses. In addition, co-dominant markers were developed by mixing the two polymorphic primer pairs specific for each parent in the multiplex PCR, which allowed zygosity to be determined in the F₂ populations. The SI/SC allele-specific markers have proven to be very useful for the selection of the desirable SC genotypes in our yellow mustard breeding program.

Evolution of S-domain receptor-like kinases in land plants and origination of S-locus receptor kinases in Brassicaceae

Background

The S-domain serine/threonine receptor-like kinases (SRLKs) comprise one of the largest and most rapidly expanding subfamilies in the plant receptor-like/Pelle kinase (RLKs) family. The founding member of this subfamily, the S-locus receptor kinase (SRK), functions as the female determinant of specificity in the self-incompatibility (SI) responses of crucifers. Two classes of proteins resembling the extracellular S domain (designated S-domain receptor-like proteins, SRLPs) or the intracellular kinase domain (designated S-domain receptor-like cytoplasmic kinases, SRLCKs) of SRK are also ubiquitous in land plants, indicating that the SRLKs are composite molecules that originated by domain fusion of the two component proteins. Here, we explored the origin and diversification of SRLKs by phylogenomic methods.

Results

Based on the distribution patterns of SRLKs and SRLCKs in a reconciled species-domain tree, a maximum parsimony model was then established for simultaneously inferring and dating gene duplication/loss and fusion /fission events in SRLK evolution. Various SRK alleles from crucifer species were then included in our phylogenetic analyses to infer the origination of SRKs by identifying the proper outgroups.

Conclusions

Two gene fusion events were inferred and the major gene fusion event occurred in the common ancestor of land plants generated almost all of extant SRLKs. The functional diversification of duplicated SRLKs was illustrated by molecular evolution analyses of SRKs. Our findings support that SRKs originated as two ancient haplotypes derived from a pair of tandem duplicate genes through random regulatory neo-/sub- functionalization in the common ancestor of the Brassicaceae.

Contrasted patterns of molecular evolution in dominant and recessive self-incompatibility haplotypes in Arabidopsis

Self-incompatibility has been considered by geneticists a model system for reproductive biology and balancing selection, but our understanding of the genetic basis and evolution of this molecular lock-and-key system has remained limited by the extreme level of sequence divergence among haplotypes, resulting in a lack of appropriate genomic sequences. In this study, we report and analyze the full sequence of eleven distinct haplotypes of the self-incompatibility locus (S-locus) in two closely related Arabidopsis species, obtained from individual BAC libraries. We use this extensive dataset to highlight sharply contrasted patterns of molecular evolution of each of the two genes controlling self-incompatibility themselves, as well as of the genomic region surrounding them. We find strong collinearity of the flanking regions among haplotypes on each side of the S-locus together with high levels of sequence similarity. In contrast, the S-locus region itself shows spectacularly deep gene genealogies, high variability in size and gene organization, as well as complete absence of sequence similarity in intergenic sequences and striking accumulation of transposable elements. Of particular interest, we demonstrate that dominant and recessive S-haplotypes experience sharply contrasted patterns of molecular evolution. Indeed, dominant haplotypes exhibit larger size and a much higher density of transposable elements, being matched only by that in the centromere. Overall, these properties highlight that the S-locus presents many striking similarities with other regions involved in the determination of mating-types, such as sex chromosomes in animals or in plants, or the mating-type locus in fungi and green algae.

Self-incompatibility is a common genetic system preventing selfing through recognition and rejection of self-pollen in hermaphroditic flowering plants. In the Brassicaceae family, this system is controlled by a single genomic region, called the S-locus, where many distinct specificities segregate in natural populations. In this study, we obtained genomic sequences comprising the S-locus in two closely related Brassicaceae species, Arabidopsis lyrata and A. halleri, and analyzed their diversity and patterns of molecular evolution. We report compelling evidence that the S-locus presents many similar properties with other genomic regions involved in the determination of mating-types in mammals, insects, plants, or fungi. In particular, in spite of their diversity, these genomic regions all show absence of similarity in intergenic sequences, large depth of genealogies, highly divergent organization, and accumulation of transposable elements. Moreover, some of these features were found to vary according to dominance of the S-locus specificities, suggesting that dominance/recessivity interactions are key drivers of the evolution of this genomic region.

Mapping of markers related to self-incompatibility, disease resistance, and quality traits in Lolium perenne L

Several linkage maps, mainly based on anonymous markers, are now available for Lolium perenne. The saturation of these maps with markers derived from expressed sequences would provide information useful for QTL mapping and map alignment. Therefore, we initiated a study to develop and map DNA markers in genes related to self-incompatibility, disease resistance, and quality traits such as digestibility and sugar content in two L. perenne families. In total, 483 and 504 primer pairs were designed and used to screen the ILGI and CLO-DvP mapping populations, respectively, for length polymorphisms. Finally, we were able to map 67 EST markers in at least one mapping population. Several of these markers coincide with previously reported QTL regions for the traits considered or are located in the neighbourhood of the self-incompatibility loci, S and Z. The markers developed expand the set of gene-derived markers available for genetic mapping in ryegrasses.

Distribution of S haplotypes and its relationship with restorer-maintainers of self-incompatibility in cultivated Brassica napus

Brassica napus (AACC, 2n = 38) is a self-compatible amphidiploid plant that arose from the interspecies hybridization of two self-incompatible species, B. rapa (AA, 2n = 20) and B. oleracea (CC, 2n = 18). Self-incompatibility (S) haplotypes in one self-incompatible line and 124 cultivated B. napus lines were detected using S-locus-specific primers, and their relationships with restorer-maintainers were investigated. Two class I (S-I ( SLG ) a and S-I ( SLG ) b) and four class II (S-II ( SLG ) a, S-II ( SLG ) b, S-II ( SP11 ) a and S-II ( SP11 ) b) S haplotypes were observed, of which S-II ( SP11 ) b was newly identified. The nucleotide sequence of SP11 showed little similarity to the reported SP11 alleles. The lines were found to express a total of eleven S genotypes. The self-incompatible line had a specific genotype consisting of S-II ( SP11 ) a, similar to B. rapa S-60, and S-II ( SLG ) a, similar to B. oleracea S-15. Restorers expressed six genotypes: the most common genotype contained S-I ( SLG ) a, similar to B. rapa S-47, and S-II ( SLG ) b, similar to B. oleracea S-15. Maintainers expressed nine genotypes: the predominant genotype was homozygous for two S haplotypes, S-II ( SLG ) a and S-II ( SP11 ) b. One genotype was specific to restorers and four genotypes were specific to maintainers, whereas five genotypes were expressed in both restorers and maintainers. This suggests that there is no definitive correlation between the distribution of S genotypes and restorer-maintainers of self-incompatibility. The finding that restorers and maintainers express unique genotypes, and share some common genotypes, would be valuable for detecting the interaction of S haplotypes in inter- or intra-genomes as well as for developing markers-assisted selection in self-incompatibility hybrid breeding.

How far are we from unravelling self-incompatibility in grasses?

The genetic and physiological mechanisms involved in limiting self-fertilization in angiosperms, referred to as self-incompatibility (SI), have significant effects on population structure and have potential diversification and evolutionary consequences. Up to now, details of the underlying genetic control and physiological basis of SI have been elucidated in two different gametophytic SI (GSI) systems, the S-RNase SI and the Papaver SI systems, and the sporophytic SI (SSI) system (Brassica). In the grass family (Poaceae), which contains all the cereal and major forage crops, SI has been known for half a century to be controlled gametophytically by two multiallelic and independent loci, S and Z. But still none of the gene products for S and Z is known and only limited information on related biochemical responses is available. Here we compare current knowledge of grass SI with that of other well-characterized SI systems and speculate about the relationship between SSI and grass SI. Additionally, we discuss comparative mapping as a tool for the further investigation of grass SI.

Comparative analysis of the S-intergenic region in class-II S haplotypes of self-incompatible Brassica rapa (syn. campestris)

In the Brassica self-incompatibility (SI) system, a pollen determinant, SP11, is involved in dominance/recessive relationships in pollen SI phenotypes. In order to gain some insights into the genomic structure around the SP11 and the mechanisms that give dominance/recessive relationships, we characterized the genomic region containing SP11 and SRK genes in three pollen recessive class-II S haplotypes. The direction of transcription of S genes was completely conserved among class-II S haplotypes. However, the region between SP11 and SRK (S-intergenic region) was highly polymorphic without short repetitive sequences. In addition, we found a sequence similarity between the short repetitive sequence and 5'-upstream region of SP11. This sequence similarity was found to be potentially related to the expression of dominance relationships through the change of chromatin structure.

Effects of recombination on hitchhiking diversity in the Brassica self-incompatibility locus complex

In self-incompatibility, a number of S haplotypes are maintained by frequency-dependent selection, which results in trans-specific S haplotypes. The region of several kilobases (approximately 40-60 kb) from SP6 to SP2, including self-incompatibility-related genes and some adjacent genes in Brassica rapa, has high nucleotide diversity due to the hitchhiking effect, and therefore we call this region the "S-locus complex." Recombination in the S-locus complex is considered to be suppressed. We sequenced regions of >50 kb of the S-locus complex of three S haplotypes in B. rapa and found higher nucleotide diversity in intergenic regions than in coding regions. Two highly similar regions of >10 kb were found between BrS-8 and BrS-46. Phylogenetic analysis using trans-specific S haplotypes (called interspecific pairs) of B. rapa and B. oleracea suggested that recombination reduced the nucleotide diversity in these two regions and that the genes not involved in self-incompatibility in the S-locus complex and the kinase domain, but not the S domain, of SRK have also experienced recombination. Recombination may reduce hitchhiking diversity in the S-locus complex, whereas the region from the S domain to SP11 would disfavor recombination.

Self-compatibility in Brassica napus is caused by independent mutations in S-locus genes

Brassica napus is an amphidiploid species with the A genome from Brassica rapa and the C genome from Brassica oleracea. Although B. rapa, B. oleracea and artificially synthesized amphidiploids with the AC genome are self-incompatible, B. napus is self-compatible. Six S genotypes were identified in B. napus, five of which had class I S haplotypes from one species and a class II S haplotype from the other species, and mutations causing self-compatibility were identified in three of these S genotypes. The most predominant S genotype (BnS-1;BnS-6), which is that of cv. 'Westar', had a class I S haplotype similar to B. rapa S-47 (BrS-47) and a class II S haplotype similar to B. oleracea S-15 (BoS-15). The stigmas of 'Westar' rejected the pollen grains of both BrS-47 and BoS-15, while reciprocal crossings were compatible. Insertion of a DNA fragment of about 3.6 kb was found in the promoter region of the SP11/SCR allele of BnS-1, and transcripts of SP11/SCR were not detected in 'Westar'. The nucleotide sequence of the SP11 genomic DNA of BnS-6 was 100% identical to that of SP11 of BoS-15. Class I SP11 alleles from one species showed dominance over class II SP11 alleles from the other species in artificially synthesized B. napus lines, suggesting that the non-functional dominant SP11 allele suppressed the expression of the recessive SP11 allele in 'Westar'. Two other S genotypes in B. napus also had non-functional class I S haplotypes together with recessive BnS-6. These observations suggest independent origins of self-compatibility in B. napus.

Suppression of gene expression of a recessive SP11/SCR allele by an untranscribed SP11/SCR allele in Brassica self-incompatibility

Mutations in the S locus of a self-compatible cultivar Yellow Sarson in Brassica rapa, which has a self-compatible class-I S haplotype, S-f2, were investigated. S-28 in Brassica oleracea was found to be a member of an interspecific pair with S-f2 in B. rapa. The original S haplotype of S-f2 was identified to be S-54 in B. rapa. Sequence comparison of alleles in S-f2 with those in S-54 and B. oleracea S-28 revealed insertion of a retrotransposon-like sequence in the first intron of SRK and 89-bp deletion in the promoter region of SP11. No transcripts of SRK and SP11 were detected in S-f2 homozygotes, suggesting that the insertion and the deletion in SRK and SP11, respectively, caused the loss of the function of these genes. Promoter assay using transgenic plants indicated that the SP11 promoter of S-f2 has no activity. Heterozygotes of S-f2 and a normal class-II S haplotype, S-60, in B. rapa were found to be self-compatible. Interestingly, transcription of SP11-60 was revealed to be suppressed in the S-f2/S-60 heterozygotes, suggesting that an untranscribed class-I SP11 allele suppresses the expression of a recessive class-II SP11 allele in the anthers of S heterozygotes. Similar phenomenon was observed in heterozygotes of a self-compatible class-I S haplotype and a self-incompatible class-II S haplotype in B. oleracea.

Comparison of the genome structure of the self-incompatibility (S) locus in interspecific pairs of S haplotypes

The determinants of recognition specificity of self-incompatibility in Brassica are SRK in the stigma and SP11/SCR in the pollen, both of which are encoded in the S locus. The nucleotide sequence analyses of many SRK and SP11/SCR alleles have identified several interspecific pairs of S haplotypes having highly similar sequences between B. oleracea and B. rapa. These interspecific pairs of S haplotypes are considered to be derived from common ancestors and to have maintained the same recognition specificity after speciation. In this study, the genome structures of three interspecific pairs of S haplotypes were compared by sequencing SRK, SP11/SCR, and their flanking regions. Regions between SRK and SP11/SCR in B. oleracea were demonstrated to be much longer than those of B. rapa and several retrotransposon-like sequences were identified in the S locus in B. oleracea. Among the seven retrotransposon-like sequences, six sequences were found to belong to the ty3 gypsy group. The gag sequences of the retrotransposon-like sequences were phylogenetically different from each other. In Southern blot analysis using retrotransposon-like sequences as probes, the B. oleracea genome showed more signals than the B. rapa genome did. These findings suggest a role for the S locus and genome evolution in self-incompatible plant species.

Interspecific pairs of class II S haplotypes having different recognition specificities between Brassica oleracea and Brassica rapa

There are several pairs of similar class I S haplotypes between Brassica oleracea and Brassica rapa. The similar S halotypes in these interspecific pairs have been reported to have the same recognition specificities. In the present study, three interspecific pairs showing a high sequence similarity were found in class II S haplotypes, i.e. between BoS-2b (B. oleracea S-2b) and BrS-44 (B. rapa S-44), between BoS-5 and BrS-40, and between BoS-15 and BrS-60. By pollination tests using interspecific hybrids between B. oleracea and B. rapa, BoS-5 and BoS-2b were revealed to have slightly and completely different recognition specificities from those of BrS-40 and BrS-44, respectively. The recognition reaction between SP11 and SRK of BoS-15 was suggested to be incomplete. The regions of class II SP11 and SRK important for self-recognition specificity and the diversification of class II S haplotypes are discussed herein.

Genomic organization of the AODEF gene in Asparagus officinalis L

The perianths of Liliaceae plants, such as lily and tulip, have two whorls of almost identical petaloid organs, which are called tepals. According to the modified ABC model proposed in tulip, the class B genes are expressed in whorl 1 as well as whorls 2 and 3, so that the organs of whorls 1 and 2 have the same petaloid structure. The floral structure of asparagus (Asparagus officinalis L.) is similar to that of Liliaceae plants, however, the expression of B-class genes (AODEF, AOGLOA, AOGLOB) was not found in whorl 1, but was confined to whorls 2 and 3. This result does not support the modified ABC model in asparagus. In order to gain a better understanding of asparagus flower development, we have characterized a genomic clone of the AODEF gene. We compared the genomic organization and promoter sequence of AODEF with three well-studied DEF-like genes, DEFICIENS (Antirrhinum), APETALA3 (Arabidopsis), and OSMADS16 (rice). Exon-intron structures of these genes are well-conserved except for the large fifth intron in the AODEF gene and the OSMADS16 gene. Putative cis-elements including CArG-boxes were found in the promoter region and forty-two microsatellites were found in the AODEF genomic sequence.

The evolutionary dynamics of self-incompatibility systems

Self-incompatible flowering plants reject pollen that expresses the same mating specificity as the pistil (female reproductive tract). In most plant families, pollen and pistil mating specificities segregate as a single locus, the S locus. In at least two self-incompatibility systems, distinct pollen and pistil specificity genes are embedded in an extensive nonrecombining tract. To facilitate consideration of how new S locus specificities arise in systems with distinct pollen and pistil genes, we present a graphical model for the generation of hypotheses. It incorporates the evolutionary principle that nonreciprocal siring success (cross-pollinations between two plants produce seeds in only one direction) tends to favor the rejecting partner. This model suggests that selection within S-allele specificity classes could accelerate the rate of nonsynonymous (amino acid-changing) substitutions, with periodic selective sweeps removing segregating variation within classes. Accelerated substitution within specificity classes could also promote the origin of new S-allele specificities.

Evolution under tight linkage to mating type

Recent large-scale sequencing studies of mating type loci in a number of organisms offer insight into the origin and evolution of these genomic regions. Extensive tracts containing genes with a wide diversity of functions typically cosegregate with mating type. Cases in which mating type determination entails complementarity between distinct transcription units may descend from systems in which close physical linkage facilitated the coordinated expression and cosegregation of the interacting genes. In response to the particular selection pressures associated with the maintenance of more than one mating type, this nucleus of low recombination may expand over evolutionary time, engulfing neighboring tracts bearing genes with no direct role in reproduction. This scenario is consistent with the present-day structure of some mating type loci, including regulators of homomorphic self-incompatibility in angiosperms (S-loci). Recombination suppression and enforced S-locus heterozygosity accelerate the accumulation of genetic load and promote genetic associations between S-alleles and degenerating genes in cosegregating tracts. This S-allele-specific load may influence the evolution of self-incompatibility systems.