Evolution of self-compatibility in Arabidopsis by a mutation in the male specificity gene

Ever since Darwin's pioneering research, the evolution of self-fertilisation (selfing) has been regarded as one of the most prevalent evolutionary transitions in flowering plants. A major mechanism to prevent selfing is the self-incompatibility (SI) recognition system, which consists of male and female specificity genes at the S-locus and SI modifier genes. Under conditions that favour selfing, mutations disabling the male recognition component are predicted to enjoy a relative advantage over those disabling the female component, because male mutations would increase through both pollen and seeds whereas female mutations would increase only through seeds. Despite many studies on the genetic basis of loss of SI in the predominantly selfing plant Arabidopsis thaliana, it remains unknown whether selfing arose through mutations in the female specificity gene (S-receptor kinase, SRK), male specificity gene (S-locus cysteine-rich protein, SCR; also known as S-locus protein 11, SP11) or modifier genes, and whether any of them rose to high frequency across large geographic regions. Here we report that a disruptive 213-base-pair (bp) inversion in the SCR gene (or its derivative haplotypes with deletions encompassing the entire SCR-A and a large portion of SRK-A) is found in 95% of European accessions, which contrasts with the genome-wide pattern of polymorphism in European A. thaliana. Importantly, interspecific crossings using Arabidopsis halleri as a pollen donor reveal that some A. thaliana accessions, including Wei-1, retain the female SI reaction, suggesting that all female components including SRK are still functional. Moreover, when the 213-bp inversion in SCR was inverted and expressed in transgenic Wei-1 plants, the functional SCR restored the SI reaction. The inversion within SCR is the first mutation disrupting SI shown to be nearly fixed in geographically wide samples, and its prevalence is consistent with theoretical predictions regarding the evolutionary advantage of mutations in male components

Construction of an integrated genetic linkage map for the A genome of Brassica napus using SSR markers derived from sequenced BACs in B. rapa

Background The Multinational Brassica rapa Genome Sequencing Project (BrGSP) has developed valuable genomic resources, including BAC libraries, BAC-end sequences, genetic and physical maps, and seed BAC sequences for Brassica rapa. An integrated linkage map between the amphidiploid B. napus and diploid B. rapa will facilitate the rapid transfer of these valuable resources from B. rapa to B. napus (Oilseed rape, Canola). Results In this study, we identified over 23,000 simple sequence repeats (SSRs) from 536 sequenced BACs. 890 SSR markers (designated as BrGMS) were developed and used for the construction of an integrated linkage map for the A genome in B. rapa and B. napus. Two hundred and nineteen BrGMS markers were integrated to an existing B. napus linkage map (BnaNZDH). Among these mapped BrGMS markers, 168 were only distributed on the A genome linkage groups (LGs), 18 distrubuted both on the A and C genome LGs, and 33 only distributed on the C genome LGs. Most of the A genome LGs in B. napus were collinear with the homoeologous LGs in B. rapa, although minor inversions or rearrangements occurred on A2 and A9. The mapping of these BAC-specific SSR markers enabled assignment of 161 sequenced B. rapa BACs, as well as the associated BAC contigs to the A genome LGs of B. napus. Conclusion The genetic mapping of SSR markers derived from sequenced BACs in B. rapa enabled direct links to be established between the B. napus linkage map and a B. rapa physical map, and thus the assignment of B. rapa BACs and the associated BAC contigs to the B. napus linkage map. This integrated genetic linkage map will facilitate exploitation of the B. rapa annotated genomic resources for gene tagging and map-based cloning in B. napus, and for comparative analysis of the A genome within Brassica species

Dominance in self-compatibility between subgenomes of allopolyploid Arabidopsis kamchatica shown by transgenic restoration of self-incompatibility

The evolutionary transition to self-compatibility facilitates polyploid speciation. In Arabidopsis relatives, the self-incompatibility system is characterized by epigenetic dominance modifiers, among which small RNAs suppress the expression of a recessive SCR/SP11 haplogroup. Although the contribution of dominance to polyploid self-compatibility is speculated, little functional evidence has been reported. Here we employ transgenic techniques to the allotetraploid plant A. kamchatica. We find that when the dominant SCR-B is repaired by removing a transposable element insertion, self-incompatibility is restored. This suggests that SCR was responsible for the evolution of self-compatibility. By contrast, the reconstruction of recessive SCR-D cannot restore self-incompatibility. These data indicate that the insertion in SCR-B conferred dominant self-compatibility to A. kamchatica. Dominant self-compatibility supports the prediction that dominant mutations increasing selfing rate can pass through Haldane’s sieve against recessive mutations. The dominance regulation between subgenomes inherited from progenitors contrasts with previous studies on novel epigenetic mutations at polyploidization termed genome shock

Mapping the Anthocyaninless (anl) Locus in Rapid-Cycling Brassica rapa (RBr) to Linkage Group R9

<p>Abstract</p> <p>Background</p> <p>Anthocyanins are flavonoid pigments that are responsible for purple coloration in the stems and leaves of a variety of plant species. <it>Anthocyaninless </it>(<it>anl</it>) mutants of <it>Brassica rapa </it>fail to produce anthocyanin pigments. In rapid-cycling <it>Brassica rapa</it>, also known as Wisconsin Fast Plants, the anthocyaninless trait, also called non-purple stem, is widely used as a model recessive trait for teaching genetics. Although anthocyanin genes have been mapped in other plants such as <it>Arabidopsis thaliana</it>, the <it>anl </it>locus has not been mapped in any <it>Brassica </it>species.</p> <p>Results</p> <p>We tested primer pairs known to amplify microsatellites in <it>Brassicas </it>and identified 37 that amplified a product in rapid-cycling <it>Brassica rapa</it>. We then developed three-generation pedigrees to assess linkage between the microsatellite markers and <it>anl</it>. 22 of the markers that we tested were polymorphic in our crosses. Based on 177 F₂offspring, we identified three markers linked to <it>anl </it>with LOD scores ≥ 5.0, forming a linkage group spanning 46.9 cM. Because one of these markers has been assigned to a known <it>B. rapa </it>linkage group, we can now assign the <it>anl </it>locus to <it>B. rapa </it>linkage group R9.</p> <p>Conclusion</p> <p>This study is the first to identify the chromosomal location of an anthocyanin pigment gene among the <it>Brassicas</it>. It also connects a classical mutant frequently used in genetics education with molecular markers and a known chromosomal location.</p

Transcriptional Characteristics and Differences in Arabidopsis Stigmatic Papilla Cells Pre- and Post-Pollination

Pollination is an important early step in sexual plant reproduction. In Arabidopsis thaliana, sequential pollination events, from pollen adhesion onto the stigma surface to pollen tube germination and elongation, occur on the stigmatic papilla cells. Following successful completion of these events, the pollen tube penetrates the stigma and finally fertilizes a female gametophyte. The pollination events are thought to be initiated and regulated by interactions between papilla cells and pollen. Here, we report the characterization of gene expression profiles of unpollinated (UP), compatible pollinated (CP) and incompatible pollinated (IP) papilla cells in A. thaliana. Based on cell type-specific transcriptome analysis from a combination of laser microdissection and RNA sequencing, 15,475, 17,360 and 16,918 genes were identified as expressed in UP, CP and IP papilla cells, respectively, and, of these, 14,392 genes were present in all three data sets. Differentially expressed gene (DEG) analyses identified 147 and 71 genes up-regulated in CP and IP papilla cells, respectively, and 115 and 46 genes down-regulated. Gene Ontology and metabolic pathway analyses revealed that papilla cells play an active role as the female reproductive component in pollination, particularly in information exchange, signal transduction, internal physiological changes and external morphological modification. This study provides fundamental information on the molecular mechanisms involved in pollination in papilla cells, furthering our understanding of the reproductive role of papilla cell

A sequence-based genetic linkage map as a reference for Brassica rapa pseudochromosome assembly

<p>Abstract</p> <p>Background</p> <p><it>Brassica rapa </it>is an economically important crop and a model plant for studies concerning polyploidization and the evolution of extreme morphology. The multinational <it>B. rapa </it>Genome Sequencing Project (BrGSP) was launched in 2003. In 2008, next generation sequencing technology was used to sequence the <it>B. rapa </it>genome. Several maps concerning <it>B. rapa </it>pseudochromosome assembly have been published but their coverage of the genome is incomplete, anchoring approximately 73.6% of the scaffolds on to chromosomes. Therefore, a new genetic map to aid pseudochromosome assembly is required.</p> <p>Results</p> <p>This study concerns the construction of a reference genetic linkage map for <it>Brassica rapa</it>, forming the backbone for anchoring sequence scaffolds of the <it>B. rapa </it>genome resulting from recent sequencing efforts. One hundred and nineteen doubled haploid (DH) lines derived from microspore cultures of an F1 cross between a Chinese cabbage (<it>B. rapa </it>ssp. <it>pekinensis</it>) DH line (Z16) and a rapid cycling inbred line (L144) were used to construct the linkage map. PCR-based insertion/deletion (InDel) markers were developed by re-sequencing the two parental lines. The map comprises a total of 507 markers including 415 InDels and 92 SSRs. Alignment and orientation using SSR markers in common with existing <it>B. rapa </it>linkage maps allowed ten linkage groups to be identified, designated A01-A10. The total length of the linkage map was 1234.2 cM, with an average distance of 2.43 cM between adjacent marker loci. The lengths of linkage groups ranged from 71.5 cM to 188.5 cM for A08 and A09, respectively. Using the developed linkage map, 152 scaffolds were anchored on to the chromosomes, encompassing more than 82.9% of the <it>B. rapa </it>genome. Taken together with the previously available linkage maps, 183 scaffolds were anchored on to the chromosomes and the total coverage of the genome was 88.9%.</p> <p>Conclusions</p> <p>The development of this linkage map is vital for the integration of genome sequences and genetic information, and provides a useful resource for the international <it>Brassica </it>research community.</p

The AtXTH28 Gene, a Xyloglucan Endotransglucosylase/Hydrolase, is Involved in Automatic Self-Pollination in Arabidopsis thaliana

Successful automatic self-pollination in flowering plants is dependent on the correct development of reproductive organs. In the stamen, the appropriate growth of the filament, which largely depends on the mechanical properties of the cell wall, is required to position the anther correctly close to the stigma at the pollination stage. Xyloglucan endotransglucosylase/hydrolases (XTHs) are a family of enzymes that mediate the construction and restructuring of xyloglucan cross-links, thereby controlling the extensibility or mechanical properties of the cell wall in a wide variety of plant tissues. Our reverse genetic analysis has revealed that a loss-of-function mutation of an Arabidopsis XTH family gene, AtXTH28, led to a decrease in capability for self-pollination, probably due to inhibition of stamen filament growth. Our results also suggest that the role of AtXTH28 in the development of the stamen is not functionally redundant with its closest paralog, AtXTH27. Thus, our finding indicates that AtXTH28 is specifically involved in the growth of stamen filaments, and is required for successful automatic self-pollination in certain flowers in Arabidopsis thaliana

Cell Type-Specific Transcriptome of Brassicaceae Stigmatic Papilla Cells From a Combination of Laser Microdissection and RNA Sequencing

Pollination is an early and critical step in plant reproduction, leading to successful fertilization. It consists of many sequential processes, including adhesion of pollen grains onto the surface of stigmatic papilla cells, foot formation to strengthen pollen-stigma interaction, pollen hydration and germination, and pollen tube elongation and penetration. We have focused on an examination of the expressed genes in papilla cells, to increase understanding of the molecular systems of pollination. From three representative species of Brassicaceae (Arabidopsis thaliana, A. halleri and Brassica rapa), stigmatic papilla cells were isolated precisely by laser microdissection, and cell type-specific gene expression in papilla cells was determined by RNA sequencing. As a result, 17,240, 19,260 and 21,026 unigenes were defined in papilla cells of A. thaliana, A. halleri and B. rapa, respectively, and, among these, 12,311 genes were common to all three species. Among the17,240 genes predicted in A. thaliana, one-third were papilla specific while approximately half of the genes were detected in all tissues examined. Bioinformatics analysis revealed that genes related to a wide range of reproduction and development functions are expressed in papilla cells, particularly metabolism, transcription and membrane-mediated information exchange. These results reflect the conserved features of general cellular function and also the specific reproductive role of papilla cells, highlighting a complex cellular system regulated by a diverse range of molecules in these cells. This study provides fundamental biological knowledge to dissect the molecular mechanisms of pollination in papilla cells and will shed light on our understanding of plant reproduction mechanism

Development of Full-Length cDNAs from Chinese Cabbage (Brassica rapa Subsp. pekinensis) and Identification of Marker Genes for Defence Response

Arabidopsis belongs to the Brassicaceae family and plays an important role as a model plant for which researchers have developed fine-tuned genome resources. Genome sequencing projects have been initiated for other members of the Brassicaceae family. Among these projects, research on Chinese cabbage (Brassica rapa subsp. pekinensis) started early because of strong interest in this species. Here, we report the development of a library of Chinese cabbage full-length cDNA clones, the RIKEN BRC B. rapa full-length cDNA (BBRAF) resource, to accelerate research on Brassica species. We sequenced 10 000 BBRAF clones and confirmed 5476 independent clones. Most of these cDNAs showed high homology to Arabidopsis genes, but we also obtained more than 200 cDNA clones that lacked any sequence homology to Arabidopsis genes. We also successfully identified several possible candidate marker genes for plant defence responses from our analysis of the expression of the Brassica counterparts of Arabidopsis marker genes in response to salicylic acid and jasmonic acid. We compared gene expression of these markers in several Chinese cabbage cultivars. Our BBRAF cDNA resource will be publicly available from the RIKEN Bioresource Center and will help researchers to transfer Arabidopsis-related knowledge to Brassica crops

Electrical properties and gustatory responses of various taste disk cells of frog fungiform papillae

We compared the electrical properties and gustatory response profiles of types Ia cell (mucus cell), Ib cell (wing cell), and II/III cell (receptor cell) in the taste disks of the frog fungiform papillae. The large depolarizing responses of all types of cell induced by 1 M NaCl were accompanied by a large decrease in the membrane resistance and had the same reversal potential of approximately +5 mV. The large depolarizing responses of all cell types for 1 mM acetic acid were accompanied by a small decrease in the membrane resistance. The small depolarizing responses of all cell types for 10 mM quinine--HCl (Q-HCl) were accompanied by an increase in the membrane resistance, but those for 1 M sucrose were accompanied by a decrease in the membrane resistance. The reversal potential of sucrose responses in all cell types were approximately +12 mV. Taken together, depolarizing responses of Ia, Ib, and II/III cells for each taste stimulus are likely to be generated by the same mechanisms. Gustatory depolarizing response profiles indicated that 1) each of Ia, Ib, and II/III cells responded 100% to 1 M NaCl and 1 mM acetic acid with depolarizing responses, 2) approximately 50% of each cell type responded to 10 mM Q-HCl with depolarizations, and 3) each approximately 40% of Ia and Ib cells and approximately 90% of II/III cells responded to 1 M sucrose with depolarizations. These results suggest that the receptor molecules for NaCl, acid, and Q-HCl stimuli are equivalently distributed on all cell types, but the receptor molecules for sugar stimuli are richer on II/III cells than on Ia and Ib cells. Type III cells having afferent synapses may play a main role in gustatory transduction and transmission