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The foundation of ancient, invented writing systems lies in the predominant iconicity of their sign shapes. However, these shapes are often used not for their referential meaning but in a metaphorical way, whereby one entity stands for another. Metaphor, including its subcategories pars pro toto and metonymy, plays a crucial role in the formation of the earliest pristine invented scripts, yet this mechanism has been understudied from a cognitive, contextual, and comparative perspective. This article aims to address issues pertaining to the definition, development, and application of these mechanisms in the formation of the Mesopotamian, Egyptian, and Chinese scripts. We analyze the local cases of metaphor-in-action in primary inventions, focusing first on visual metaphors and, second, on the typical or idiosyncratic uses of metonyms., (© 2024 The Author(s). Topics in Cognitive Science published by Wiley Periodicals LLC on behalf of Cognitive Science Society.)

Purpose: Small cell lung cancer (SCLC) has an extremely poor prognosis. Despite high initial response rates to chemotherapy and modest survival improvements with the addition of immune checkpoint inhibitors (ICI), almost all patients experience relapse and fatal outcomes. Recent genomic insights uncovered extensive molecular heterogeneity in addition to the almost uniform loss of RB1 and TRP53. Additionally, defective DNA mismatch repair (MMR) has recently been described in some SCLC cases. Here, we generated a novel SCLC mouse model capturing MMR deficiency and assessed immunotherapy responses., Methods: We developed an MMR-deficient genetically engineered mouse model (GEMM) of SCLC by introducing a conditional Msh2 gene, crucial for maintaining MMR integrity, into the standard Rb1 fl/fl ;Trp53 fl/fl (RP) model. Genomic characteristics and preclinical therapy responses were evaluated by focusing on overall survival and whole exome sequencing (WES) analyses., Results: MMR-defective SCLC tumors (Rb1 fl/fl ;Trp53 fl/fl ;Msh2 fl/fl (RPM)) developed later than tumors in MMR-proficient mice. However, the time from tumor manifestation to death of the affected animals was substantially shortened (median survival 55 days in RP vs. 46.5 days in RPM), indicating increased aggressiveness of MMR-defective tumors. RPM tumors exhibited MMR deficiency, high tumor mutational burden (TMB), and an elevated load of candidate neoantigens, compared to RP lesions (p = 0.0106), suggesting increased immunogenicity. Importantly, the overall survival of RPM animals was significantly improved when exposed to ICI., Conclusion: We propose a novel RPM mouse model as a suitable system to mimic MMR-defective SCLC and tumors with high TMB. We provide in vivo evidence that Msh2 deficiency enhances ICI sensitivity. These findings could contribute to stratifying SCLC patients to immunotherapy, thereby improving treatment outcomes., Competing Interests: Declarations Conflict of interest H.C.R. received consulting and lecture fees from Abbvie, AstraZeneca, Vertex, and Merck. H.C.R. received research funding from AstraZeneca and Gilead Pharmaceuticals. H.C.R. is a co-founder of CDL Therapeutics GmbH. R.K.T. is a founder of Disco Pharmaceuticals GmbH, PearlRiver Bio (now part of Centessa), a shareholder of Centessa, founder and shareholder of Epiphanes Inc. and a consultant to PearlRiver Bio and Epiphanes Inc. R.K.T. has received research support from Roche. The remaining authors declare no competing financial interest. Animal ethics declaration Animal experiments in this study were approved by the local Ethics Committee of Animal Experiments authorities (LANUV, North Rhine-Westphalia, Germany) under license number 81-02.04.2019-A491. All mice were maintained according to FELASA recommendations and in compliance with the European Union and German guidelines., (© 2024. The Author(s).)

Purpose: The clinical course of pulmonary carcinoids ranges from indolent to fatal disease, suggesting that specific molecular alterations drive progression toward the fully malignant state. A similar spectrum of clinical phenotypes occurs in pediatric neuroblastoma, in which activation of telomerase reverse transcriptase ( TERT ) is decisive in determining the course of disease. We therefore investigated whether TERT expression defines the clinical fate of patients with pulmonary carcinoid., Methods: TERT expression was examined by RNA sequencing in a test cohort and a validation cohort of pulmonary carcinoids (n = 88 and n = 105, respectively). A natural TERT expression cutoff was determined in the test cohort on the basis of the distribution of TERT expression, and its prognostic value was assessed by Kaplan-Meier survival estimates and multivariable analyses. Telomerase activity was validated by telomere repeat amplification protocol assay., Results: Similar to neuroblastoma, TERT expression exhibited a bimodal distribution in pulmonary carcinoids, separating tumors into TERT -high and TERT- low subgroups. A natural TERT cutoff discriminated unfavorable from favorable clinical courses with high accuracy both in the test cohort (5-year overall survival [OS], 0.547 ± 0.132 v 1.0; P < .001) and the validation cohort (5-year OS, 0.788 ± 0.063 v 0.913 ± 0.048; P < .001). In line with these findings, telomerase activity was largely absent in TERT -low tumors, whereas it was readily detectable in TERT- high carcinoids. In multivariable analysis considering TERT expression, histology (typical v atypical carcinoid), and stage (≤IIA v ≥IIB), high TERT expression was an independent prognostic marker for poor survival, with a hazard ratio of 5.243 (95% CI, 1.943 to 14.148; P = .001)., Conclusion: Our data demonstrate that high TERT expression defines clinically aggressive pulmonary carcinoids with fatal outcome, similar to neuroblastoma, indicating that activation of TERT may be a defining feature of lethal cancers.

The evolutionary processes that underlie the marked sensitivity of small cell lung cancer (SCLC) to chemotherapy and rapid relapse are unknown 1-3 . Here we determined tumour phylogenies at diagnosis and throughout chemotherapy and immunotherapy by multiregion sequencing of 160 tumours from 65 patients. Treatment-naive SCLC exhibited clonal homogeneity at distinct tumour sites, whereas first-line platinum-based chemotherapy led to a burst in genomic intratumour heterogeneity and spatial clonal diversity. We observed branched evolution and a shift to ancestral clones underlying tumour relapse. Effective radio- or immunotherapy induced a re-expansion of founder clones with acquired genomic damage from first-line chemotherapy. Whereas TP53 and RB1 alterations were exclusively part of the common ancestor, MYC family amplifications were frequently not constituents of the founder clone. At relapse, emerging subclonal mutations affected key genes associated with SCLC biology, and tumours harbouring clonal CREBBP/EP300 alterations underwent genome duplications. Gene-damaging TP53 alterations and co-alterations of TP53 missense mutations with TP73, CREBBP/EP300 or FMN2 were significantly associated with shorter disease relapse following chemotherapy. In summary, we uncover key processes of the genomic evolution of SCLC under therapy, identify the common ancestor as the source of clonal diversity at relapse and show central genomic patterns associated with sensitivity and resistance to chemotherapy., (© 2024. The Author(s).)

The emergence of resistance to targeted therapies restrains their efficacy. The development of rationally guided drug combinations could overcome this currently insurmountable clinical challenge. However, our limited understanding of the trajectories that drive the outgrowth of resistant clones in cancer cell populations precludes design of drug combinations to forestall resistance. Here, we propose an iterative treatment strategy coupled with genomic profiling and genome-wide CRISPR activation screening to systematically extract and define preexisting resistant subpopulations in an EGFR-driven lung cancer cell line. Integrating these modalities identifies several resistance mechanisms, including activation of YAP/TAZ signaling by WWTR1 amplification, and estimates the associated cellular fitness for mathematical population modeling. These observations led to the development of a combination therapy that eradicated resistant clones in large cancer cell line populations by exhausting the spectrum of genomic resistance mechanisms. However, a small fraction of cancer cells was able to enter a reversible nonproliferative state of drug tolerance. This subpopulation exhibited mesenchymal properties, NRF2 target gene expression, and sensitivity to ferroptotic cell death. Exploiting this induced collateral sensitivity by GPX4 inhibition clears drug-tolerant populations and leads to tumor cell eradication. Overall, this experimental in vitro data and theoretical modeling demonstrate why targeted mono- and dual therapies will likely fail in sufficiently large cancer cell populations to limit long-term efficacy. Our approach is not tied to a particular driver mechanism and can be used to systematically assess and ideally exhaust the resistance landscape for different cancer types to rationally design combination therapies., Significance: Unraveling the trajectories of preexisting resistant and drug-tolerant persister cells facilitates the rational design of multidrug combination or sequential therapies, presenting an approach to explore for treating EGFR-mutant lung cancer., (©2023 American Association for Cancer Research.)

We study natural DNA polymorphisms and associated phenotypes in the Arabidopsis relative Cardamine hirsuta. We observed strong genetic differentiation among several ancestry groups and broader distribution of Iberian relict strains in European C. hirsuta compared to Arabidopsis. We found synchronization between vegetative and reproductive development and a pervasive role for heterochronic pathways in shaping C. hirsuta natural variation. A single, fast-cycling ChFRIGIDA allele evolved adaptively allowing range expansion from glacial refugia, unlike Arabidopsis where multiple FRIGIDA haplotypes were involved. The Azores islands, where Arabidopsis is scarce, are a hotspot for C. hirsuta diversity. We identified a quantitative trait locus (QTL) in the heterochronic SPL9 transcription factor as a determinant of an Azorean morphotype. This QTL shows evidence for positive selection, and its distribution mirrors a climate gradient that broadly shaped the Azorean flora. Overall, we establish a framework to explore how the interplay of adaptation, demography, and development shaped diversity patterns of 2 related plant species., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Baumgarten et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Background: Genomic alterations of the anaplastic lymphoma kinase gene (ALK) occur recurrently in neuroblastoma, a pediatric malignancy of the sympathetic nervous system. However, information on their development over time has remained sparse., Methods: ALK alterations were assessed in neuroblastomas at diagnosis and/or relapse from a total of 943 patients, covering all stages of disease. Longitudinal information on diagnostic and relapsed samples from individual patients was available in 101 and 102 cases for mutation and amplification status, respectively., Results: At diagnosis, ALK point mutations occurred in 10.5% of all cases, with highest frequencies in stage 4 patients <18 months. At relapse, ALK alteration frequency increased by 70%, both in high-risk and non-high-risk cases. The increase was most likely due to de novo mutations, frequently leading to R1275Q substitutions, which are sensitive to pharmacological ALK inhibition. By contrast, the frequency of ALK amplifications did not change over the course of the disease. ALK amplifications, but not mutations, were associated with poor patient outcome., Conclusions: The considerably increased frequency of ALK mutations at relapse and their high prevalence in young stage 4 patients suggest surveying the genomic ALK status regularly in these patient cohorts, and to evaluate ALK-targeted treatment also in intermediate-risk patients., (© 2023. The Author(s).)

The mechanisms behind the evolution of complex genomic amplifications in cancer have remained largely unclear. Using whole-genome sequencing data of the pediatric tumor neuroblastoma, we here identified a type of amplification, termed 'seismic amplification', that is characterized by multiple rearrangements and discontinuous copy number levels. Overall, seismic amplifications occurred in 9.9% (274 of 2,756) of cases across 38 cancer types, and were associated with massively increased copy numbers and elevated oncogene expression. Reconstruction of the development of seismic amplification showed a stepwise evolution, starting with a chromothripsis event, followed by formation of circular extrachromosomal DNA that subsequently underwent repetitive rounds of circular recombination. The resulting amplicons persisted as extrachromosomal DNA circles or had reintegrated into the genome in overt tumors. Together, our data indicate that the sequential occurrence of chromothripsis and circular recombination drives oncogene amplification and overexpression in a substantial fraction of human malignancies., (© 2021. The Author(s), under exclusive licence to Springer Nature America, Inc.)

The spectrum of somatic genetic variation in colorectal adenomas caused by biallelic pathogenic germline variants in the MSH3 gene, was comprehensively analysed to characterise mutational signatures and identify potential driver genes and pathways of MSH3-related tumourigenesis. Three patients from two families with MSH3-associated polyposis were included. Whole exome sequencing of nine adenomas and matched normal tissue was performed. The amount of somatic variants in the MSH3-deficient adenomas and the pattern of single nucleotide variants (SNVs) was similar to sporadic adenomas, whereas the fraction of small insertions/deletions (indels) (21-42% of all small variants) was significantly higher. Interestingly, pathogenic somatic APC variants were found in all but one adenoma. The vast majority (12/13) of these were di-, tetra-, or penta-base pair (bp) deletions. The fraction of APC indels was significantly higher than that reported in patients with familial adenomatous polyposis (FAP) (p < 0.01) or in sporadic adenomas (p < 0.0001). In MSH3-deficient adenomas, the occurrence of APC indels in a repetitive sequence context was significantly higher than in FAP patients (p < 0.01). In addition, the MSH3-deficient adenomas harboured one to five (recurrent) somatic variants in 13 established or candidate driver genes for early colorectal carcinogenesis, including ACVR2A and ARID genes. Our data suggest that MSH3-related colorectal carcinogenesis seems to follow the classical APC-driven pathway. In line with the specific function of MSH3 in the mismatch repair (MMR) system, we identified a characteristic APC mutational pattern in MSH3-deficient adenomas, and confirmed further driver genes for colorectal tumourigenesis., Competing Interests: The authors have declared that no competing interests exist.

Due to the redundancy of the genetic code most amino acids are encoded by multiple synonymous codons. It has been proposed that a biased frequency of synonymous codons can affect the function of proteins by modulating distinct steps in transcription, translation and folding. Here, we use two similar prototype K + channels as model systems to examine whether codon choice has an impact on protein sorting. By monitoring transient expression of GFP-tagged channels in mammalian cells, we find that one of the two channels is sorted in a codon and cell cycle-dependent manner either to mitochondria or the secretory pathway. The data establish that a gene with either rare or frequent codons serves, together with a cell-state-dependent decoding mechanism, as a secondary code for sorting intracellular membrane proteins.

The identification of the mutational processes operating in tumour cells has implications for cancer diagnosis and therapy. These processes leave mutational patterns on the cancer genomes, which are referred to as mutational signatures. Recently, 81 mutational signatures have been inferred using computational algorithms on sequencing data of 23,879 samples. However, these published signatures may not always offer a comprehensive view on the biological processes underlying tumour types that are not included or underrepresented in the reference studies. To circumvent this problem, we designed CaMuS (Cancer Mutational Signatures) to construct de novo signatures while simultaneously fitting publicly available mutational signatures. Furthermore, we propose to estimate signature similarity by comparing probability distributions using the Hellinger distance. We applied CaMuS to infer signatures of mutational processes in poorly studied cancer types. We used whole genome sequencing data of 56 neuroblastoma, thus providing evidence for the versatility of CaMuS. Using simulated data, we compared the performance of CaMuS to sigfit, a recently developed algorithm with comparable inference functionalities. CaMuS and sigfit reconstructed the simulated datasets with similar accuracy; however two main features may argue for CaMuS over sigfit: (i) superior computational performance and (ii) a reliable parameter selection method to avoid spurious signatures.

Purpose: Pleomorphic dermal sarcoma (PDS) is a rare malignant cutaneous tumor with an unknown cell of origin. Locally defined tumors can be treated by curative excisions, whereas advanced stages of the disease are difficult to treat, using standard regimens., Experimental Design: We performed whole-exome sequencing on a cohort of 28 individuals and corresponding transcriptomic analysis on 21 patients, as well as quantitative IHC image analysis on 27 patients., Results: PDS exhibits a universally high mutational load (42.7 mutations/mega base) with an inflamed, immunogenic tumor microenvironment. Three cases of PDS showed response to immune checkpoint blockade. Local mutation rate variation together with mRNA expression data demonstrate that PDS form a distinct entity, with PDGFRB as a lineage marker. In addition, we found that PDS is of mesenchymal, fibroblastic differentiation., Conclusions: PDS is of fibroblastic differentiation and exhibits a strong susceptibility to immunotherapy, including a high mutational burden and an inflamed tumor microenvironment., (©2020 American Association for Cancer Research.)

Neuroblastoma is a pediatric tumor of the sympathetic nervous system. Its clinical course ranges from spontaneous tumor regression to fatal progression. To investigate the molecular features of the divergent tumor subtypes, we performed genome sequencing on 416 pretreatment neuroblastomas and assessed telomere maintenance mechanisms in 208 of these tumors. We found that patients whose tumors lacked telomere maintenance mechanisms had an excellent prognosis, whereas the prognosis of patients whose tumors harbored telomere maintenance mechanisms was substantially worse. Survival rates were lowest for neuroblastoma patients whose tumors harbored telomere maintenance mechanisms in combination with RAS and/or p53 pathway mutations. Spontaneous tumor regression occurred both in the presence and absence of these mutations in patients with telomere maintenance-negative tumors. On the basis of these data, we propose a mechanistic classification of neuroblastoma that may benefit the clinical management of patients., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Background: Accurate and automated phenotyping of leaf images is necessary for high throughput studies of leaf form like genome-wide association analysis and other forms of quantitative trait locus mapping. Dissected leaves (also referred to as compound) that are subdivided into individual units are an attractive system to study diversification of form. However, there are only few software tools for their automated analysis. Thus, high-throughput image processing algorithms are needed that can partition these leaves in their phenotypically relevant units and calculate morphological features based on these units., Results: We have developed MowJoe, an image processing algorithm that dissects a dissected leaf into leaflets, petiolule, rachis and petioles. It employs image skeletonization to convert leaves into graphs, and thereafter applies algorithms operating on graph structures. This partitioning of a leaf allows the derivation of morphological features such as leaf size, or eccentricity of leaflets. Furthermore, MowJoe automatically places landmarks onto the terminal leaflet that can be used for further leaf shape analysis. It generates specific output files that can directly be imported into downstream shape analysis tools. We applied the algorithm to two accessions of Cardamine hirsuta and show that our features are able to robustly discriminate between these accessions., Conclusion: MowJoe is a tool for the semi-automated, quantitative high throughput shape analysis of dissected leaf images. It provides the statistical power for the detection of the genetic basis of quantitative morphological variations.

Deciphering the evolution of cancer cells under therapeutic pressure is a crucial step to understand the mechanisms that lead to treatment resistance. To this end, we analyzed whole-exome sequencing data of eight chronic lymphocytic leukemia (CLL) patients that developed resistance upon BCL2-inhibition by venetoclax. Here, we report recurrent mutations in BTG1 (2 patients) and homozygous deletions affecting CDKN2A/B (3 patients) that developed during treatment, as well as a mutation in BRAF and a high-level focal amplification of CD274 (PD-L1) that might pinpoint molecular aberrations offering structures for further therapeutic interventions.

A conserved genetic toolkit underlies the development of diverse floral forms among angiosperms. However, the degree of conservation vs divergence in the configuration of these gene regulatory networks is less clear. We addressed this question in a parallel genetic study between the closely related species Arabidopsis thaliana and Cardamine hirsuta. We identified leafy (lfy) and apetala1 (ap1) alleles in a mutant screen for floral regulators in C. hirsuta. C. hirsuta lfy mutants showed a complete homeotic conversion of flowers to leafy shoots, mimicking lfy ap1 double mutants in A. thaliana. Through genetic and molecular experiments, we showed that AP1 activation is fully dependent on LFY in C. hirsuta, by contrast to A. thaliana. Additionally, we found that LFY influences heteroblasty in C. hirsuta, such that loss or gain of LFY function affects its progression. Overexpression of UNUSUAL FLORAL ORGANS also alters C. hirsuta leaf shape in an LFY-dependent manner. We found that LFY and AP1 are conserved floral regulators that act nonredundantly in C. hirsuta, such that LFY has more obvious roles in floral and leaf development in C. hirsuta than in A. thaliana., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Finding causal relationships between genotypic and phenotypic variation is a key focus of evolutionary biology, human genetics and plant breeding. To identify genome-wide patterns underlying trait diversity, we assembled a high-quality reference genome of Cardamine hirsuta, a close relative of the model plant Arabidopsis thaliana. We combined comparative genome and transcriptome analyses with the experimental tools available in C. hirsuta to investigate gene function and phenotypic diversification. Our findings highlight the prevalent role of transcription factors and tandem gene duplications in morphological evolution. We identified a specific role for the transcriptional regulators PLETHORA5/7 in shaping leaf diversity and link tandem gene duplication with differential gene expression in the explosive seed pod of C. hirsuta. Our work highlights the value of comparative approaches in genetically tractable species to understand the genetic basis for evolutionary change.

The utility of genome assemblies does not only rely on the quality of the assembled genome sequence, but also on the quality of the gene annotations. The Pacific Biosciences Iso-Seq technology is a powerful support for accurate eukaryotic gene model annotation as it allows for direct readout of full-length cDNA sequences without the need for noisy short read-based transcript assembly. We propose the implementation of the TeloPrime Full Length cDNA Amplification kit to the Pacific Biosciences Iso-Seq technology in order to enrich for genuine full-length transcripts in the cDNA libraries. We provide evidence that TeloPrime outperforms the commonly used SMARTer PCR cDNA Synthesis Kit in identifying transcription start and end sites in Arabidopsis thaliana. Furthermore, we show that TeloPrime-based Pacific Biosciences Iso-Seq can be successfully applied to the polyploid genome of bread wheat (Triticum aestivum) not only to efficiently annotate gene models, but also to identify novel transcription sites, gene homeologs, splicing isoforms and previously unidentified gene loci.

A key problem in biology is whether the same processes underlie morphological variation between and within species. Here, by using plant leaves as an example, we show that the causes of diversity at these two evolutionary scales can be divergent. Some species like the model plant Arabidopsis thaliana have simple leaves, whereas others like the A. thaliana relative Cardamine hirsuta bear complex leaves comprising leaflets. Previous work has shown that these interspecific differences result mostly from variation in local tissue growth and patterning. Now, by cloning and characterizing a quantitative trait locus (QTL) for C. hirsuta leaf shape, we find that a different process, age-dependent progression of leaf form, underlies variation in this trait within species. This QTL effect is caused by cis-regulatory variation in the floral repressor ChFLC, such that genotypes with low-expressing ChFLC alleles show both early flowering and accelerated age-dependent changes in leaf form, including faster leaflet production. We provide evidence that this mechanism coordinates leaf development with reproductive timing and may help to optimize resource allocation to the next generation.

A major goal in biology is to identify the genetic basis for phenotypic diversity. This goal underpins research in areas as diverse as evolutionary biology, plant breeding and human genetics. A limitation for this research is no longer the availability of sequence information but the development of functional genetic tools to understand the link between changes in sequence and phenotype. Here we describe Cardamine hirsuta, a close relative of the reference plant Arabidopsis thaliana, as an experimental system in which genetic and transgenic approaches can be deployed effectively for comparative studies. We present high-resolution genetic and cytogenetic maps for C. hirsuta and show that the genome structure of C. hirsuta closely resembles the eight chromosomes of the ancestral crucifer karyotype and provides a good reference point for comparative genome studies across the Brassicaceae. We compared morphological and physiological traits between C. hirsuta and A. thaliana and analysed natural variation in stamen number in which lateral stamen loss is a species characteristic of C. hirsuta. We constructed a set of recombinant inbred lines and detected eight quantitative trait loci that can explain stamen number variation in this population. We found clear phylogeographic structure to the genetic variation in C. hirsuta, thus providing a context within which to address questions about evolutionary changes that link genotype with phenotype and the environment., (© 2014 The Authors The Plant Journal © 2014 John Wiley & Sons Ltd.)

Leaves show considerable variation in shape, and may be described as simple, when the leaf is entire, or dissected, when the leaf is divided into individual leaflets. Here, we report that the SIMPLE LEAF3 (SIL3) gene is a novel determinant of leaf shape in Cardamine hirsuta - a dissected-leaved relative of the simple-leaved model species Arabidopsis thaliana. We show that SIL3 is required for leaf growth and leaflet formation but leaf initiation is less sensitive to perturbation of SIL3 activity. SIL3 is further required for KNOX (knotted1-like homeobox) gene expression and localized auxin activity maxima, both of which are known to promote leaflet formation. We cloned SIL3 and showed that it encodes RLI2 (RNase L inhibitor 2), an ATP binding cassette-type ATPase with important roles in ribosome recycling and translation termination that are conserved in eukaryotes and archaea. RLI mutants have not been described in plants to date, and this paper highlights the potential of genetic studies in C. hirsuta to uncover novel gene functions. Our data indicate that leaflet development is sensitive to perturbation of RLI2-dependent aspects of cellular growth, and link ribosome function with dissected-leaf development., (© 2012 The Authors The Plant Journal © 2012 Blackwell Publishing Ltd.)

Morphological diversity is often caused by altered gene expression of key developmental regulators. However, the precise developmental trajectories through which morphologies evolved remain poorly understood. It is also unclear to what degree genetic changes contributing to morphological divergence were fixed by natural selection. Here we investigate these problems in the context of evolutionary developmental transitions that produced the simple unlobed leaf of the model species Arabidopsis thaliana. We demonstrate that A. thaliana leaf shape likely derived from a more complex lobed ancestral state that persists in extant Arabidopsis species. We also show that evolution of the unlobed leaf form in A. thaliana involved loss of expression of the knotted1-like homeobox gene SHOOTMERISTEMLESS (STM) in leaves and that cis-regulatory divergence contributed to this process. Further, we provide evidence for a selective sweep at the A. thaliana STM locus, indicating that loss of STM expression in A. thaliana leaves may have been fixed by positive selection. In summary, our data provide key information as to when and how the characteristic leaf form of A. thaliana evolved.

Spatial regulation of C-function genes controlling reproductive organ identity in the centre of the flower can be achieved by adjusting the level of their expression within the genuine central expression domain in Antirrhinum and Petunia. Loss of this control in mutants is revealed by enhanced C-gene expression in the centre and by lateral expansion of the C-domain. In order to test whether the level of central C-gene expression and hence the principle of 'regulation by tuning' also applies to spatial regulation of the C-function gene AGAMOUS (AG) in Arabidopsis, we generated transgenic plants with enhanced central AG expression by using stem cell-specific CLAVATA3 (CLV3) regulatory sequences to drive transcription of the AG cDNA. The youngest terminal flowers on inflorescences of CLV3::AG plants displayed homeotic features in their outer whorls indicating ectopic AG expression. Dependence of the homeotic feature on the age of the plant is attributed to the known overall weakening of repressive mechanisms controlling AG. Monitoring AG with an AG-I::GUS reporter construct suggests ectopic AG expression in CLV3::AG flowers when AG in the inflorescence is still repressed, although in terminating inflorescence meristems, AG expression expands to all tissues. Supported by genetic tests, we conclude that upon enhanced central AG expression, the C-domain laterally expands necessitating tuning of the expression level of C-function genes in the wild type. The tuning mechanism in C-gene regulation in Arabidopsis is discussed as a late security switch that ensures wild-type C-domain control when other repressive mechanism starts to fade and fail.

It is commonly thought that deep phylogenetic conservation of plant microRNAs (miRNAs) and their targets indicates conserved regulatory functions. We show that the blind (bl) mutant of Petunia hybrida and the fistulata (fis) mutant of Antirrhinum majus, which have similar homeotic phenotypes, are recessive alleles of two homologous miRNA-encoding genes. The BL and FIS genes control the spatial restriction of homeotic class C genes to the inner floral whorls, but their ubiquitous early floral expression patterns are in contradiction with a potential role in patterning C gene expression. We provide genetic evidence for the unexpected function of the MIRFIS and MIRBL genes in the center of the flower and propose a dynamic mechanism underlying their regulatory role. Notably, Arabidopsis thaliana, a more distantly related species, also contains this miRNA module but does not seem to use it to confine early C gene expression to the center of the flower.