Thomas Städler

Genomic analyses provide insights into the history of tomato breeding

The histories of crop domestication and breeding are recorded in genomes. Although tomato is a model species for plant biology and breeding, the nature of human selection that altered its genome remains largely unknown. Here we report a comprehensive analysis of tomato evolution based on the genome sequences of 360 accessions. We provide evidence that domestication and improvement focused on two independent sets of quantitative trait loci (QTLs), resulting in modern tomato fruit ∼100 times larger than its ancestor. Furthermore, we discovered a major genomic signature for modern processing tomatoes, identified the causative variants that confer pink fruit color and precisely visualized the linkage drag associated with wild introgressions. This study outlines the accomplishments as well as the costs of historical selection and provides molecular insights toward further improvement.

The Impact of Sampling Schemes on the Site Frequency Spectrum in Nonequilibrium Subdivided Populations

Using coalescent simulations, we study the impact of three different sampling schemes on patterns of neutral diversity in structured populations. Specifically, we are interested in two summary statistics based on the site frequency spectrum as a function of migration rate, demographic history of the entire substructured population (including timing and magnitude of specieswide expansions), and the sampling scheme. Using simulations implementing both finite-island and two-dimensional stepping-stone spatial structure, we demonstrate strong effects of the sampling scheme on Tajimas D (DT) and Fu and Lis D (DFL) statistics, particularly under specieswide (range) expansions. Pooled samples yield average DT and DFL values that are generally intermediate between those of local and scattered samples. Local samples (and to a lesser extent, pooled samples) are influenced by local, rapid coalescence events in the underlying coalescent process. These processes result in lower proportions of external branch lengths and hence lower proportions of singletons, explaining our finding that the sampling scheme affects DFL more than it does DT. Under specieswide expansion scenarios, these effects of spatial sampling may persist up to very high levels of gene flow (Nm > 25), implying that local samples cannot be regarded as being drawn from a panmictic population. Importantly, many data sets on humans, Drosophila, and plants contain signatures of specieswide expansions and effects of sampling scheme that are predicted by our simulation results. This suggests that validating the assumption of panmixia is crucial if robust demographic inferences are to be made from local or pooled samples. However, future studies should consider adopting a framework that explicitly accounts for the genealogical effects of population subdivision and empirical sampling schemes.

A genomic variation map provides insights into the genetic basis of cucumber domestication and diversity

Most fruits in our daily diet are the products of domestication and breeding. Here we report a map of genome variation for a major fruit that encompasses ∼3.6 million variants, generated by deep resequencing of 115 cucumber lines sampled from 3,342 accessions worldwide. Comparative analysis suggests that fruit crops underwent narrower bottlenecks during domestication than grain crops. We identified 112 putative domestication sweeps; 1 of these regions contains a gene involved in the loss of bitterness in fruits, an essential domestication trait of cucumber. We also investigated the genomic basis of divergence among the cultivated populations and discovered a natural genetic variant in a β-carotene hydroxylase gene that could be used to breed cucumbers with enhanced nutritional value. The genomic history of cucumber evolution uncovered here provides the basis for future genomics-enabled breeding.

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.

Fitness effects of derived deleterious mutations in four closely related wild tomato species with spatial structure.

A key issue in evolutionary biology is an improved understanding of the genetic mechanisms by which species adapt to various environments. Using DNA sequence data, it is possible to quantify the number of adaptive and deleterious mutations, and the distribution of fitness effects of new mutations (its mean and variance) by simultaneously taking into account the demography of a given species. We investigated how selection functions at eight housekeeping genes of four closely related, outcrossing species of wild tomatoes that are native to diverse environments in western South America (Solanum arcanum, S. chilense, S. habrochaites and S. peruvianum). We found little evidence for adaptive mutations but pervasive evidence for strong purifying selection in coding regions of the four species. In contrast, the strength of purifying selection seems to vary among the four species in non-coding (NC) regions (introns). Using FST-based measures of fixation in subdivided populations, we suggest that weak purifying selection has affected the NC regions of S. habrochaites, S. chilense and S. peruvianum. In contrast, NC regions in S. arcanum show a distribution of fitness effects with mutations being either nearly neutral or very strongly deleterious. These results suggest that closely related species with similar genetic backgrounds but experiencing contrasting environments differ in the variance of deleterious fitness effects.

Genomic Imprinting in the Endosperm Is Systematically Perturbed in Abortive Hybrid Tomato Seeds.

Hybrid seed failure represents an important postzygotic barrier to interbreeding among species of wild tomatoes (Solanum section Lycopersicon) and other flowering plants. We studied genome-wide changes associated with hybrid seed abortion in the closely related Solanum peruvianum and S. chilense where hybrid crosses yield high proportions of inviable seeds due to endosperm failure and arrested embryo development. Based on differences of seed size in reciprocal hybrid crosses and developmental evidence implicating endosperm failure, we hypothesized that perturbed genomic imprinting is involved in this strong postzygotic barrier. Consequently, we surveyed the transcriptomes of developing endosperms from intra- and inter-specific crosses using tissues isolated by laser-assisted microdissection. We implemented a novel approach to estimate parent-of-origin–specific expression using both homozygous and heterozygous nucleotide differences between parental individuals and identified candidate imprinted genes. Importantly, we uncovered systematic shifts of “normal” (intraspecific) maternal:paternal transcript proportions in hybrid endosperms; the average maternal proportion of gene expression increased in both crossing directions but was stronger with S. peruvianum in the maternal role. These genome-wide shifts almost entirely eliminated paternally expressed imprinted genes in S. peruvianum hybrid endosperm but also affected maternally expressed imprinted genes and all other assessed genes. These profound, systematic changes in parental expression proportions suggest that core processes of transcriptional regulation are functionally compromised in hybrid endosperm and contribute to hybrid seed failure.

Estimating parameters of speciation models based on refined summaries of the joint site-frequency spectrum

Understanding the processes and conditions under which populations diverge to give rise to distinct species is a central question in evolutionary biology. Since recently diverged populations have high levels of shared polymorphisms, it is challenging to distinguish between recent divergence with no (or very low) inter-population gene flow and older splitting events with subsequent gene flow. Recently published methods to infer speciation parameters under the isolation-migration framework are based on summarizing polymorphism data at multiple loci in two species using the joint site-frequency spectrum (JSFS). We have developed two improvements of these methods based on a more extensive use of the JSFS classes of polymorphisms for species with high intra-locus recombination rates. First, using a likelihood based method, we demonstrate that taking into account low-frequency polymorphisms shared between species significantly improves the joint estimation of the divergence time and gene flow between species. Second, we introduce a local linear regression algorithm that considerably reduces the computational time and allows for the estimation of unequal rates of gene flow between species. We also investigate which summary statistics from the JSFS allow the greatest estimation accuracy for divergence time and migration rates for low (around 10) and high (around 100) numbers of loci. Focusing on cases with low numbers of loci and high intra-locus recombination rates we show that our methods for the estimation of divergence time and migration rates are more precise than existing approaches.

Adaptive Reduction of Male Gamete Number in a Selfing Species

The number of male gametes produced is critical for reproductive success and varies greatly between and within species1–3. Evolutionary reduction of male gamete production has been widely reported in plants as a hallmark of the selfing syndrome, as well as in humans. Such a reduction may simply represent deleterious decay4–7, but evolutionary theory predicts that breeding systems could act as a major selective force on male gamete number: while large numbers of sperm should be produced in highly promiscuous species because of male–male gamete competition1, reduced sperm numbers may be advantageous at lower outcrossing rates because of the cost of gamete production. Here we used genome-wide association study (GWAS) to show a signature of polygenic selection on pollen number in the predominantly selfing plant Arabidopsis thaliana. The top associations with pollen number were significantly more strongly enriched for signatures of selection than those for ovule number and 107 phenotypes analyzed previously, indicating polygenic selection8. Underlying the strongest association, responsible for 20% of total pollen number variation, we identified the gene REDUCED POLLEN NUMBER 1 affecting cell proliferation in the male germ line. We validated its subtle but causal allelic effects using a quantitative complementation test with CRISPR-Cas9-generated null mutants in a nonstandard wild accession. Our results support polygenic adaptation underlying reduced male gamete numbers.

Wild tomato endosperm transcriptomes reveal common roles of genomic imprinting in both nuclear and cellular endosperm

Genomic imprinting is a conspicuous feature of the endosperm, a triploid tissue nurturing the embryo and synchronizing angiosperm seed development. An unknown subset of imprinted genes (IGs) is critical for successful seed development and should have highly conserved functions. Recent genome-wide studies have found limited conservation of IGs among distantly related species, but there is a paucity of data from closely related lineages. Moreover, most studies focused on model plants with nuclear endosperm development, and comparisons with properties of IGs in cellular-type endosperm development are lacking. Using laser-assisted microdissection, we characterized parent-specific expression in the cellular endosperm of three wild tomato lineages (Solanum section Lycopersicon). We identified 1025 candidate IGs and 167 with putative homologs previously identified as imprinted in distantly related taxa with nuclear-type endosperm. Forty-two maternally expressed genes (MEGs) and 17 paternally expressed genes (PEGs) exhibited conserved imprinting status across all three lineages, but differences in power to assess imprinted expression imply that the actual degree of conservation might be higher than that directly estimated (20.7% for PEGs and 10.4% for MEGs). Regardless, the level of shared imprinting status was higher for PEGs than for MEGs, indicating dissimilar evolutionary trajectories. Expression-level data suggest distinct epigenetic modulation of MEGs and PEGs, and gene ontology analyses revealed MEGs and PEGs to be enriched for different functions. Importantly, our data provide evidence that MEGs and PEGs interact in modulating both gene expression and the endosperm cell cycle, and uncovered conserved cellular functions of IGs uniting taxa with cellular- and nuclear-type endosperm.

Molecular ecology in Vienna: hot topics in a chilly place

The international VIPCA conference on ‘Molecular Ecology’ took place in Vienna, Austria, in early February 2012. The meeting showcased the diversity of molecular tools and conceptual approaches at the disposal of practitioners in this flourishing field, which lies at the interface of ecology, evolution and molecular biology.