Loss of daylength sensitivity by splice site mutation in Cannabis pseudo-response regulator.

Photoperiod insensitivity (auto-flowering) in drug-type Cannabis sativa circumvents the need for short day (SD) flowering requirements making outdoor cultivation in high latitudes possible. However, the benefits of photoperiod insensitivity are counterbalanced by low cannabinoid content and poor flower quality in auto-flowering genotypes. Despite recent studies in cannabis flowering, a mechanistic understanding of photoperiod insensitivity is still lacking. We used a combination of genome-wide association study and genetic fine-mapping to identify the genetic cause of auto-flowering in cannabis. We then used gene expression analyses and transient transformation assays to characterize flowering time control. Herein, we identify a splice site mutation within circadian clock gene PSEUDO-RESPONSE REGULATOR 37 (CsPRR37) in auto-flowering cannabis. We show that CsPRR37 represses FT expression and its circadian oscillations transition to a less repressive state during SD as compared to long days (LD). We identify several key circadian clock genes whose expression is altered in auto-flowering cannabis, particularly under non-inductive LD. Research into the pervasiveness of this mutation and others affecting flowering time will help elucidate cannabis domestication history and advance cannabis breeding toward a more sustainable outdoor cultivation system.

Both mechanism and age of duplications contribute to biased gene retention patterns in plants.

BACKGROUND: All extant seed plants are successful paleopolyploids, whose genomes carry duplicate genes that have survived repeated episodes of diploidization. However, the survival of gene duplicates is biased with respect to gene function and mechanism of duplication. Transcription factors, in particular, are reported to be preferentially retained following whole-genome duplications (WGDs), but disproportionately lost when duplicated by tandem events. An explanation for this pattern is provided by the Gene Balance Hypothesis (GBH), which posits that duplicates of highly connected genes are retained following WGDs to maintain optimal stoichiometry among gene products; but such connected gene duplicates are disfavored following tandem duplications. RESULTS: We used genomic data from 25 taxonomically diverse plant species to investigate the roles of duplication mechanism, gene function, and age of duplication in the retention of duplicate genes. Enrichment analyses were conducted to identify Gene Ontology (GO) functional categories that were overrepresented in either WGD or tandem duplications, or across ranges of divergence times. Tandem paralogs were much younger, on average, than WGD paralogs and the most frequently overrepresented GO categories were not shared between tandem and WGD paralogs. Transcription factors were overrepresented among ancient paralogs regardless of mechanism of origin or presence of a WGD. Also, in many cases, there was no bias toward transcription factor retention following recent WGDs. CONCLUSIONS: Both the fixation and the retention of duplicated genes in plant genomes are context-dependent events. The strong bias toward ancient transcription factor duplicates can be reconciled with the GBH if selection for optimal stoichiometry among gene products is strongest following the earliest polyploidization events and becomes increasingly relaxed as gene families expand.

The genetic architecture of UV floral patterning in sunflower.

Background and Aims: The patterning of floral ultraviolet (UV) pigmentation varies both intra- and interspecifically in sunflowers and many other plant species, impacts pollinator attraction, and can be critical to reproductive success and crop yields. However, the genetic basis for variation in UV patterning is largely unknown. This study examines the genetic architecture for proportional and absolute size of the UV bullseye in Helianthus argophyllus , a close relative of the domesticated sunflower. Methods: A camera modified to capture UV light (320-380 nm) was used to phenotype floral UV patterning in an F 2 mapping population, then quantitative trait loci (QTL) were identified using genotyping-by-sequencing and linkage mapping. The ability of these QTL to predict the UV patterning of natural population individuals was also assessed. Key Results: Proportional UV pigmentation is additively controlled by six moderate effect QTL that are predictive of this phenotype in natural populations. In contrast, UV bullseye size is controlled by a single large effect QTL that also controls flowerhead size and co-localizes with a major flowering time QTL in Helianthus . Conclusions: The co-localization of the UV bullseye size QTL, flowerhead size QTL and a previously known flowering time QTL may indicate a single highly pleiotropic locus or several closely linked loci, which could inhibit UV bullseye size from responding to selection without change in correlated characters. The genetic architecture of proportional UV pigmentation is relatively simple and different from that of UV bullseye size, and so should be able to respond to natural or artificial selection independently.

Revisiting a classic case of introgression: hybridization and gene flow in Californian sunflowers.

During invasion, colonizing species can hybridize with native species, potentially swamping out native genomes. However, theory predicts that introgression will often be biased into the invading species. Thus, empirical estimates of gene flow between native and invasive species are important to quantify the actual threat of hybridization with invasive species. One classic example of introgression occurs in California, where Helianthus bolanderi was thought to be a hybrid between the serpentine endemic Helianthus exilis and the congeneric invader Helianthus annuus. We used genotyping by sequencing to look for signals of introgression and population structure. We find that H. bolanderi and H. exilis form one genetic clade, with weak population structure that is associated with geographic location rather than soil composition and likely represent a single species, not two. Additionally, while our results confirmed early molecular analysis and failed to support the hybrid origin of H. bolanderi, we did find evidence for introgression mainly into the invader H. annuus, as predicted by theory.

Genome-wide genotyping-by-sequencing data provide a high-resolution view of wild Helianthus diversity, genetic structure, and interspecies gene flow.

PREMISE: Wild sunflowers harbor considerable genetic diversity and are a major resource for improvement of the cultivated sunflower, Helianthus annuus. The Helianthus genus is also well known for its propensity for gene flow between taxa. METHODS: We surveyed genomic diversity of 292 samples of wild Helianthus from 22 taxa that are cross-compatible with the cultivar using genotyping by sequencing. With these data, we derived a high-resolution phylogeny of the taxa, interrogated genome-wide levels of diversity, explored H. annuus population structure, and identified localized gene flow between H. annuus and its close relatives. KEY RESULTS: Our phylogenomic analyses confirmed a number of previously established interspecific relationships and indicated for the first time that a newly described annual sunflower, H. winteri, is nested within H. annuus. Principal component analyses showed that H. annuus has geographic population structure with most notable subpopulations occurring in California and Texas. While gene flow was identified between H. annuus and H. bolanderi in California and between H. annuus and H. argophyllus in Texas, this genetic exchange does not appear to drive observed patterns of H. annuus population structure. CONCLUSIONS: Wild H. annuus remains an excellent resource for cultivated sunflower breeding effort because of its diversity and the ease with which it can be crossed with cultivated H. annuus. Cases of interspecific gene flow such as those documented here also indicate wild H. annuus can act as a bridge to capture alleles from other wild taxa; continued breeding efforts with it may therefore reap the largest rewards.

Genome scans reveal candidate domestication and improvement genes in cultivated sunflower, as well as post-domestication introgression with wild relatives.

The development of modern crops typically involves both selection and hybridization, but to date most studies have focused on the former. In the present study, we explore how both processes, and their interactions, have molded the genome of the cultivated sunflower (Helianthus annuus), a globally important oilseed. To identify genes targeted by selection during the domestication and improvement of sunflower, and to detect post-domestication hybridization with wild species, we analyzed transcriptome sequences of 80 genotypes, including wild, landrace, and modern lines of H. annuus, as well as two cross-compatible wild relatives, Helianthus argophyllus and Helianthus petiolaris. Outlier analyses identified 122 and 15 candidate genes associated with domestication and improvement, respectively. As in several previous studies, genes putatively involved in oil biosynthesis were the most extreme outliers. Additionally, several promising associations were observed with previously mapped quantitative trait loci (QTLs), such as branching. Admixture analyses revealed that all the modern cultivar genomes we examined contained one or more introgressions from wild populations, with every chromosome having evidence of introgression in at least one modern line. Cumulatively, introgressions cover c. 10% of the cultivated sunflower genome. Surprisingly, introgressions do not avoid candidate domestication genes, probably because of the reintroduction of branching.

Genomic sequence and copy number evolution during hybrid crop development in sunflowers.

Hybrid crops, an important part of modern agriculture, rely on the development of male and female heterotic gene pools. In sunflowers, heterotic gene pools were developed through the use of crop-wild relatives to produce cytoplasmic male sterile female and branching, fertility restoring male lines. Here, we use genomic data from a diversity panel of male, female, and open-pollinated lines to explore the genetic changes brought during modern improvement. We find the male lines have diverged most from their open-pollinated progenitors and that genetic differentiation is concentrated in chromosomes, 8, 10 and 13, due to introgressions from wild relatives. Ancestral variation from open-pollinated varieties almost universally evolved in parallel for both male and female lines suggesting little or no selection for heterotic overdominance. Furthermore, we show that gene content differs between the male and female lines and that differentiation in gene content is concentrated in high FST regions. This means that the introgressions that brought branching and fertility restoration to the male lines, brought with them different gene content from the ancestral haplotypes, including the removal of some genes. Although we find no evidence that gene complementation genomewide is responsible for heterosis between male and female lines, several of the genes that are largely absent in either the male or female lines are associated with pathogen defense, suggesting complementation may be functionally relevant for crop breeders.

Sunflower pan-genome analysis shows that hybridization altered gene content and disease resistance.

Domesticated plants and animals often display dramatic responses to selection, but the origins of the genetic diversity underlying these responses remain poorly understood. Despite domestication and improvement bottlenecks, the cultivated sunflower remains highly variable genetically, possibly due to hybridization with wild relatives. To characterize genetic diversity in the sunflower and to quantify contributions from wild relatives, we sequenced 287 cultivated lines, 17 Native American landraces and 189 wild accessions representing 11 compatible wild species. Cultivar sequences failing to map to the sunflower reference were assembled de novo for each genotype to determine the gene repertoire, or 'pan-genome', of the cultivated sunflower. Assembled genes were then compared to the wild species to estimate origins. Results indicate that the cultivated sunflower pan-genome comprises 61,205 genes, of which 27% vary across genotypes. Approximately 10% of the cultivated sunflower pan-genome is derived through introgression from wild sunflower species, and 1.5% of genes originated solely through introgression. Gene ontology functional analyses further indicate that genes associated with biotic resistance are over-represented among introgressed regions, an observation consistent with breeding records. Analyses of allelic variation associated with downy mildew resistance provide an example in which such introgressions have contributed to resistance to a globally challenging disease.

Genomic variation in Helianthus: learning from the past and looking to the future.

Helianthus is an economically important and genetically diverse genus, containing both evolutionary model species and cultivated species. Genetic variation within this genus has been examined at many different scales, from genome size changes to chromosomal structure to nucleotide variation. The growing amount of genomic resources within the genus has yielded insights into the importance of paleopolyploid events, and how transposable elements can cause rapid genome size increases. The rapidly evolving chromosomes in Helianthus have provided a system whereby it has been possible to study how chromosomal rearrangements impact speciation, adaptation and introgression. Population and quantitative genetic studies have used the abundant nucleotide variation to identify a number of candidate genes which may be involved in both local adaptation and domestication. The results from these investigations have provided basic knowledge about evolution and how to utilize genetic resources for both agriculture and conservation. Targeting Helianthus for further study as new technologies emerge will allow for a better understanding of how different types of genomic variation interact and contribute to phenotypic variation in a complex system that is ecologically and economically significant.

Organ and cell type-specific complementary expression patterns and regulatory neofunctionalization between duplicated genes in Arabidopsis thaliana.

Duplicated genes can contribute to the evolution of new functions and they are common in eukaryotic genomes. After duplication, genes can show divergence in their sequence and/or expression patterns. Qualitative complementary expression, or reciprocal expression, is when only one copy is expressed in some organ or tissue types and only the other copy is expressed in others, indicative of regulatory subfunctionalization or neofunctionalization. From analyses of two microarray data sets with 83 different organ types, developmental stages, and cell types in Arabidopsis thaliana, we determined that 30% of whole-genome duplicate pairs and 38% of tandem duplicate pairs show reciprocal expression patterns. We reconstructed the ancestral state of expression patterns to infer that considerably more cases of reciprocal expression resulted from gain of a new expression pattern (regulatory neofunctionalization) than from partitioning of ancestral expression patterns (regulatory subfunctionalization). Pollen was an especially common organ type for expression gain, resulting in contrasting expression of some duplicates in pollen. Many of the gene pairs with reciprocal expression showed asymmetric sequence rate evolution, consistent with neofunctionalization, and the more rapidly evolving copy often showed a more restricted expression pattern. A gene with reciprocal expression in pollen, involved in brassinosteroid signal transduction, has evolved more rapidly than its paralog, and it shows evidence for a new function in pollen. This study indicates the evolutionary importance of reciprocal expression patterns between gene duplicates, showing that they are common, often associated with regulatory neofunctionalization, and may be a factor allowing for retention and divergence of duplicated genes.