Biology subjects

Johan Reimegård

Publications and source records attributed to Johan Reimegård.

3 recordsLinked to original sources

bioRxiv2016

The role of transposable elements for gene expression in Capsella hybrids and allopolyploids

The formation of an allopolyploid species involves the merger of two genomes with separate evolutionary histories. In allopolyploids, genes derived from one progenitor species are often expressed at higher levels than those from the other progenitor. It has been suggested that this could be due to differences in transposable element (TE) content among progenitors, as silencing of TEs can affect expression of nearby genes. Here, we examine the role of TEs for expression biases in the widespread allotetraploid Capsella bursa-pastoris and in diploid F1 hybrids generated by crossing Capsella orientalis and Capsella rubella, two close relatives of the progenitors of C. bursa-pastoris. As C. rubella harbors more TEs than C. orientalis, we expect C. orientalis alleles to be expressed at higher levels if TE content is key for expression biases. To test this hypothesis, we quantified expression biases at approximately 5800 genes in flower buds and leaves, while correcting for read mapping biases using genomic data. While three of four C. bursa-pastoris accessions exhibited a shift toward higher relative expression of C. orientalis alleles, the fourth C. bursa-pastoris accession had the opposite direction of expression bias, as did diploid F1 hybrids. Associations between TE polymorphism and expression bias were weak, and the effect of TEs on expression bias was small. These results suggest that differences in TE content alone cannot fully explain expression biases in these species. Future studies should investigate the role of differences in TE silencing efficacy, as well as a broader set of other factors. Our results are important for a more general understanding of the role of TEs for cis-regulatory evolution in plants.

Evolutionary Biology↗

bioRxiv2015

Genomic analysis reveals major determinants of cis-regulatory variation in Capsella grandiflora

Understanding the causes of cis-regulatory variation is a long-standing aim in evolutionary biology. Although cis-regulatory variation has long been considered important for adaptation, we still have a limited understanding of the selective importance and genomic determinants of standing cis-regulatory variation. To address these questions, we studied the prevalence, genomic determinants and selective forces shaping cis-regulatory variation in the outcrossing plant Capsella grandiflora. We first identified a set of 1,010 genes with common cis-regulatory variation using analyses of allele-specific expression (ASE). Population genomic analyses of whole-genome sequences from 32 individuals showed that genes with common cis-regulatory variation are 1) under weaker purifying selection and 2) undergo less frequent positive selection than other genes. We further identified genomic determinants of cis-regulatory variation. Gene-body methylation (gbM) was a major factor constraining cis-regulatory variation, whereas presence of nearby TEs and tissue specificity of expression increased the odds of ASE. Our results suggest that most common cis-regulatory variation in C. grandiflora is under weak purifying selection, and that gene-specific functional constraints are more important for the maintenance of cis-regulatory variation than genome-scale variation in the intensity of selection. Our results agree with previous findings that suggest TE silencing affects nearby gene expression, and provide novel evidence for a link between gbM and cis-regulatory constraint, possibly reflecting greater dosage-sensitivity of body-methylated genes. Given the extensive conservation of gene-body methylation in flowering plants, this suggests that gene-body methylation could be an important predictor of cis-regulatory variation in a wide range of plant species.

Evolutionary Biology↗

bioRxiv2015

Cis-regulatory changes associated with a recent mating system shift and floral adaptation in Capsella

The selfing syndrome constitutes a suite of floral and reproductive trait changes that have evolved repeatedly across many evolutionary lineages in response to the shift to selfing. Convergent evolution of the selfing syndrome suggests that these changes are adaptive, yet our understanding of the detailed molecular genetic basis of the selfing syndrome remains limited. Here, we investigate the role of cis-regulatory changes during the recent evolution of the selfing syndrome in Capsella rubella, which split from the outcrosser Capsella grandiflora less than 200 kya. We assess allele-specific expression (ASE) in leaves and flower buds at a total of 18,452 genes in three interspecific F1 C. grandiflora x C. rubella hybrids. Using a hierarchical Bayesian approach that accounts for technical variation using genomic reads, we find evidence for extensive cis-regulatory changes. On average, 44% of the assayed genes show evidence of ASE, however only 6% show strong allelic expression biases. Flower buds, but not leaves, show an enrichment of cis-regulatory changes in genomic regions responsible for floral and reproductive trait divergence between C. rubella and C. grandiflora. We further detected an excess of heterozygous transposable element (TE) insertions near genes with ASE, and TE insertions targeted by uniquely mapping 24-nt small RNAs were associated with reduced expression of nearby genes. Our results suggest that cis-regulatory changes have been important during the recent adaptive floral evolution in Capsella and that differences in TE dynamics between selfing and outcrossing species could be important for rapid regulatory divergence in association with mating system shifts.

Evolutionary Biology↗