Most diploid organisms have polyploid ancestors. The evolutionary process of polyploidization is poorly understood but has frequently been conjectured to involve some form of ‘genome shock’, such as genome reorganization and subgenome expression dominance. Here we study polyploidization in Arabidopsis suecica, a post-glacial allopolyploid species formed via hybridization of Arabidopsis thaliana and Arabidopsis arenosa. We generated a chromosome-level genome assembly of A. suecica and complemented it with polymorphism and transcriptome data from all species. Despite a divergence around 6 million years ago (Ma) between the ancestral species and differences in their genome composition, we see no evidence of a genome shock: the A. suecica genome is colinear with the ancestral genomes; there is no subgenome dominance in expression; and transposon dynamics appear stable. However, we find changes suggesting gradual adaptation to polyploidy. In particular, the A. thaliana subgenome shows upregulation of meiosis-related genes, possibly to prevent aneuploidy and undesirable homeologous exchanges that are observed in synthetic A. suecica, and the A. arenosa subgenome shows upregulation of cyto-nuclear processes, possibly in response to the new cytoplasmic environment of A. suecica, with plastids maternally inherited from A. thaliana. These changes are not seen in synthetic hybrids, and thus are likely to represent subsequent evolution.

 Ustilago maydis (U. maydis) is the causal agent of maize smut disease. During the colonization process, the fungus secretes effector proteins which suppress immune responses and redirect the host metabolism in favor of the pathogen. As effectors play a critical role during plant colonization, their identification and functional characterization is essential to understanding biotrophy and disease.  Using biochemical, molecular, and transcriptomic techniques, we performed a functional characterization of the U. maydis effector Jasmonate/Ethylene signaling inducer 1 (Jsi1).  Jsi1 interacts with several members of the plant co-repressor family Topless/Topless related (TPL/TPR). Jsi1 expression in Zea mays (Z. mays) and Arabidopsis thaliana (A. thaliana) leads to transcriptional induction of the ethylene response factor (ERF) branch of the jasmonate/ethylene (JA/ET) signaling pathway. In A. thaliana, activation of the ERFbranch leads to biotrophic susceptibility. Jsi1 likely activates the ERF-branch via an EAR motif, which resembles EAR motifs from plant ERF transcription factors, that interacts with TPL/TPR proteins.  EAR motif-containing effector candidates were identified from different fungal species including Magnaporthe oryzae, Sporisorium scitamineum, and Sporisorium reilianum. Interaction between plant TPL proteins and these effector candidates from biotrophic and hemibiotrophic fungi indicates the convergent evolution of effectors modulating the TPL/TPR co-repressor hub.

Embryonic polarity of invertebrates, amphibians and fish is specified largely by maternal determinants, which fixes cell fates early in development. In contrast, amniote embryos remain plastic and can form multiple individuals until gastrulation. How is their polarity determined? In the chick embryo, the earliest known factor is cVg1 (homologous to mammalian growth differentiation factor 1, GDF1), a transforming growth factor beta (TGFβ) signal expressed posteriorly before gastrulation. A molecular screen to find upstream regulators of cVg1 in normal embryos and in embryos manipulated to form twins now uncovers the transcription factor Pitx2 as a candidate. We show that Pitx2 is essential for axis formation, and that it acts as a direct regulator of cVg1 expression by binding to enhancers within neighbouring genes. Pitx2, Vg1/GDF1 and Nodal are also key actors in left–right asymmetry, suggesting that the same ancient polarity determination mechanism has been co-opted to different functions during evolution.DOI: http://dx.doi.org/10.7554/eLife.03743.001