The transition to selfing in Capsella rubella accompanies its recent divergence from the ancestral outcrossing C. grandiflora species about 100,000 years ago. Whether the change in mating system was accompanied by the evolution of additional reproductive barriers that enforced species divergence remained unknown. Here, we show that C. rubella and C. grandiflora are reproductively separated by an endosperm-based, non-reciprocal postzygotic hybridization barrier. While hybridizations of C. rubella maternal plants with C. grandiflora pollen donors resulted in complete seed abortion caused by endosperm cellularization failure, the reciprocal hybridization resulted in the formation of small seeds with precociously cellularized endosperm. Strikingly, the transcriptomic response of both hybridizations mimicked respectively the response of paternal and maternal excess hybridizations in Arabidopsis thaliana, suggesting unbalanced genome strength causes hybridization failure in both species. These results provide strong support for the theory that crosses between plants of different mating systems will be unbalanced, with the outcrosser behaving like a plant of increased ploidy, evoking a response that resembles an interploidy-type seed failure. Seed incompatilibity of C. rubella pollinated by C. grandiflora followed the Bateson-Dobzhansky-Muller model, involving negative genetic interaction of multiple paternal C. grandiflora loci with at least one maternal C. rubella locus. Given that both species only recently diverged, our data suggest that a fast evolving mechanism underlies the post-zygotic hybridization barrier(s) separating both species.

Genomic imprinting is an epigenetic phenomenon occurring in mammals and flowering plants that causes genes to adopt a parent-of-origin-specific mode of expression. While the imprinting status of genes is well conserved in mammals, clear estimates for the degree of conservation were lacking in plants. We therefore analyzed the genome-wide imprinting status of Capsella rubella, which shared a common recent ancestor with Arabidopsis thaliana ∼10 to 14 million years ago. However, only ∼14% of maternally expressed genes (MEGs) and ∼29% of paternally expressed genes (PEGs) in C. rubella were commonly imprinted in both species, revealing that genomic imprinting is a rapidly evolving phenomenon in plants. Nevertheless, conserved PEGs exhibited signs of selection, suggesting that a subset of imprinted genes play an important functional role and are therefore maintained in plants. Like in Arabidopsis, PEGs in C. rubella are frequently associated with the presence of transposable elements that preferentially belong to helitron and MuDR families. Our data further reveal that MEGs and PEGs differ in their targeting by 24-nucleotide small RNAs and asymmetric DNA methylation, suggesting different mechanisms establishing DNA methylation at MEGs and PEGs.

Hybrid seed lethality is a widespread type of reproductive barrier among angiosperm taxa that contributes to species divergence by preventing gene flow between natural populations. Besides its ecological importance, it is an important obstacle to plant breeding strategies . Hybrid seed lethality is mostly due to a failure of the nourishing endosperm tissue, resulting in embryo arrest. The cause of this failure is a parental dosage imbalance in the endosperm that can be a consequence of either differences in parental ploidy levels or differences in the 'effective ploidy', also known as the endosperm balance number (EBN). Hybrid seed defects exhibit a parent-of-origin pattern, suggesting that differences in number or expression strength of parent-of-origin-specific imprinted genes underpin, as the primary or the secondary cause, the molecular basis of the EBN. Here, we have tested this concept in the genus Capsella and show that the effective ploidy of three Capsella species correlates with the number and expression level of paternally expressed genes (PEGs). Importantly, the number of PEGs and the effective ploidy decrease with the selfing history of a species: the obligate outbreeder Capsella grandiflora had the highest effective ploidy, followed by the recent selfer Capsella rubella and the ancient selfer Capsella orientalis. PEGs were associated with the presence of transposable elements and their silencing mark, DNA methylation in CHH context (where H denotes any base except C). This suggests that transposable elements have driven the imprintome divergence between Capsella species. Together, we propose that variation in transposable element insertions, the resulting differences in PEG number and divergence in their expression level form one component of the effective ploidy variation between species of different breeding system histories, and, as a consequence, allow the establishment of endosperm-based hybridization barriers.

Promoter sequences of 13 Phosphate Transporter genes of Oryza sativa L. (OsPTs) have been analyzed in silico to identify their cis-regulatory elements (CREs). The DNA sequences of these OsPT genes were mined from NCBI gene databases. MEGABLAST program was used to align these sequences to Rice Genome Database in obtaining their complete sequences. The upstream region (-1 to -400) of the complete gene sequences were analyzed using SIGNALSCAN program provided by PLACE database of cis-regulatory element motives. From this procedure, 153 types of CRE were identified. Four of these CREs were found on all of the OsPT genes: ARR1AT, CAATBOX1, CACTFTPPCA1, and DOFCOREZM. Among these CREs, CACTFTPPCA1 was found with 3 to 15 duplications in each cis-regulatory sequence. In addition, each OsPT gene has typical CREs that can only be found in the respective genes. The total number of these typical CREs is 54, and one of them was a binding site for a bHLH-like protein, CACGTGMOTIF or the G-box. Moreover, several E-boxes which also functioned as a binding site for a bHLH-like protein were identified in all OsPTs except in OsPT1, OsPT6, and OsPT8. There were significant correlations (p < 0.05) between mRNA levels of OsPT1 to OsPT11 in rice root with or without inoculation of Glomus intradices reported by Paszkowskidagger et al. (2002) and the duplication numbers of ARR1AT, CAATBOX1, CACTFTPPCA1, CURECORECR, and WRKY71OS.