When fathers are instant losers: homogenization of rDNA loci in recently formed Cardamine × schulzii trigenomic allopolyploid.

Recently formed allopolyploids represent an excellent system to study the impacts of hybridization and genomic duplication on genome structure and evolution. Here we explored the 35SrRNA genes (rDNA) in the Cardamine × schulzii allohexaploid that was formed by two subsequent hybridization events within the past c. 150 yr. The rDNA loci were analyzed by cloning, next generation sequencing (NGS), RT-PCR and FISH methods. The primary C. × insueta triploid hybrid derived from C. rivularis (♀) and C. amara (♂) had gene ratios highly skewed towards maternal sequences. Similarly, C. × schulzii, originating from the secondary hybridization event involving C. × insueta (♀) and C. pratensis (♂), showed a reduction in paternal rDNA homeologs despite an excess of chromosomes inherited from C. pratensis. We also identified novel rDNA loci in C. × schulzii, suggesting that lost loci might be slowly reinstalled by translocation (but not recombination) of genes from partner genomes. Prevalent clonal propagation of allopolyploids, C. × insueta and C. × schulzii, indicates that concerted evolution of rDNA may occur in the absence of extensive meiotic cycles. Adoption of NGS in rDNA variant analysis is highly informative for deciphering the evolutionary histories of allopolyploid species with ongoing homogenization processes.

Evidence that an evolutionary transition from dehiscent to indehiscent fruits in Lepidium (Brassicaceae) was caused by a change in the control of valve margin identity genes.

In the Brassicaceae, indehiscent fruits evolved from dehiscent fruits several times independently. Here we use closely related wild species of the genus Lepidium as a model system to analyse the underlying developmental genetic mechanisms in a candidate gene approach. ALCATRAZ (ALC), INDEHISCENT (IND), SHATTERPROOF1 (SHP1) and SHATTERPROOF2 (SHP2) are known fruit developmental genes of Arabidopsis thaliana that are expressed in the fruit valve margin governing dehiscence zone formation. Comparative expression analysis by quantitative RT-PCR, Northern blot and in situ hybridization show that their orthologues from Lepidium campestre (dehiscent fruits) are similarly expressed at valve margins. In sharp contrast, expression of the respective orthologues is abolished in the corresponding tissue of indehiscent Lepidium appelianum fruits, indicating that changes in the genetic pathway identified in A. thaliana caused the transition from dehiscent to indehiscent fruits in the investigated species. As parallel mutations in different genes are quite unlikely, we conclude that the changes in gene expression patterns are probably caused by changes in upstream regulators of ALC, IND and SHP1/2, possible candidates from A. thaliana being FRUITFULL (FUL), REPLUMLESS (RPL) and APETALA2 (AP2). However, neither expression analyses nor functional tests in transgenic plants provided any evidence that the FUL or RPL orthologues of Lepidium were involved in evolution of fruit indehiscence in Lepidium. In contrast, stronger expression of AP2 in indehiscent compared to dehiscent fruits identifies AP2 as a candidate gene that deserves further investigation.

Corolla monosymmetry: evolution of a morphological novelty in the Brassicaceae family.

Evolution of floral monosymmetry is thought to be a major driving force of angiosperm radiation, making angiosperms the most successful land plant group in terms of species richness. Monosymmetry evolved from a polysymmetric ancestor repeatedly in different angiosperm lineages, where it likely facilitated diversification through the interaction with insects. Most monosymmetric taxa are thus dominated by monosymmetric members. However, in the Brassicaceae, only few members develop a monosymmetric corolla with two petal pairs of unequal size, making them an ideal system to study the evolution of molecular mechanisms enhancing flower complexity. Monosymmetry is controlled by the TCP transcription factors that belong to the CYC2 clade in distantly related taxa. In Iberis amara, the first crucifer analyzed in terms of monosymmetry development, unequal corolla formation is due to a stronger CYC2 clade gene expression in the smaller adaxial petals compared with the larger abaxial ones. Phylogenetic reconstruction of the crucifer family reveals that the monosymmetric genera Iberis, Calepina, and Teesdalia belong to one major crucifer lineage. Monosymmetry is most pronounced in Iberis and less so in Calepina and Teesdalia, with a positive dosage-dependent correlation between the strength of a CYC2 expression difference and the extent of monosymmetry formation. An early adaxial CYC2 expression in floral meristems, observed in many distantly related taxa, might have facilitated the repeated evolution of CYC2-controlled monosymmetry. Comparison of early and late CYC2 expression in monosymmetric and polysymmetric crucifers representative for the four major crucifer lineages reveals that an adaxial CYC2 expression in floral meristems is likely ancestral for the Brassicaceae. However, it got lost in all analyzed monosymmetric members and is, as such, not a prerequisite for the establishment of corolla monosymmetry in crucifers. Here, monosymmetry evolved via a heterochronic CYC2 expression shift from an ancestral early adaxial expression in floral meristems to an adaxial CYC2 transcript accumulation later in petal development. This study emphasizes the potential of regulatory changes in the evolution of morphological novelties, like corolla monosymmetry in the Brassicaceae. In combination with a corymboid inflorescence, monosymmetry might have served as a key invention driving diversification in the genus Iberis comprising more than 20 monosymmetric species.

Lepidium as a model system for studying the evolution of fruit development in Brassicaceae.

Fruits represent a key innovation of the flowering plants that facilitates seed dispersal. In many species of the plant family Brassicaceae dehiscent fruits develop in which seed dispersal occurs through a process termed 'pod-shatter'. In the case of dehiscence, the fruit opens during fruit maturation. Phylogeny reconstructions using molecular markers indicate that the development of dehiscent fruits is the ancestral condition within the genus Lepidium s.l., but that indehiscent fruits evolved independently several times from dehiscent fruits. With Lepidium campestre and Cardaria pubescens (also known as Lepidium appelianum), very closely related taxa with dehiscent and indehiscent fruits, respectively, were identified which constitute a well-suited model system to determine the molecular genetic basis of evolutionary changes in fruit dehiscence. Following the rationale of evolutionary developmental biology ('evo-devo') phylomimicking mutants with indehiscent fruits of the close relative Arabidopsis have been used to define the candidate genes ALC, FUL, IND, RPL, and SHP1/2 which might be involved in the origin of indehiscent fruits in Cardaria. Comparative expression studies in L. campestre and C. pubescens are used to identify differentially expressed genes and thus to narrow down the number of candidate genes. Reciprocal heterologous transformation experiments may help us to distinguish direct from indirect developmental genetic causes of fruit indehiscence, and to assess the contribution of cis- and trans-regulatory changes.