Hybridity and Polyploidy

Interspecies hybridization and polyploidy are prominent in the plant kingdom. Wheat for example is an hexaploid, whose genome is an hybrid combining the genome of three diploid progenitors. Polyploidization can occur overnight, for example through inter-specific hybridization followed by genome doubling via unreduced gametes. It is thus one of the most efficient and rapid way to generate a new species and is a driving force in plant genome evolution.

The paradigm to explain the success of polyploidy was that the increased range of gene dosage, the new heterotic interactions between alleles, homeoalleles or genes and the buffering of the mutation load resulting from gene duplication facilitate the formation of novel genes and the establishment of the new species. While this long-held view is still valid, there are now new twists to the paradigm. Recent studies done in collaboration with Prof. Moshe Feldman, have emphasized the importance of non-Mendelian processes and described their time course. In these studies, synthetic polyploids were made and analyzed immediately after formation. These studies show that a new, non-additive variation, not previously present in the diploid progenitors, can be induced immediately upon polyploidization rather than on an evolutionary scale.

The basis of this new variation is both genetic and epigenetic. The types of non-Mendelian changes observed were: programmed elimination of sequences (coding and non-coding); gene silencing associated with cytosine methylation and transcriptional activation of retrotransposons. These findings were confirmed by a recent bioinformatics analysis using public data on the mapping and expression of DNA sequences in natural hexaploid wheat rather than in synthetic polyploids. This rapid reorganization of the genome structure and expression is now investigated in wheat as well as in model systems, Arabidopsis and budding yeast. The current work in wheat aims at determining the precise time course and the mechanism of programmed DNA elimination. In Arabidopsis, we are making use of a new method of gene targeting recently developed in our laboratory to follow the expression of alleles as a function of hybridity and of gene dosage variation achieved through polyploidization. We expect that epigenetic changes will be triggered as a result of hybridity and/or polyploidization, that may lead to the silencing of specific alleles.

In budding yeast, we are collaborating with Prof. Naama Barkai to analyze the new patterns of gene expression in yeast hybrids and in polyploids derived from interspecific crosses. Moreover, we are analyzing the genetic basis for the heterosis (Hybrid vigor) that has been observed in some of the hybrids.