Abstract: The review focuses on the genetic consequences of interspecific hybridization and discusses its role in speciation and increasing the genetic diversity of plants, including the diversity of species and varieties of cultivated crops and garden plants. The combination of two or more genomes of different origin in the first-generation hybrids is usually accompanied by a phenomenon referred to as genomic shock, which results in different genetic and epigenetic changes. As a result, a material appears which is unique in providing multiple variants for natural selection of organisms, in different degrees adapted to new environmental conditions. In isolated plant populations with various destabilized genomes of hybrid origin under the influence of natural selection and due to genetic drift, gene and chromosomal differences will accumulate, triggering new reproductive isolating mechanisms that increase genetic isolation of a new race or species. Progressive establishment of genome stabilization at the eupolyploid stage and subsequent repeated genome and karyotype diploidization facilitate the retention of selected new genomic and epigenomic combinations. It seems likely that the evolutionary history of all agricultural crops and garden plant varieties, as well as of many invasive alien species and the components of adventive flora, was marked by interspecific crossings purposefully carried out by breeders during the creation of modern varieties and crossings between previously geographically isolated plants that were unintentionally united in apothecary and botanical gardens, in fields and backyards, and on disturbed lands around settlements. At the same time, phenotypic and genetic diversity of cultivars not only results from the combination of different alleles that already existed in the genomes of the parental species but also is a consequence of the appearance in the first-generation hybrids of new genomic and epigenomic variants that are direct or distant effects of post-hybridization genomic shock, the creative role of which we would like to emphasize.
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