Concerted Evolution of Subgenomes in Polyploids

Genome duplication is thought to be central to the evolution of morphological complexity, and some polyploids enjoy a variety of capabilities that transgress those of their diploid progenitors.  Spread of repetitive DNA to new subgenomes has long been known to be an element of genome size fluctuations.  Comparison of genomic sequences from several tetraploid (A_tD_t) Gossypium species and genotypes with putative diploid A and D genome progenitor species revealed that unidirectional DNA exchanges between homeologous genes were the predominant mechanism responsible for allelic differences between the Gossypium tetraploids and their diploid progenitors. Preservation of homeologous gene conversion events (HeGCE) is asymmetric in the two subgenomes of tetraploid cottons. A_t to D_t conversion is far more abundant than the reciprocal, is enriched in heterochromatin, is highly correlated with GC content and transposon distribution, and may silence abundant A-genome-derived retrotransposons. D_t to A_t conversion is abundant in euchromatin and genes, frequently reversing losses of gene function. Now known to be widespread, even in the post-Neolithic polyploid B. napus, HeGCE may provide an alternative to (rare) reciprocal DNA exchanges between chromosomes in heterochromatin, and is a natural non-invasive means of gene transfer with the precision of transformation, potentially  important in genetic improvement of many crop plants.