Multispecies polyploidization, chromosome shuffling, and genome extraction in Zea/Tripsacum hybrids

By hybridization and special sexual reproduction, we sequentially aggregated Zea mays, Zea perennis, and Tripsacum dactyloides in an allohexaploid, backcrossed it with maize, derived self-fertile allotetraploids of maize and Z. perennis by natural genome extraction, extended their first six selfed generations, and finally constructed amphitetraploid maize using nascent allotetraploids as a genetic bridge. Transgenerational chromosome inheritance, subgenome stability, chromosome pairings and rearrangements, and their impacts on an organism's fitness were investigated by fertility phenotyping and molecular cytogenetic techniques genomic in situ hybridization (GISH) and fluorescence in situ hybridization (FISH). Results showed that diversified sexual reproductive methods produced highly differentiated progenies (2n = 35-84) with varying proportions of subgenomic chromosomes, of which one individual (2n = 54, MMMPT) overcame self-incompatibility barriers and produced a self-fertile nascent near-allotetraploid by preferentially eliminating Tripsacum chromosomes. Nascent near-allotetraploid progenies showed persistent chromosome changes, intergenomic translocations, and rDNA variations for at least up to the first six selfed generations; however, the mean chromosome number preferably maintained at the near-tetraploid level (2n = 40) with full integrity of 45S rDNA pairs, and a trend of decreasing variations by advancing generations with an average of 25.53, 14.14, and 0.37 for maize, Z. perennis, and T. dactyloides chromosomes, respectively. The mechanisms for three genome stabilities and karyotype evolution for formatting new polyploid species were discussed.