A comparative transcriptomic study of an allotetraploid and its diploid progenitors illustrates the unique advantages and challenges of RNA-seq in plant species

Premise of the study: RNA-seq analysis of plant transcriptomes poses unique challenges due to the highly duplicated nature of plant genomes. We address these challenges in the context of recently formed polyploid species and detail an RNA-seq experiment comparing the leaf transcriptome profile of an allopolyploid relative of soybean with the diploid species that contributed its homoeologous genomes. Methods: RNA-seq reads were obtained from the three species and were aligned against the genome sequence of Glycine max. Transcript levels were estimated for each gene, relative contributions of pol y ploidy-duplicated loci (homoeologues) in the tetraploid were identified, and comparisons of transcript profiles and individual genes were used to analyze the regulation of transcript levels. Key results: We present a novel metric developed to address issues arising from high degrees of gene space duplication and a method for dissecting a gene's measured transcript level in a polyploid species into the relative contribution of its homoeologues. We identify the gene family likely contributing to differences in photosynthetic rate between the allotetraploid and its progenitors and show that the tetraploid appears to be using the "redundant" gene copies in novel ways. Conclusions: Given the prevalence of polyploidy events in plants, we believe many of the approaches developed here to be applicable, and often necessary, in most plant RNA-seq experiments. The deep sampling provided by RNA-seq allows us to dissect the genetic underpinnings of specific phenotypes as well as examine complex interactions within polyploid genomes.