High-quality genomes of Bombax ceiba and Ceiba pentandra provide insights into the evolution of Malvaceae species and differences in their natural fiber development

Members of the Malvaceae family, including Corchorus spp., Gossypium spp., Bombax spp., and Ceiba spp., are important sources of natural fibers. In the past decade, the genomes of several Malvaceae species have been assembled; however, the evolutionary history of Malvaceae species and the differences in their fiber development remain to be clarified. Here, we report the genome assembly and annotation of two natural fiber plants from the Malvaceae, Bombax ceiba and Ceiba pentandra, whose assembled genome sizes are 783.56 Mb and 1575.47 Mb, respectively. Comparative analysis revealed that whole-genome duplication and Gypsy long terminal repeat retroelements have been the major causes of differences in chromosome number (2n = 14 to 2n = 96) and genome size (234 Mb to 2676 Mb) among Malvaceae species. We also used comparative genomic analyses to reconstruct the ancestral Malvaceae karyotype with 11 proto-chromosomes, providing new insights into the evolutionary trajectories of Malvaceae species. MYB-MIXTA-like 3 is relatively conserved among the Malvaceae and functions in fiber cell-fate determination in the epidermis. It appears to perform this function in any tissue where it is expressed, i.e. in fibers on the endocarp of B. ceiba and in ovule fibers of cotton. We identified a structural variation in a cellulose synthase gene and a higher copy number of cellulose synthase-like genes as possible causes of the finer, less spinnable, weaker fibers of B. ceiba. Our study provides two high-quality genomes of natural fiber plants and offers insights into the evolution of Malvaceae species and differences in their natural fiber formation and development through multi-omics analysis.