Genomic analyses of three Acanthus L. species provide insight into polyploidization-driven speciation and evolution

Allopolyploidy fundamentally influences plant evolution, yet the genomic dynamics of allotetraploidization remain incompletely understood. We investigated Acanthus tetraploideus (2n = 4x = 96), an ecologically significant allotetraploid true mangrove from Indo-West Pacific intertidal zones. Our prior integrative investigations indicate that A. tetraploideus originated through hybridization of the diploid species A. ilicifolius and A. ebracteatus with subsequent chromosome doubling. Here, we present complete chromosome-scale genome assemblies for all three species, representing the first genomic resources for true mangrove polyploid research. Our analysis reveals that the three species have experienced at least four rounds of polyploidization events, with the most recent, approximately 53 mya, possibly an Acanthus-specific event. The allotetraploid A. tetraploideus, which emerged between 1.5 and 2.2 mya, has A. ebracteatus as its maternal progenitor and A. ilicifolius as its paternal one. Through a comprehensive genomic comparison and analysis of homoeologous gene expression, we propose a gradual evolutionary trajectory for allotetraploidy in A. tetraploideus. Despite the allotetraploidization event dating back to around 2 mya, A. tetraploideus retains a high degree of colinearity with its ancestral genomes, with the majority (76.2%) of duplicated genes retained and no significant sub-genome bias in gene expression. Furthermore, we have identified positive selection in specific genes that may facilitate the adaptation of Acanthus mangrove species to their intertidal habitats. These findings establish A. tetraploideus as a model for studying allopolyploid evolution while providing new insights into mangrove speciation processes.