The Winner Takes It All: a single genotype of Kalanchoe xhoughtonii is a global invader

Background and AimsInvasive alien plant species pose a global challenge, and their impact is amplified by globalisation and the accelerating pace of climate change. In mild-climate regions, drought-tolerant invasive plants showing broad environmental tolerance have a competitive advantage. One example is Kalanchoe xhoughtonii (Crassulaceae), popularly known as "mother of millions". It is a hybrid resulting from the interploid cross between K. daigremontiana and K. delagoensis, both native to Madagascar. Kalanchoe xhoughtonii, propagated as an ornamental plant, has emerged as a global invader in less than a century. Four morphotypes of this hybrid have been identified, with different ploidy levels and varying invasive capacities. Here we aim to investigate the genomic variability behind the invasion success of Kalanchoe xhoughtonii. MethodsWe sampled 57 accessions of Kalanchoe xhoughtonii, K. daigremontiana, K. delagoensis and closely related taxa, including old herbarium materials, from all over the world. We analysed genome size, chromosome numbers, sequenced the whole genome, analysed the complete plastome sequence of each accession, and studied the diversity of the ribosomal RNA genes. We also performed a detailed phylogenomic study using nuclear BUSCO genes. Key ResultsOur study reveals the genetic and cytogenetic variability between morphotypes, and shows that a single tetraploid genotype (morphotype A) dominates all populations, emerging as the first reported clonal hybrid capable of worldwide colonisation. Morphotype A shows a striking genetic uniformity, high phenotypic plasticity, and extremely high rates of vegetative reproduction, representing an example of a "general-purpose genotype". ConclusionsThe astonishing reproductive capacity, broad adaptability and the speed at which K. xhoughtonii is colonising new regions by clonal spread highlight the importance of understanding hybridisation and polyploidy in the invasion of ecosystems. Our findings call for the need for risk assessments before developing new hybrids for ornamental plant breeding that may exhibit invasive characteristics.