The winner takes it all: a single genotype of Kalanchoe × houghtonii is a global invader

Invasive alien plant species pose a global challenge, and their impact is amplified by globalization and the accelerating pace of climate change. In regions with mild climates, drought-tolerant invasive plants showing broad environmental tolerance have a competitive advantage. One example is Kalanchoe × houghtonii (Crassulaceae), popularly known as 'mother of millions'. It is a hybrid resulting from the interploid cross between Kalanchoe daigremontiana and Kalanchoe delagoensis, both native to Madagascar. Kalanchoe × houghtonii, 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 success of invasion of Kalanchoe × houghtonii.

We sampled 57 accessions of Kalanchoe × houghtonii, K. daigremontiana, K. delagoensis and closely related taxa, including old herbarium materials, from all over the world. We analysed the genome size and 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.

Our study reveals 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 colonizing mild-climate regions worldwide. 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'.

The astonishing reproductive capacity, broad adaptability and speed at which K. × houghtonii is colonizing new regions by clonal spread highlight the importance of understanding hybridization and polyploidy in the invasion of ecosystems. Our findings underscore the need to recognize and monitor the potential invasive risks of new hybrids developed through ornamental plant breeding.