The widespread symbiotic interaction between plants and arbuscular mycorrhizal (AM) fungi relies on a complex molecular dialog with reciprocal benefits in terms of nutrition, growth, and protection. Approximately 29% of all vascular plant species do not host AM symbiosis, including major crops. Under certain conditions, however, presumed non-host plants can become colonized by AM fungi and develop rudimentary AM (RAM) phenotypes. Here we zoom in on the mustard family (Brassicaceae), which harbors AM hosts, non-hosts, and presumed non-host species such as Arabidopsis thaliana, for which conditional RAM colonization has been described. We advocate that RAM phenotypes and redundant genomic elements of the symbiotic ‘toolkit’ are missing links that can help to unravel genetic constraints that drive the evolution of symbiotic incompatibility.