More than the sum of its parts: reproductive addiction in the ecology and evolution of fungal-bacterial symbioses

SUMMARY Reproductive control exerted by endosymbionts over their hosts is an adaptation for maintaining host-symbiont associations across generations. This strategy is particularly well-characterized in endosymbiotic bacteria (EB) associated with arthropods, in which EB of disparate species have independently evolved diverse mechanisms to manipulate host reproduction, thereby ensuring their reliable transmission across host generations. Like arthropods, fungi in the phylum Mucoromycota commonly harbor ancient and highly coevolved EB that significantly affect fungal life histories, including reproduction. In particular, the symbiosis between the mucoromycete Rhizopus microsporus and its Mycetohabitans spp. endosymbionts is significantly contingent on reproductive addiction, as removal of the symbiont results in the loss or severe impairment of host asexual and sexual reproduction. Here, we synthesize the current knowledge on sexual reproduction in mucoromycete fungi and the Rhizopus microsporus–Mycetohabitans symbiosis. We highlight current gaps in the knowledge of mucoromycete reproduction as well as present novel hypotheses for studying these interactions based on the understanding of reproductive manipulation in arthropods.