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Symbiosis is one of the most important evolutionary processes shaping the biodiversity on Earth. Symbiotic associations often bring together organisms from different domains of life, which can provide an unparalleled route to evolutionary innovation. 1–4 The phylum Apicomplexa encompasses 6,000 ubiquitous animal parasites; however, species in the recently described apicomplexan family, Nephromycidae, are reportedly non-virulent. 5,6 The members of the genus Nephromyces live within a specialized organ of tunicates, called the renal sac, in which they use concentrated uric acid as a primary nitrogen source. 7,8 Here, we report genomic and transcriptomic data from the diverse genus Nephromyces , as well as the three bacterial symbionts that live within this species complex. We show that the diversity of Nephromyces is unexpectedly high within each renal sac, with as many as 20 different species inhabiting the renal sacs in wild populations. The many species of Nephromyces can host three different types of bacterial endosymbionts; however, FISH microscopy allowed us to demonstrate that each individual Nephromyces cell hosts only a single bacterial type. Through the reconstruction and analyses of the endosymbiont bacterial genomes, we infer that each bacterial type supplies its host with different metabolites. No individual species of Nephromyces , in combination with its endosymbiont, can produce a complete set of essential amino acids, and culture experiments demonstrate that individual Nephromyces species cannot form a viable infection. Therefore, we hypothesize that Nephromyces spp. depend on co-infection with congeners containing different bacterial symbionts in order to exchange metabolites to meet their needs. • Nephromyces is a diverse apicomplexan that can harbor three bacterial endosymbionts • Each individual Nephromyces contains only a single bacterial species • No bacterial endosymbiont completes the metabolic needs of Nephromyces • Nephromyces with different endosymbionts must likely exchange metabolites Paight et al. demonstrate that the members of the apicomplexan genus Nephromyces can host three different types of highly reduced bacterial endosymbionts; however, none produce all necessary metabolites. Therefore, metabolite exchange between congeners harboring different endosymbionts likely explains universal multispecies Nephromyces infections. [ABSTRACT FROM AUTHOR]

Summary Climate change has been implicated as driving shifts of hybridizing species’ range limits [ 1, 2 ]. Whether and how much hybrid zones move depends on the relative fitness of hybridzing species under changing conditions [ 3, 4 ]. However, fitness is rarely linked to both climatic conditions and movement of hybrid zones, such that the relationship between climate change and hybrid zone dynamics remains tenuous [ 5 ]. Here we report how interactions between climate (seasonal precipitation) and competitor densities result in steep differentials in survival, which in turn drive hybrid zone movement for two woodrat species ( Neotoma fuscipes and N. macrotis ) in central California, USA. Using 6 years of capture-mark-recapture data, we found that the smaller-bodied species, N. macrotis , and hybrids had survival advantages over the larger-bodied N. fuscipes in the contact region during dry winters and wet springs. This pattern of differential survival, with N. macrotis having a consistent advantage over N. fuscipes during our study period, matched the spatial dynamics of the hybrid zone, which moved steadily north into N. fuscipes territory, with its estimated center moving ∼150 m north in 6 years. Our findings provide a unique demonstration of range movements emerging from a complex interplay between climate and competition. Although all study site areas experienced the same climatic conditions, competitive effects created a complex spatial pattern of survival differentials, which in turn influenced hybrid zone movement. Characterization of fitness differentials derived from replicated demographic studies of contact regions between competitors should greatly improve our ability to understand and forecast climate-driven range dynamics. [ABSTRACT FROM AUTHOR]