Most Populations Are Significantly Different When Comparing Bray-Curtis, But Not Unifrac Distances.
We did not find evidence for any core community of ASVs shared broadly across the Florida Gulf Coast range of R. mangle, though most samples had at least one ASV from each of the phyla Proteobacteria, Actinobacteria, and Bacteroidetes. These phyla may also be very abundant in seawater and coastal sediments [8, 38], suggesting that the propagule communities could be significantly influenced by these environmental sources. We found variation in the number of core ASVs shared between individuals within each population (Table 3). These results are consistent with patterns found across leaves, stems, flowers, and propagules from three populations of red mangroves in the Florida Panhandle in terms of the dearth of shared ASVs (Unpublished Data). Interestingly, work on the microbiome of Rhizophora stylosa found no bacteria associated with the leaf or stem tissue [39].
Most work exploring core communities within plants has focused on terrestrial species, where core communities sometimes occur across broad ranges. Bacterial communities associated with ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) were found to display lower intraspecies variability than interspecies variability when compared to those of a congener and two other tree species, even where samples of Pinus ponderosa originated on different continents [40]. Furthermore, four bacterial species were found in seeds from 14 cultivars of maize (Zea mays L.) as well as its wild ancestor teosinte (Zea diploperennis H.H.Iltis et al.); several additional bacterial species were found in most of the cultivars sampled [10].
The lack of a core propagule bacterial community in R. mangle could be related to their viviparous nature. Morphologically, anatomically, and physiologically the propagules are similar to seedlings rather than seeds. As germinated seedlings, propagules interact with their dispersal environment in ways that seeds do not, suggesting the potential for environmental influence in ways not experienced by bacterial communities of seeds.
Carposphere bacterial community composition varied across mangrove populations. That we found Bray-Curtis to differ significantly between most pairwise comparisons of populations while no comparisons based on Unifrac distances were significantly different suggests that although the identities and relative abundances of ASVs associated with each population were different, the higher-level taxonomic diversity is relatively similar. This can be restated as bacterial communities differ in the precise ASVs found within them, but those ASVs are closely related to one another. These communities could contain similar taxonomic groups due to the similarity in the morphology, anatomy, and physiology of red mangrove propagules which are themselves a product of host evolution, even if the bacteria themselves are not faithfully inherited across generations.
The seeds of terrestrial plants, and perhaps endophytic communities in general, tend to be dominated by Proteobacteria, Firmicutes, Bacterioidetes, and Actinobacteria [41]. Our results generally align with this pattern, though we found few ASVs from the phylum Firmicutes (RA = 2.1%). While there is ample evidence in the literature of inheritance of microbes in plants [9, 10], we think it is unlikely that the patterns we describe here are the result of inheritance, for reasons explored below.
Host Geography, But Not Maternal Genotype, Is Correlated With Beta Diversity
We found Bray-Curtis dissimilarity, but not Generalized Unifrac distance, to be correlated with geographic distance between samples. Samples located close together tended to be more similar in terms of the identities and relative abundances of their ASVs, but not in terms of the genetic similarity of the bacterial communities. Together, these results suggest that geography might affect propagule-associated bacterial diversity at fine taxonomic scales, but broader taxonomic groupings remain relatively unaffected across geographic distance. This is broadly consistent with our understanding of plant-associated microbial communities [10, 40] and with the results of our pairwise PERMANOVA (Table 4).
Additionally, neither beta diversity metric was correlated with maternal genetic distance, suggesting that maternal genotype did not have a significant influence on bacterial community composition. These results are somewhat inconsistent with the literature. Bacterial endophytes of corn seeds were shown to be influenced by host genotype, and a core community of bacteria was identified across cultivars and even in the wild ancestor, Zea mays [10]. The bacterial and fungal leaf communities of grapes were shown to be influenced more heavily by geographic region (and the associated differences in environmental conditions) than by maternal genotype [14], suggesting that microbial communities found across different plant systems may differ in the relative influence of environment, geography, and genotype. Our sampling methods captured bacteria from the propagule surface and within the plant tissue. Bacterial communities associated with plant surfaces may be more readily influenced by differences in environmental conditions such as UV radiation, humidity, exposure to seawater, and ambient temperature than those found within plant tissues [42], and therefore may vary more strongly over geographic distances.
Genetic and geographic distance among mangroves were correlated, suggesting that more closely located individuals tended to be more closely related. We found evidence for population differentiation, with geographic distance accounting for nearly 67% of genetic variation (Figure 4). This is broadly consistent with previous work which found significant genetic variation between populations using 8 and 7 microsatellite loci, respectively [21, 22]. However, additional analysis with larger RADseq datasets found lower FST between populations [43].