Data pre-processing
The metabarcoding sequencing of the 16S rDNA of the microbiota associated with eggs and adult females of T. urticae and with adult females of N. californicus yielded a total of 2,902,789 reads, with a mean of 80,633 reads per sample. After quality filtering, the total number of paired reads was 591,495 with a mean length of 454 bp. Rarefaction curves demonstrated that the sampling efforts were adequate to fully represent the richness of the communities analyzed, as no increase in ASV diversity was observed or unique ASVs detected after sampling 3,013 sequences (Fig. S1).
Taxonomic profiles
The host plants did not affect the relative abundance of the microbiota associated with eggs and adult females of T. urticae and adult females of N. californicus at the phylum level. In the microbiota of T. urticae, Proteobacteria comprised nearly 99% of all bacteria detected, with Firmicutes and Bacteroidota comprising less than 1% of ASVs. Proteobacteria was also the most prevalent (> 85% of all ASVs) in N. californicus, with a higher relative abundance of Firmicutes, mainly in samples from maize and tomato, and Bacteroidota in samples from cotton and pinto bean (Fig. S2).
Proteobacteria was represented only by Alphaproteobacteria and Gammaproteobacteria. Alphaproteobacteria comprised more than 90% of the total microbiota associated with adult females of T. urticae, except for the microbiota of adults from cotton, in which the abundance of Gammaproteobacteria increased to around 35% of the total ASVs detected (Fig. 1a). The microbiota of eggs of T. urticae was dominated by Gammaproteobacteria, which comprised more than 95% of the relative abundance of all classes of bacteria detected, regardless of the host plant (Fig. 1a). Gammaproteobacteria was also the most prevalent in adult females of N. californicus, but with a lower relative abundance than observed in eggs of T. urticae. Alphaproteobacteria predominated in the microbiota of N. californicus from maize, while Gammaproteobacteria predominated in the microbiota of predatory mites from cotton (Fig. 1a). Firmicutes was consistently recorded in samples of predatory mites from maize and tomato, but only sporadically represented in samples of predatory mites from cotton and pinto bean (Fig. 1a).
Serratia (Gammaproteobacteria, Enterobacteriaceae) dominated the microbiota of T. urticae eggs from all host plants (Fig. 1b). In contrast to eggs, the microbiota of adult females of T. urticae was dominated by the alphaproteobacterium Rickettsia (Rickettsiaceae), comprising more than 75% of the relative bacterial diversity (Fig. 1b). The prevalence of Rickettsia in the microbiota associated with T. urticae females was shared only with Pseudomonas (Gammaproteobacteria, Pseudomonadaceae) in cotton and with Wolbachia (Alphaproteobacteria, Anaplasmataceae) in tomato (Fig. 1b). The microbiota associated with adult females of N. californicus was more diversified, but Serratia, Rickettsia, and Pseudomonas were the most abundant bacteria, changing in relative abundance from host plant to host plant (Fig. 1b). Several other genera were detected in N. californicus females, including Paenibacillus (Bacilli, Paenibacillaceae), common in the microbiota of this mite from tomato (Fig. 1b).
Alpha and beta diversities
The alpha-diversity analysis indicated that host plants did not affect the microbial community diversity associated with eggs and adult females of T. urticae or with adults of N. californicus, except for the microbiota associated with T. urticae females on pinto bean (Shannon index values: F = 6.05; p = 0.04). The alpha-diversity of the microbiota associated with eggs of T. urticae on pinto bean was also lower than the diversity observed in adults of N. californicus (Shannon index values: F = ˗ 4.36; p = 0.0471) (Fig. 2). Other comparisons did not result in significant differences for alpha diversity (Table S1).
PCoA analysis using Jaccard indices indicated that the microbiota of T. urticae from maize, pinto bean and tomato were closely clustered, while the microbiota from cotton was more dispersed (Fig. 3a; 3b). PCoA analysis indicated no differences in the beta diversity of the microbiota of N. californicus in the host plant-prey systems evaluated (Fig. 3c). PCoA analysis using the distances based on weighted Unifrac showed that the microbiota associated with adult females of T. urticae from cotton differed most from the others (Fig. 3d), while samples from eggs of T. urticae were grouped (Fig. 3e), and samples from N. californicus were dispersed (Fig. 3f). PCoA analyses based on the unweighted Unifrac index separated the microbiota of adult females of T. urticae from maize and tomato into one group, and those from cotton and pinto bean in another group (Fig. 3g). However, the microbiota of eggs of T. urticae (Fig. 3h) and the predator N. californicus (Fig. 3i) clustered in accordance with the host plants.
Clustering analysis
Differences in ASV counts between species (T. urticae and N. californicus) and stages of development of T. urticae (eggs and adult females) contributed more in the clustering analysis than the differences in ASV counts among the host plants (cotton, maize, pinto bean, and tomato) (Fig. 4a). The members of Proteobacteria were over-represented in the microbiota of T. urticae compared to the microbiota associated with the predatory mite N. californicus, with Gammaproteobacteria predominating in eggs, and Alphaproteobacteria in adult females of T. urticae (Fig. 4b). The most abundant ASVs of the microbiota of N. californicus belonged to Bacilli and Bacteroidia, which were among the least represented ASVs in eggs and adult females of T. urticae.
Serratia was the most abundant ASV in the microbiota of eggs of T. urticae, but showed low abundances in the microbiota of adult females of T. urticae and N. californicus (Fig. 4c). Rickettsia was the most abundant ASV in adult females of T. urticae, followed by Wolbachia, with the exception of the microbiota of T. urticae reared on cotton and maize, in which Wolbachia had low abundances. Pseudomonas also appeared in high numbers in the microbiota of adult females of T. urticae, but was abundant only in mites reared on cotton plants (Fig. 4c).
The abundance of ASVs belonging to the microbiota associated with the predatory mite varied according to the host plant. Pseudomonas and Flavobacterium were the most abundant ASVs in N. californicus from cotton and pinto bean, while Enterococcus, Bacillus, and Paenibacillus were the most abundant in maize, and Bacillus and Paenibacillus in predatory mites reared on prey from tomato plants (Fig. 4c). Stenotrophomonas was also an abundant ASV in the microbial community associated with females of N. califonrnicus from cotton, pinto bean, and tomato (Fig. 4c).
The dendrograms produced using the distances based on weighted Unifrac showed that all samples of T. urticae eggs clustered in a clade separated from adult females of T. urticae (Fig. 5a) and N. californicus (Fig. 5b), except for one replicate of T. urticae eggs from cotton. This replicate clustered with one replicate of N. californicus from cotton (Fig. 5a) and two replicates of T. urticae eggs from cotton and tomato (Figure 5b). Adult females of T. urticae clustered according to the host plants, except for cotton leaves, while eggs of T. urticae and adult females of N. californicus did not cluster according to the host plants (Fig. 5a, 5b).
Comparative analysis
Comparative analysis showed that the microbiota associated with eggs of T. urticae was affected by the rearing host plant, while we detected no evidence that the microbiota associated with the predatory mite N. californicus and adult females of T. urticae suffered any effect from the host plant. The effect of the host plant on the diversity and richness of the microbiota associated with eggs of T. urticae was observed in mites reared on cotton. One unassigned genus was more abundant in the microbiota of eggs from cotton than from the other host plants.
Comparative analysis from maize indicated 4.5 times more Serratia in the microbiota of T. urticae eggs than in the microbiota of N. californicus females, but the microbiota of N. californicus had 4.3–6.7 times more Rickettsia, Methylophilus, and Pseudomonas than the microbiota of T. urticae eggs (Fig. 6). Eggs of T. urticae from pinto bean had 10 times more Serratia and 4 times more Wolbachia than females of N. californicus (Fig. 6). The abundance of Serratia and Wolbachia in the microbiota of eggs of T. urticae from tomato was also higher than in the microbiota of N. californicus, with Candidatus Hamiltonella showing an opposite trend (Fig. 6).
The microbiota of eggs had higher abundance of Serratia than the microbiota of adult females of T. urticae from all host plants (Fig. 7). The microbiota of adult females of T. urticae had from 4.5 to 6.2 times more Rickettsia than the microbiota of eggs from cotton (Fig. 7) and maize (Fig. 7). Wolbachia and Enterococcus were also more abundant in the microbiota of adult females than in the microbiota of eggs of T. urticae from cotton (Fig. 7), while the opposite was observed for the abundance of the ASV that represented an unassigned genus (Fig. 7).
LDA Effect Size (LEfSe) analyses identified biomarkers in the microbiota associated with T. urticae and N. californicus reared on different host plants. Serratia was identified as a biomarker for the microbiota of eggs of T. urticae regardless of the host-plant association, while the microbiota of adult females of T. urticae was characterized by Rickettsia in females from cotton and maize, Rickettsia and Wolbachia from pinto bean, and Wolbachia from tomato (Fig. 8). Biomarkers for the microbiota of the predatory mite were identified only when feeding on pinto bean- (Pseudomonas) and tomato-prey systems (Paenibacillus) (Fig. 8).
Functional prediction analysis
The relative abundance of the pathways predicted from the microbiota associated with eggs and adult females of T. urticae and adult females of N. californicus was similar regardless of the host-plant association (Fig. 9), except for the lower relative abundance of the amino-acid metabolism (AAM) and xenobiotic biodegradation and metabolism (XBM) pathways. The relative abundance of the AAM and XBM pathways of the microbiota from T. urticae females from maize, pinto bean, and tomato was half that in the remaining samples (Fig. 9).
Comparisons of the potential functional contribution between the microbiota of eggs of two˗spotted spider mite and that of N. californicus females from each host plant indicated significant differences in the potential contribution of each microbiota (Table S2). Differences in the functional contribution changed from plant to plant. In cotton, we did not detect differences in the potential functional contribution for the microbiota of the predatory mite N. californicus compared to that of eggs of T. urticae. However, most comparisons between the microbiota of the predator and prey eggs indicated a higher potential functional contribution of the predator microbiota than the microbiota associated with eggs of T. urticae (Fig. 10). The contribution of the microbiota of eggs of T. urticae exceeded that of the microbiota of the predatory mite only in pathways related to carbohydrate digestion and absorption in maize, pinto bean, and tomato; glycosphingolipid biosynthesis, linoleic acid metabolism, and various types of N-glycan biosynthesis in maize; arabinogalactan biosynthesis in pinto bean; and the calcium signaling and prolactin signaling pathways in maize and pinto bean (Fig. 10).
We also observed differences in the functional contribution of the microbiota between eggs and adult females of T. urticae from all host plants tested (Fig. 10, Table S3). In general, a higher potential functional contribution was observed for microbiota of eggs of T. urticae than for that of adult females, particularly on maize, pinto bean, and tomato (Fig. 10). The egg microbiota functional contribution was higher than that of the microbiota of females in all host plants for the phosphotransferase system (PTS), other glycan degradation, various types of N-glycan biosynthesis, arabinogalactan biosynthesis, and glycosphingolipid biosynthesis (Fig. 10). But the opposite was observed on maize, pinto bean, and tomato plants for pathways involved in thermogenesis, zeatin biosynthesis, protein processing in the endoplasmic reticulum, the p53 signaling pathway, and apoptosis (Fig. 10).