Bacterial Communities Associated with Venom Glands in Different Ant Species and Different Castes of S. invicta
A total of 2,334,172 reads were obtained after filtering the sequencing data, averaging 129,676 reads per sample, and 1,453 OTUs were identified from all samples. A sample-based species dilution curve showed that the sequencing depth was sufficient to cover all bacterial species and reflect the richness level, which also ensured that the sample sequencing data in each group were reasonable (Figure S1). The alpha diversity, including the Shannon diversity index, was analyzed to assess the species diversity of each individual sample. As shown in Figure S2, there was no significant difference between the flora diversities of samples (one-way ANOVA F5,12=0.786, df = 5, p = 0.579, Figure S2). Microbial composition analysis of each group showed that the numbers of phyla in the different samples were not the same: 21 in DrW, 18 in GZSiQ, 30 in GZSiG, 18 in GZSiW, 18 in GXSiW, and 20 in GXSgW. Proteobacteria (mean ± SE) were the dominant bacteria in GXSiW (49.85 ± 13.88%), GXSgW (85.65 ± 14.77%), GZSiG (74.35 ± 35.74%), GXSiQ (51.12 ± 32.76%), and GZSiW (98.08 ± 1.23%) but less abundant in DrW (35.27 ± 6.62%). Firmicutes were dominant in DrW (46.67 ± 22.92%) compared to GXSiW (22.30 ± 22.30%), GXSgW (7.61 ± 8.15%), and GZSiW (11.70 ± 14.71%). Tenericutes (19.81 ± 34.21%) occupied a secondary position (Fig. 1) in the Guangzhou red fire ant queens.
At the genus level, higher percentages of Pseudomonas were associated with venom glands of DrW (11.56 ± 9.59%), followed by GXSgW (11.44 ± 11.11%) and GZSiW (0.97 ± 0.66%). Pseudomonas associated with the glands of GZSiQ (5.29 ± 1.64%) and GZSiG (2.23 ± 0.69%) was the second and third most abundant genus, respectively. Mesoplasma was most abundant in the venom glands of GXSiW (19.80 ± 34.20%) and GZSiQ (20.82 ± 36.02%). The percentage of Streptococcus associated with GZSiG (4.48 ± 7.42%) was higher than that associated with other ants. Exiguobacterium was most associated with the glands of GXSiW (16.03 ± 24.90%), GXSgW (5.87 ± 7.14%), and GZSiW (0.35 ± 0.07%) ants. The percentage of the genus Proteus was significantly higher in the glands of DrW (8.56 ± 14.80) than those of all other groups of ants (Table 1).
Table 1
Relative abundances (mean ± SE) of the most common bacterial genera associated with the venom glands of Solenopsis invicta, Solenopsis geminata and Diacamma rugosum
Taxon | DrW (%) | GXSiW (%) | GXSgW (%) | GZSiG (%) | GZSiQ (%) | GZSiW (%) |
Mesoplasma | 0.06 ± 0.03 | 19.8 ± 34.2 | 0.75 ± 0.98 | 0.03 ± 0.01 | 20.83 ± 36.03 | 0.04 ± 0.03 |
Pseudomonas | 11.56 ± 9.59 | 8.04 ± 3.48 | 11.45 ± 11.11 | 2.23 ± 0.69 | 5.29 ± 1.64 | 0.97 ± 0.66 |
Exiguobacterium | 1.2 ± 0.91 | 16.03 ± 24.91 | 5.87 ± 7.14 | 2.09 ± 1.59 | 1.48 ± 1.22 | 0.35 ± 0.07 |
Acinetobacter | 2.87 ± 2.76 | 5.19 ± 6.01 | 0.42 ± 0.38 | 1.83 ± 2.14 | 1.75 ± 1.82 | 0.14 ± 0.05 |
Proteus | 8.56 ± 14.8 | 0.03 ± 0.02 | 0.02 ± 0.01 | 0.02 ± 0 | 0.01 ± 0.01 | 0.02 ± 0 |
Streptococcus | 0.51 ± 0.85 | 0.44 ± 0.52 | 0.01 ± 0 | 4.48 ± 7.42 | 2.22 ± 2.31 | 0.01 ± 0 |
Bacillus | 0.01 ± 0.01 | 0.61 ± 0.19 | 0.03 ± 0.04 | 0.88 ± 1.05 | 2.44 ± 4.18 | 0.06 ± 0.07 |
Enterococcus | 0.5 ± 0.83 | 2.85 ± 4.15 | 0.01 ± 0 | 0.08 ± 0.13 | 0 | 0 |
Rothia | 0.01 ± 0 | 0.09 ± 0.15 | 0.01 ± 0 | 1.58 ± 2.73 | 1 ± 1.15 | 0.01 ± 0 |
Stenotrophomonas | 1.83 ± 2.23 | 0.23 ± 0.22 | 0.58 ± 0.95 | 0.01 ± 0.01 | 0.01 ± 0.01 | 0.01 ± 0 |
For each sample, the abundance of each taxon was compared to total abundance of the microbial community members. DrW: D. rugosum workers, GXSgW: Guangxi S. geminata workers from, GZSiW: Guangzhou S. invicta workers, GXSiW: Guangxi S. invicta workers, GZSiG: Guangzhou S. invicta alates, GZSiQ: Guangzhou S. invicta queens. |
Beta Diversity Analysis inside Venom Glands of Different Ant Species or Castes of S. invicta
PCoA was used to further compare species diversity differences in bacterial communities associated with the ant venom glands. PCoA for workers of the three ant species, GZSiW, GXSgW, and DrW, showed significant differences in the microbial community diversity of the three ant species (PERMANOVA F = 5.822, df = 2, p = 0.002, Fig. 2A). PCoA performed on the venom gland-associated bacterial communities of workers ants of S. invicta collected from different locations (GXSiW and GZSiW) (Fig. 2B) also showed significant differences in diversity. There were considerable diversity differences in bacterial communities present in the venom glands of castes (workers, alates, and queens) of S. invicta collected from Guangzhou (PERMANOVA F = 1.190, df = 2, p = 0.033), as depicted in the PCoA graph (Fig. 2C).
Relative Abundances in Bacterial Communities Associated with Venom Glands of Three Ant Species and Different Castes of S. invicta
The PERMANOVA test was carried out on the bacterial communities of the venom glands of GZSiW, GZSiG and GZSiQ. ANOVA of bacterial populations with relative abundances greater than 1% in at least one replicate showed that the relative abundance of Pseudomonas in the venom glands of S. invicta queens (GZSiQ) was significantly higher than those of workers (GZSiW) and alates (GZSiG) of this ant species (ANOVA F = 12.280, df = 2, p = 0.008, Fig. 3A). The relative abundance of Spiroplasma in GXSgW was significantly higher than those in DrW and GZSiW (Kruskal-Wallis H = 6.006, df = 2, p = 0.05, Fig. 3B), while the relative abundance of Lactococcus associated with the venom gland of DrW was considerably higher than those in GXSgW and GZSiW (Kruskal-Wallis H = 6.489, df = 2, p = 0.039, Fig. 3C).
Relative Abundances in Bacterial Communities of S. invicta Workers Collected from Different Locations
Comparing the whole bacterial communities in the venom gland of S. invicta workers collected from two populations (GZSiW and GXSiW) showed differences between GZSiW and GXSiW. A PCoA map (Fig. 2B) showed that the relative abundances of Bacillus and Lactococcus in the venom glands of GXSiW were significantly higher than those of GZSiW (t-test, t1 = 4.689, df1 = 4, p1 = 0.009; t2 = 5.462, df2 = 4, p2 = 0.005, Fig. 4A, B).