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Endosymbiotic bacteria of the boar louse Haematopinus apri (Insecta: Phthiraptera: Anoplura) - PubMed

  • ️Sat Jan 01 2022

Endosymbiotic bacteria of the boar louse Haematopinus apri (Insecta: Phthiraptera: Anoplura)

Yudai Nishide et al. Front Microbiol. 2022.

Abstract

Insects exclusively feeding on vertebrate blood are usually dependent on symbiotic bacteria for provisioning of B vitamins. Among them, sucking lice are prominent in that their symbiotic bacteria as well as their symbiotic organs exhibit striking diversity. Here we investigated the bacterial diversity associated with the boar louse Haematopinus apri in comparison with the hog louse Haematopinus suis. Amplicon sequencing analysis identified the primary endosymbiont predominantly detected from all populations of H. apri with some minor secondary bacterial associates. Sequencing and phylogenetic analysis of bacterial 16S rRNA gene confirmed that the endosymbionts of the boar louse H. apri, the hog louse H. suis and the cattle louse Haematopinus eurysternus form a distinct clade in the Gammaproteobacteria. The endosymbiont clade of Haematopinus spp. was phylogenetically distinct from the primary endosymbionts of other louse lineages. Fluorescence in situ hybridization visualized the endosymbiont localization within midgut epithelium, ovarial ampulla and posterior oocyte of H. apri, which were substantially the same as the endosymbiont localization previously described in H. suis and H. eurysternus. Mitochondrial haplotype analysis revealed that, although the domestic pig was derived from the wild boar over the past 8,000 years of human history, the populations of H. apri constituted a distinct sister clade to the populations of H. suis. Based on these results, we discussed possible evolutionary trajectories of the boar louse, the hog louse and their endosymbionts in the context of swine domestication. We proposed 'Candidatus Haematopinicola symbiotica' for the distinct clade of the endosymbionts of Haematopinus spp.

Keywords: Haematopinus apri; Haematopinus suis; boar louse; domestication; evolution; hog louse; symbiont; symbiotic organ.

Copyright © 2022 Nishide, Oguchi, Murakami, Moriyama, Koga and Fukatsu.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1

Bacterial diversity associated with the boar louse Haematopinus apri and the hog louse Haematopinus suis based on amplicon sequencing data of the hypervariable V3/V4 region of 16S rRNA gene. Assigned bacterial taxa are color-coded as shown on the right side. As for sample information and amplicon sequencing data, see Supplementary Table S1.

Figure 2
Figure 2

Phylogenetic placement of the primary endosymbionts of the boar louse H. apri and the hog louse H. suis based on 16S rRNA gene sequences. A maximum-likelihood phylogeny inferred from 1,013 aligned nucleotide sites is shown with bootstrap probability at each node (model: K2 + G + I). Host insect information is shown in parentheses, whereas accession number is indicated in brackets. The louse endosymbiont sequences determined in this study are highlighted in red, whereas the louse endosymbiont sequences reported in previous studies are underlined.

Figure 3
Figure 3

FISH visualization of in vivo localization of the primary endosymbiont in H. apri. (A) Dorsal view of an adult female. (B) Internal organs dissected from an adult female. (C–E) Whole-mount FISH visualization of the symbiont in dissected organs. (C) Localization of the symbiont in the ovarial ampulla of an adult female. The sample is the same as (B). (D) Scattered localization of the symbiont across the midgut epithelial region of an adult female. (E) Similar localization pattern of the symbiont in the midgut epithelium of an adult male. (F–H) Magnified confocal images of the symbiont in the midgut epithelium. (F) Optical section image parallel to the epithelial plane. (G) Optical section image perpendicular to the epithelial plane. (H) Optical section image crossing the epithelial plane, in which a bacteriocyte-like structure with an external pit is seen. The bacteriocyte-like structures are actually extracellular cavities round in shape, full of tubular bacterial cells, and surrounded by the midgut epithelial cells. (I–K) Localization of the symbiont in ovaries and oocytes. (I) Localization of the symbiont in the ovarial ampulla located at the interface of lateral oviduct and oocyte-containing ovarioles. (J) Optical section image of the ovarial ampulla, in which peculiar host cells are densely populated by the symbiont cells. (K) Optical section image of the posterior pole of an oocyte, to which the tubular symbiont cells are infecting through the follicle cell layer. Abbreviations: fc, follicle cell layer; hg, hindgut; mg, midgut; mt, Malpighian tubule; oa, ovarial ampulla; oo, oocyte; ov, ovary; ovd, oviduct; sv, seminal vesicle; ter, tergite; tf, terminal filament. In (B,C), arrowheads indicate the location of the ovarial ampulla. In (G,H), “out” and “in” indicate outside and inside of the midgut, respectively. In (H), an arrow highlights a pit associated with the bacteriocyte-like structure.

Figure 4
Figure 4

Phylogenetic placement of the Neisseria/Snodgrassella-allied bacterium of the boar louse H. apri in the Neisseriaceae based on 16S rRNA gene sequences. A maximum-likelihood phylogeny inferred from 1,063 aligned nucleotide sites is shown with bootstrap probability at each node (model: HKY + G + I). Host insect information is shown in parentheses, whereas accession number is indicated in brackets. The louse derived sequence determined in this study is highlighted in red, whereas the louse derived sequence reported in a previous study is underlined.

Figure 5
Figure 5

(A) Haplotype network of mitochondrial COI gene haplotypes representing 44 individuals from five collection localities of H. apri and two individuals from a collection locality of H. suis. (B) Phylogenetic relationship of the COI haplotypes of H. apri and H. suis. COI gene sequences of H. apri from Japan and China, and those of H. suis from Australia, were retrieved from the DNA databases and analyzed together. A maximum-likelihood phylogeny inferred from 516 aligned nucleotide sites is shown with bootstrap probability at each node (model: HKY + G + I). Collection locality information is shown in parentheses, whereas accession number is indicated in brackets. The collection localities are depicted on the map of Japan. The sequences determined in this study are highlighted in red.

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