Insight into the Transmission Biology and Species-Specific Functional Capabilities of Tsetse (Diptera: Glossinidae) Obligate Symbiont Wigglesworthia

Abstract

Ancient endosymbionts have been associated with extreme genome structural stability with little differentiation in gene inventory between sister species. Tsetse flies (Diptera: Glossinidae) harbor an obligate endosymbiont, Wigglesworthia, which has coevolved with the Glossina radiation. We report on the ~720-kb Wigglesworthia genome and its associated plasmid from Glossina morsitans morsitans and compare them to those of the symbiont from Glossina brevipalpis. While there was overall high synteny between the two genomes, a large inversion was noted. Furthermore, symbiont transcriptional analyses demonstrated host tissue and development-specific gene expression supporting robust transcriptional regulation in Wigglesworthia, an unprecedented observation in other obligate mutualist endosymbionts. Expression and immunohistochemistry confirmed the role of flagella during the vertical transmission process from mother to intrauterine progeny. The expression of nutrient provisioning genes (thiC and hemH) suggests that Wigglesworthia may function in dietary supplementation tailored toward host development. Furthermore, despite extensive conservation, unique genes were identified within both symbiont genomes that may result in distinct metabolomes impacting host physiology. One of these differences involves the chorismate, phenylalanine, and folate biosynthetic pathways, which are uniquely present in Wigglesworthia morsitans. Interestingly, African trypanosomes are auxotrophs for phenylalanine and folate and salvage both exogenously. It is possible that W. morsitans contributes to the higher parasite susceptibility of its host species.

Interwoven Biology of the Tsetse Holobiont

“…An additional extracellular population is found in the female milk glands which is maternally transmitted to offspring (40,41). Described roles of this symbiont include both nutrient provisioning, where Wigglesworthia supplements B vitamins lacking in the tsetse blood diet (38,(53)(54)(55), and contributions to the maturation of host immunity (35,56,57). Insight into these roles has been found by examining tsetse biology upon removal of the symbiont.…”

“…Genome adaptations by Wigglesworthia, while in association with tsetse, are believed to have resulted in the loss of many capabilities required for a free-living lifestyle through reductive evolution (53,54). Despite its small genome size, the majority of B-vitamin biosynthesis pathways remain intact, supporting the nutritional mutualism.…”

Signatures of host/symbiont genome coevolution in insect nutritional endosymbioses

“…In Sternorrhyncha, BCA mediates the terminal step in branched-chain amino acid biosynthesis, complementing the missing symbiont gene, ilvE (14,16). On the basis of the Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations, we propose that in the divergent blood feeders P. humanus and G. morsitans, BCA also complements the missing symbiont ilvE, this time mediating conversion of valine to pantoate in panthothenate biosynthesis (4,12,44). These examples demonstrate that at least three times, symbionts have independently lost the same gene (ilvE) whose function was complemented by a conserved host gene, indicating that symbiont gene loss is indeed constrained by host gene repertoire.…”

“…Given the common patterns of collaboration in amino acid biosynthesis across holobionts (reviewed in refs. 9 and 17), it is notable that the endosymbionts of P. citri (8), Homalodisca vitripennis (19), P. humanus (4), and G. morsitans (12), like Buchnera APS, have lost panD and therefore the ability to synthesize β-alanine. At the same time their role in pantothenate biosynthesis is strongly supported by retention of panC, a gene encoding pantothenate synthetase (E.C.…”

“…The second was that of the human body louse, Pediculus humanus humanus and its primary bacterial endosymbiont Candidatus Riesia pediculicola (4), quickly followed by the carpenter ant, Camponotus floridanus (5) and its endosymbiont Candidatus Blochmannia floridanus (6). Other insect holobiont genomes now include the citrus mealybug, Planococcus citri with its dual endosymbionts (7,8); the hackberry petiole gall psyllid Pachypsylla venusta with its endosymbiont Candidatus Carsonella ruddii (9); and the tsetse fly, Glossina morsitans (10) with its endosymbiont Wigglesworthia glossinidia (11,12). This recent accumulation of holobiont genomes facilitates elucidation of the patterns that characterize coevolution in these ancient, intimate symbiotic associations.…”