Abstract
While strict vertical transmission insures the durability of intracellular symbioses, phylogenetic incongruences between hosts and endosymbionts suggest horizontal transmission must also occur. These horizontal acquisitions can have important implications for the biology of the host. Wolbachia is one of the most ecologically successful prokaryotes in arthropods, infecting an estimated 50–70% of all insect species. Much of this success is likely due to the fact that, in arthropods, Wolbachia is notorious for manipulating host reproduction to favor transmission through the female germline. However, its natural potential for horizontal transmission remains poorly understood. Here we evaluate the fundamental prerequisites for successful horizontal transfer, including necessary environmental conditions, genetic potential of bacterial strains, and means of mediating transfers. Furthermore, we revisit the relatedness of Wolbachia strains infecting the Panamanian leaf-cutting ant, Acromyrmex echinatior, and its inquiline social parasite, Acromyrmex insinuator, and compare our results to a study published more than 15 years ago by Van Borm et al. (2003). The results of this pilot study prompt us to reevaluate previous notions that obligate social parasitism reliably facilitates horizontal transfer and suggest that not all Wolbachia strains associated with ants have the same genetic potential for horizontal transmission.
Highlights
Wolbachia pipientis is a maternally inherited α-proteobacterium widely found in arthropods (Werren et al, 2008)
The Van Borm study suggested multiple horizontal transmission (HT) events occurred for Acromyrmex ants to acquire their Wolbachia, as evidenced by their findings showing distantly related Wolbachia strains shared by closely related Acromyrmex hosts and the reverse, closely related Wolbachia present in distantly related host species
HT events are believed to have largely contributed to the Wolbachia pandemic, where an estimated 50–70% of all insect species are infected (Werren et al, 2008; Saridaki and Bourtzis, 2010; Weinert et al, 2015)
Summary
Wolbachia pipientis is a maternally inherited α-proteobacterium widely found in arthropods (Werren et al, 2008). Transmission within species is strictly vertical, Wolbachia phylogenies rarely correspond to host phylogenies, suggesting horizontal transmission (HT) occurs (Zhou et al, 1998; Vavre et al, 1999; Raychoudhury et al, 2009; Stahlhut et al, 2010; Ahmed et al, 2013). Intimate inquiline cohabitation where social parasites live with hosts across generations offers ample opportunities for HT. This idea was first explored in Acromyrmex echinatior colonies, which are considered closed systems for endosymbionts since workers are highly aggressive toward non-nestmates (Larsen et al, 2014). Van Borm et al (2003) first suggested that HT events occur between Wolbachia endosymbionts of A. echinatior and A. insinuator based on shared bacterial genotypes between cohabiting ant species. Later research suggested the same for Solenopsis daguerrei, a social parasite of S. saevissima (Dedeine et al, 2005; Martins et al, 2012) and for another fungus-growing ant, Sericomyrmex amabilis, and its social parasite Megalomyrmex symmetochus (Adams et al, 2013; Liberti et al, 2015)
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