A Freeloader? The Highly Eroded Yet Large Genome of the Serratia symbiotica Symbiont of Cinara strobi

Abstract

Genome reduction is pervasive among maternally inherited bacterial endosymbionts. This genome reduction can eventually lead to serious deterioration of essential metabolic pathways, thus rendering an obligate endosymbiont unable to provide essential nutrients to its host. This loss of essential pathways can lead to either symbiont complementation (sharing of the nutrient production with a novel co-obligate symbiont) or symbiont replacement (complete takeover of nutrient production by the novel symbiont). However, the process by which these two evolutionary events happen remains somewhat enigmatic by the lack of examples of intermediate stages of this process. Cinara aphids (Hemiptera: Aphididae) typically harbor two obligate bacterial symbionts: Buchnera and Serratia symbiotica. However, the latter has been replaced by different bacterial taxa in specific lineages, and thus species within this aphid lineage could provide important clues into the process of symbiont replacement. In the present study, using 16S rRNA high-throughput amplicon sequencing, we determined that the aphid Cinara strobi harbors not two, but three fixed bacterial symbionts: Buchnera aphidicola, a Sodalis sp., and S. symbiotica. Through genome assembly and genome-based metabolic inference, we have found that only the first two symbionts (Buchnera and Sodalis) actually contribute to the hosts’ supply of essential nutrients while S. symbiotica has become unable to contribute towards this task. We found that S. symbiotica has a rather large and highly eroded genome which codes only for a few proteins and displays extensive pseudogenization. Thus, we propose an ongoing symbiont replacement within C. strobi, in which a once “competent” S. symbiotica does no longer contribute towards the beneficial association. These results suggest that in dual symbiotic systems, when a substitute cosymbiont is available, genome deterioration can precede genome reduction and a symbiont can be maintained despite the apparent lack of benefit to its host.

Serial horizontal transfer of vitamin-biosynthetic genes enables the establishment of new nutritional symbionts in aphids’ di-symbiotic systems

“…Regarding the putative origin of these transfers, Sodalis -allied bacteria are found as facultative or obligate endosymbionts across different insect taxa [12, 72, 74, 75] and likely also in plant tissues [76]. Furthermore, Sodalis have been previously found in aphids through 16S rRNA gene sequencing surveys [14, 18, 77]. Therefore, it is likely that Erwinia and a Sodalis -related facultative endosymbiont could have co-infected aphids, and that such co-infection facilitated the horizontal transfer of these genes to the plasmid of E. haradaeae with subsequent transfers of bioD and thiI to the chromosome.…”

Six-state amino acid recoding is not an effective strategy to offset the effects of compositional heterogeneity and saturation in phylogenetic analyses

“…Organismal scope or featured taxon Cunha & Giribet (2019) yes Gastropods Laumer et al (2019) yes Animals Lemer et al (2019) yes Bivalves Lozano-Fernandez et al (2019) yes Chelicerates Marlétaz et al (2019) yes Spiralia Philippe et al (2019) yes Bilateria Narayanan Kutty et al (2019) no Calyptratae Uribe et al (2019) no Gastropods Wolfe et al (2019) no Decapod crustaceans Zverkov et al (2019) no Dicyemida and Orthonectida Aouad et al (2018) yes Archaea Laumer et al (2018) yes Placozoa Otero-Bravo et al (2018) yes Pantoea Puttick et al (2018) yes Land plants Schwentner et al (2018) yes Pancrustacea Sousa et al (2018) yes Land plants Bennett & Mao (2018) no Fulgoroidea symbionts Eitel et al (2018) no Placozoa Manzano-Marín et al (2018) no Cinara strobi symbionts Feuda et al (2017) yes Animals Szabó et al (2017) yes Pseudococcidae symbionts Williams et al (2017) yes Archaea Schwentner et al (2017) no Pancrustacea Shin et al (2017) no Curculionoidea Simion et al (2017) no Animals Yoshida et al (2017) no Tardigrades Leliaert et al (2016) yes Viridiplantae Zhang et al (2016) yes Roseobacter CHAB-I-5 lineage He et al (2016) no Rhizaria Song et al (2016) no Holometabola Domman et al (2015) yes Plastids Luo (2015) yes SAR11 Petitjean et al (2015) yes Archaea Borowiec et al (2015) no Animals Derelle et al (2015) no Eukaryotes Wang & Wu (2015) no Mitochondria…”

The protector within: Comparative genomics of APSE phages across aphids reveals rampant recombination and diverse toxin arsenals

“…For this filtration protocol circa 15 aphids from one colony were pooled together. Extracted DNA was used to prepare custom paired-end libraries in Genoscope as in (Manzano-Marín et al, 2018). These libraries were sequenced using either 151 or 251 bp paired-end reads chemistry on a HiSeq2500 Illumina sequencer.…”