Two male-killing Wolbachia from Drosophila birauraia that are closely related but distinct in genome structure

Abstract

Insects harbour diverse maternally inherited bacteria and viruses, some of which have evolved to kill the male progeny of their hosts (male killing: MK). The fly species Drosophila biauraria carries a maternally transmitted MK-inducing partiti-like virus, but it was unknown if it carries other MK-inducing endosymbionts. Here, we identified two male-killing Wolbachia strains (wBiau1 and wBiau2) from D. biauraria and compared their genomes to elucidate their evolutionary processes. The two strains were genetically closely related but had exceptionally different genome structures with considerable rearrangements compared with combinations of other Wolbachia strains. Despite substantial changes in the genome structure, the two Wolbachia strains did not experience gene losses that would disrupt the male-killing expression or persistence in the host population. The two Wolbachia-infected matrilines carried distinct mitochondrial haplotypes, suggesting that wBiau1 and wBiau2 have invaded D. biauraria independently and undergone considerable genome changes owing to unknown selective pressures in evolutionary history. This study demonstrated the presence of three male-killers from two distinct origins in one fly species and highlighted the diverse and rapid genome evolution of MK Wolbachia in the host.

Keywords: Wolbachia; evolution; genome rearrangement; male killing.

Figure 1.

Sex ratio distortion in Drosophila biauraria matrilines. (a) Morphology of D. biaurara female adult (W1 line). (b) Sex ratio of adult flies in Wolbachia-infected W1 and W2 matrilines with and without tetracycline treatment. Each treatment had two replicates. Sample sizes (number of individuals examined) are given in parentheses. (c) Hatchability of W1 and uninfected matrilines (NSR, SP11-20). The total number of replicates, hatched and unhatched individuals, are shown below the whisker plot. The horizontal bar within the box represents the median. The upper and lower hinges of the box indicate the upper and lower quartiles, respectively. Different letters indicate significant differences between groups (Wilcoxon test, p < 0.05). (d) Sex ratio of hatched larvae and unhatched embryos examined by PCR assays targeting the male-specific Y chromosome. Sample sizes (number of examined individuals) are given in parentheses. NSR, normal sex ratio.

Figure 2.

Genomic similarities between Wolbachia strains wBiau1 and wBiau2 and phylogeny of the host D. biauraria. (a) Phylogenetic tree of concatenated Wolbachia wsp and MLST sequences constructed based on maximum likelihood with bootstrap re-sampling of 1,000 replicates using MEGA7 [37]. Homologs of Wolbachia strains were quoted from the Wolbachia MLST database. (b) Protein clusters conserved within wBiau1 and wBiau2 strains. (c–j) Dot plots showing conserved syntenies between Wolbachia strains. wBiau1 and wBiau2 in D. biauraria (c), wMel and wAu in D. melanogaster and D. simulans, respectively (d), wBiau1 and wMel (e), wBiau2 and wMel (f), wAu and wRi in D. simulans (g), wMa and wNo in D. simulans (h), wHm-t in H. magnanima and wNo (i), and wPip in C. pipiens and wNo (i), were compared. (k) Phylogenetic tree of mitochondria COI sequences of D. biauraria. Drosophila auraria [GenBank AB669695.1] was used as an outgroup.