The genetic properties of the primary endosymbionts of mealybugs differ from those of other endosymbionts of plant sap-sucking insects

Abstract

Mealybugs (Hemiptera, Coccoidea, Pseudococcidae), like aphids and psyllids, are plant sap-sucking insects that have an obligate association with prokaryotic endosymbionts that are acquired through vertical, maternal transmission. We sequenced two fragments of the genome of Tremblaya princeps, the endosymbiont of mealybugs, which is a member of the beta subdivision of the Proteobacteria. Each of the fragments (35 and 30 kb) contains a copy of 16S-23S-5S rRNA genes. A total of 37 open reading frames were detected, which corresponded to putative rRNA proteins, chaperones, and enzymes of branched-chain amino acid biosynthesis, DNA replication, protein translation, and RNA synthesis. The genome of T. princeps has a number of properties that distinguish it from the genomes of Buchnera aphidicola and Carsonella ruddii, the endosymbionts of aphids and psyllids, respectively. Among these properties are a high G+C content (57.1 mol%), the same G+C content in intergenic spaces and structural genes, and similar G+C contents of the genes encoding highly and poorly conserved proteins. The high G+C content has a substantial effect on protein composition; about one-third of the residues consist of four amino acids with high-G+C-content codons. Sequence analysis of DNA fragments containing the rRNA operon and adjacent regions from endosymbionts of several mealybug species suggested that there was a single duplication of the rRNA operon and the adjacent genes in an ancestor of the present T. princeps. Subsequently, in one mealybug lineage rpS15, one of the duplicated genes, was retained, while in another lineage it decayed. These results extend the diversity of the types of endosymbiotic associations found in plant sap-sucking insects.

FIG. 1.

Genetic maps of DNA fragments from T. princeps from D. brevipes. The thick lines indicate structural genes; the arrows with one arrowhead indicate the direction of transcription; the arrows with two arrowheads indicate overlapping DNA fragments that were cloned into λZAP and sequenced; the striped lines on arrows with two arrowheads indicate the positions of probes; the dashed lines with two arrowheads indicate an identical 5.7-kb sequence; and the lines with brackets indicate DNA fragments obtained by PCR.

FIG. 2.

Genetic map of the upstream and downstream portions of the duplicated regions containing 16S and 23S rDNAs. A and B are two different clusters established on the basis of the evolutionary relationships of T. princeps (Thao et al., submitted). The numbers in the boxes indicate the lengths of fragments having identical nucleotide sequences from the 16S (upstream) and 23S (downstream) oligonucleotide primers used in the PCRs. The dashed lines indicate missing sequence; the striped lines indicate pseudogenes. T. princeps was obtained from the following mealybug species: D. brevipes (Dbr), M. albizziae (Mal), P. citri (Pci), M. australiensis (Mau), and M. hirsutus (Mhi).

FIG. 3.

Amino acid compositions and A+T and G+C contents of codons of proteins of C. ruddii (Car), B. aphidicola (Buc), and T. princeps (Tre). The G+C contents are the G+C contents for the total coding regions. The data for C. ruddii are from reference ; the data for T. princeps are from this study; and the data for B. aphidicola are data for the genes present in C. ruddii and T. princeps (40).

FIG. 4.

Comparisons of the levels of amino acid (AA) sequence identity of homologous proteins of C. ruddii and E. coli (Car), B. aphidicola and P. aeruginosa (Buc), and T. princeps and N. meningitidis (Tre) with the G+C contents of the genes of the endosymbiont.