Dynamic evolution of diaphorin biosynthetic genes against a background of genome conservation in Profftella

The Asian citrus psyllid Diaphorina citri, a significant agricultural pest, is home to the obligate bacterial symbiont Candidatus Profftella armatura, known as Profftella. This bacterium, integrated within its host, allocates a substantial portion of its highly reduced genome to the production of diaphorin, a bioactive polyketide.

To understand the evolution of this unique symbiont, researchers sequenced the genomes of Profftella and its co-resident nutritional symbiont, Candidatus Carsonella ruddii (Carsonella), from Diaphorina lycii and compared them with those from D. citri and D. cf. continua. Both Profftella and Carsonella genomes from D. lycii measured 471 and 174 kb, respectively, with Profftella containing an 8-kb plasmid.

Despite high conservation in genome organization and gene content across the three lineages, pronounced structural variations were observed in diaphorin synthases DipP and DipT. These synthases, encoded by two of the 20 diaphorin biosynthetic genes, showcased extensive amplification of acyl carrier protein (ACP) domains, with up to seven copies compared to one or two at similar positions in related pederin synthases.

This copy-number variation among Profftella lineages indicates that ACP amplification remains evolutionarily dynamic, even in highly integrated obligate symbionts. Thus, even closely associated symbionts maintain evolutionary flexibility in specialized functional systems.