Huang, Jianjian

Psyllids harbour the obligate nutritional symbiont ‘ Candidatus Carsonella ruddii’ ( Ca Cr), yet complete Ca Cr genomes remain unevenly sampled across Psylloidea, limiting both comparative analysis and temporal inference. Here, seven complete Ca Cr genomes (165–174 kb) were assembled and annotated from psyllid hosts representing four families, including lineages for which genome-grade resources had previously been unavailable. We integrated whole-genome phylogenomics, pangenome analysis, fossil-calibrated relaxed-clock dating and quantitative PCR (based on 16S rRNA) screening of secondary symbionts. Across hosts, Ca Cr retained an extremely reduced, AT-rich (>82%) and gene-dense (>90% coding) genome architecture with a conserved core of 155 genes, while the accessory fraction was limited and lineage-specific. Most variable genes belonged to amino-acid metabolism or proteins of unknown function, suggesting differential erosion of peripheral functions around a stable translational and informational core. Phylogenomically, Ca Cr diversification broadly tracked deep host diversification, although the Ca Cr lineage from Diaphorina citri grouped with Triozidae-associated lineages rather than with other sampled Psyllidae. Using two host fossil-informed soft calibrations, we estimated that major Ca Cr divergences occurred mainly from the Paleogene to the Miocene, with crown diversification of Cacopsylla -associated Ca Cr at 15.95–18.99 Ma. In contrast to the stability of the primary symbiosis, secondary symbionts showed patchy host distributions: Wolbachia- and Arsenophonus -like lineages occurred in multiple hosts, ‘ Candidatus Profftella armatura’ was restricted to D. citri , and no secondary symbiont was detected in Cacopsylla chinensis . These results provide a time-resolved comparative framework for Ca Cr evolution in psyllids and underscore the different evolutionary stability of primary and facultative associates.

‘Candidatus Liberibacter asiaticus’ is the bacterium associated with the citrus disease known as huanglongbing (HLB). This study evaluated the influence of ‘Ca. L. asiaticus’ infection on a number of key plant physiological variables concerning photosynthesis, cell integrity, reactive oxygen species scavengers’ activity, and osmoregulation of two different species of citrus—the pomelo Citrus maxima and the mandarin C. reticulata ‘Tankan’—relative to their measured ‘Ca. L. asiaticus’ infection load. Results indicated that all measured physiological variables except soluble sugar were affected by increased ‘Ca. L. asiaticus’ infection titers, wherein the variety C. maxima proved overall more resistant than C. reticulata. ‘Ca. L. asiaticus’ infection was linked in both plants to decrease in chlorophyll concentration, cell membrane permeability, and malondialdehyde, as well as increased free proline and starch contents. Chlorophyll fluorescence measurements taken 9 months after grafting the mandarin C. reticulata with ‘Ca. L. asiaticus’ scions revealed a significant decrease in the photosynthesis variables maximum photochemical quantum yield of photosystem II (PSII), effective photochemical quantum yield of PSII, and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae, whereas nonphotochemical fluorescence quenching increased significantly; C. maxima plants, on the other hand, did not show significant differences until the 12th month from infection exposure. The variables superoxide dismutase, catalase, peroxidase, and soluble protein initially increased and later decreased. In addition, progression of ‘Ca. L. asiaticus’ replication in both citrus species was accompanied by rapid changes in three reactive oxygen species scavenging enzymes in C. maxima, while the pattern was different in C. reticulata. We hypothesize that the observed interspecific differences in physiological change are related to their relative resistance against ‘Ca. L. asiaticus’ infection. These results provide a scaffold for better describing the pathogenesis, selecting the most resistant breeds, or even validating pertaining omics research; ultimately, these detailed observations can facilitate the diagnosis of ‘Ca. L. asiaticus’ infection.