Ovary structure and symbiotic associates of a ground mealybug, Rhizoecus albidus (Hemiptera, Coccomorpha: Rhizoecidae) and their phylogenetic implications

Abstract

The ovary structure and the organization of its symbiotic system of the ground mealybug, Rhizoecus albidus (Rhizoecidae), were examined by means of microscopic and molecular methods. Each of the paired elongated ovaries of R. albidus is composed of circa one hundred short telotrophic-meroistic ovarioles, which are radially arranged along the distal part of the lateral oviduct. Analysis of serial sections revealed that each ovariole contains four germ cells: three trophocytes (nurse cells) occupying the tropharium and a single oocyte in the vitellarium. The ovaries are accompanied by giant cells termed bacteriocytes which are tightly packed with large pleomorphic bacteria. Their identity as Brownia rhizoecola (Bacteroidetes) was confirmed by means of amplicon sequencing and fluorescence in situ hybridization techniques. Moreover, to our knowledge, this is the first report on the morphology and ultrastructure of the Brownia rhizoecola bacterium. In the bacteriocyte cytoplasm bacteria Brownia co-reside with sporadic rod-shaped smaller bacteria, namely Wolbachia (Proteobacteria: Alphaproteobacteria). Both symbionts are transmitted to the next generation vertically (maternally), that is, via female germline cells. We documented that, at the time when ovarioles contain oocytes at the vitellogenic stage, these symbionts leave the bacteriocytes and move toward the neck region of ovarioles (i.e. the region between tropharium and vitellarium). Next, the bacteria enter the cytoplasm of follicular cells surrounding the basal part of the tropharium, leave them and enter the space between the follicular epithelium and surface of the nutritive cord connecting the tropharium and vitellarium. Finally, they gather in the deep depression of the oolemma at the anterior pole of the oocyte in the form of a 'symbiont ball'. Our results provide further arguments strongly supporting the validity of the recent changes in the classification of mealybugs, which involved excluding ground mealybugs from the Pseudococcidae family and raising them to the rank of their own family Rhizoecidae.

Insect Bacteriocytes: Adaptation, Development, and Evolution

“…In contrast to infection at the posterior pole of the oocyte, infection at the anterior pole of the oocyte is somewhat unique among insects. For example, in the ground mealybug Rhizoecus albidus, after leaving the bacteriocytes, Brownia rhizoecola migrates toward the ovaries and infects the anterior pole of the oocyte (109). This transmission strategy also occurs in the mealybugs Phenacoccinae and Pseudococcidae (64,121); the eriococcids Greenisca brachypodii, Acanthococcus aceris, and Gossyparia spuria (65,67); and Puto superbus (Putoidae) (110).…”

Nutrient supplementation by genome-eroded Burkholderia symbionts of scale insects

“…Its phylogeny and classification are still under discussion; however, the 18S rRNA-based phylogeny of scale insects suggests a paraphyletic origin of eriococcids [11,12]. Previous studies showed that analyses of scale insect symbiotic microorganisms might help resolve relationships within and among Coccoidea families [13,14]. Results of studies on symbionts of Eriococcidae, although few, also indicate their diversity [15][16][17][18].…”