Cell Structure of the Preoral Mycangia of Xyleborus (Coleoptera: Curculiondiae) Ambrosia Beetles.

Ambrosia beetles have evolved specialized structures termed "mycangia", which house and transport symbiotic microbes. Microbial partners include at least one obligate mutualistic filamentous fungus used as food for larvae and adults, and potentially secondary filamentous fungi, yeasts, and bacteria. Beetles in the genus Xyleborus possess paired pre-oral mycangial structures located within the head on either side of the mouth parts. Mycangia develop in pupae, with newly emerged adults acquiring partners from the environment. However, information concerning the cellular structure and function of Xyleborus mycangia remains limited. We show that in X. affinis, mycangia are lined with a layer of striated dense material, enclosing layers of insect epithelial cells, with diverse spine-like structures. Larger (5-10 μm) projections were concentrated within and near the entrance of mycangia, with smaller filaments (4-8 μm) within the mycangia itself. Rows of "eyelash" structures lined the inside of mycangia, with fungal cells free-floating or in close association with these projections. Serial sections revealed mandibular articulations, and mandibular, pharyngeal, and labial muscles, along with the mycangial entry/exit channel. Sheets of comb-like spines at the mycangial entrance and opposite the mycangia attached to the roof of the labrum or epipharynx may serve as an interlocking mechanism for opening/closing the mycangia and guiding fungal cells into entry/exit channels. Additionally, mandibular fibra (muscle tissue) potentially enervating and affecting the mechanism of mycangial functioning were noted. These data add crucial mechanistic detail to the model of pre-oral mycangia in Xyleborus beetles, their cellular structures, and how they house and dispense microbial symbionts.