Transmission of symbiotic fungus with a nonsocial leaf-rolling weevil

Abstract

Transmission modes of symbionts with fungus-growing insects are closely related to the stability of symbioses. As compare to social fungus-farming insects, transmission modes of nonsocial fungus-farming insects need to be further investigated. Euops chinensis, a nonsocial leaf-rolling weevil, harbors a symbiotic fungus Penicillium herquei in the specialized mycangium. Previous works have indicated that P. herquei is cultivated to host plants by females during oviposition process, however, it is still unclear when (before or after oviposition) and how P. herquei is transmitted. In this study, we observed fungal cultivating behaviors and adult bodies by scanning electron microscopy (SEM), and compared isolation rates of P. herquei on ovary eggs, newly oviposited eggs, cradle leaves (leaf pieces cut before rolling cradles), cradles, and female mycangia. Fungal isolates were identified by internal transcribed spacers (ITS) and cytochrome oxidase I (COI) genes. We found the female's serrated tarsi and comb-like setae on the abdomen were specialized structures for fungal transmission. Newly oviposited eggs showed 81.11% frequency of fungal symbionts, but ovary eggs did not show any growth of fungal symbionts. Isolation rates of P. herquei on cradle leaves, mycangia and cradles were 76.67%, 77.71% and 87.72%, respectively. Analyses of ITS and COI genes showed that isolated fungal strains belong to the same species. We concluded that P. herquei was transmitted before oviposition, and the female's tarsi are newly found specialized structures for fungal transmission. This study elucidates the cultivar transmission mode with fungus-farming attelabid weevils, and might be useful to study of other fungiculture mutualisms.