Effects Of Unfavorable Temperature On The Whitefly Bemisia Tabaci Symbionts

The whitefly Bemisia tabaci(Hemiptera:Aleyrodidae) is a species complex including at least36cryptic species, of which the Middle East-Asia Minor1(MEAM1) and Mediterranean (MED) are notorious invasive pests. The symbiont species of whitefly is diverse, and seven symbionts have been found, which include primary symbiont "Candidatus Portiera aleyrodidarum" and six secondary symbionts "Candidatus Hamiltonella defensa", Rickettsia spp., Arsenophonus spp.,"’Candidatus Cardinium hertigii’", Wolbachia spp.,"Candidatus Fritschea bemisiae","Candidatus Hemipteriphilus asiaticus". Primary symbiont is necessary for whitefly by providing essential nutritions, secondary symbionts are not necessary but have also important functions to the host. In whitefly, all species contain primary symbiont "Ca. Portiera aleyrodidarum", each species can have one or more secondary symbionts. In our field-collected whitefly samples, MEAM1harbors an obligate primary symbiont "Ca. Portiera aleyrodidarum" and two secondary symbionts,"Ca. Hamiltonella defensa" and Rickettsia sp., whereas MED has only "Ca. Portiera aleyrodidarum "and "Ca. Hamiltonella defensa". Both "Ca. Portiera aleyrodidarum"and "Ca. Hamiltonella defensa" are intracellular endosymbionts residing in the bacteriomes, whereas Rickettsia sp. has a scattered distribution throughout the host body cavity.We examined responses of these symbionts to adverse temperatures as well as survival of the host insects. After cold treatment at5℃or10℃or heat treatment at35℃or40℃for24hours respectively, the infection rates of all symbionts were not significantly decreased based on diagnosis PCR. However, quantitative PCR assays indicated significant reduction of "Ca. Hamiltonella defensa"at40℃, other symbionts were not significanly affected. When whiteflies of the two species were treated at40℃for1-5days, the symbiont "Ca. Hamiltonella defensa" reduced significantly in first day and the reduction became greater as the duration increased. Compared with "Ca. Hamiltonella defensa", primary symbiont "Ca. Portiera aleyrodidarum" was initially less affected in the first day but then showed more rapid reduction at days3-5. The density of Rickettsia sp. fluctuated but was not reduced significantly at40℃. FISH detection verified the same results. Meanwhile, the mortality rates of the host whiteflies elevated rapidly as the duration of exposure to heat treatment increased. The differential responses of various symbionts to adverse temperatures implycomplex interactions among the symbionts inside the same host insect and highlight the importance of taking the whole bacterial community into account in studies of symbioses.When exposed persistently at35℃, both infection rates and amount of symbionts underwent some detrimental effects. At35℃for5-20days, the infection rates of "Ca. Portiera aleyrodidarum" and "Ca. Hamiltonella defensa" reduced continually in MEAM1adults, and the infection rates of the two symbionts also reduced significantly in F1adults for30days. While the infection rate of another symbiont Rickettsia sp. was not affected significantly during30days. Through the quantitative PCR dectecion in F1adults, the relative densities of "Ca. Portiera aleyrodidarum" and "Ca. Hamiltonella defensa" in MEAM1were reduced by>99%, while Rickettsia sp. were reduced by82.6%and34.2%in female and male whiteflies respectively and there was no significant difference in the reduction of this symbiont in males. For MED, the relative densities of "Ca. Portiera aleyrodidarum" and "Ca. Hamiltonella defensa" were also reduced close to or achieve99%. The4th instar nymph had the same result by FISH detection. The bacteriome and bacteriocyte of whitefly were suffered negative effects during high temperature stress. The whitefly contains two bacteriomes during1-4instar nymphs at25℃, while the proportion of individuals with two bacteriomes decreased to25-60%at35℃. Moreover, bacteriocytes differentiation was inhibited at35℃, which was showed by the reduction of numbers of bacteriocytes in3rd and4th instar nymphs and adults. The bacteriomes were dark under white transmitted-light in4th instar nymphs and adults that were reared at35℃, while they are transparent under white transmitted-light at25℃. MEAM1and MED whitefly can’t survive to the next generation at35℃. The fecundity of MEAM1and MED whitefly that were reared from egg to adults at35℃and then transferred to25℃were less than control, and their eggs could not hatch successfully. The symbionts lived in bacteriocyte were more vulnerable than those lived extracellular at high temperature stress, probably suggest the high temperature damaged the bacteriocytes and then affected intracellar symbionts. At high temperature stress, the symbionts including pramiry symbionts reduced significantly and eventually disappeared and the host insect failed to produce nex generation simultaneously in two species of whitefly implies a close association between them.