Chiel Elad, Zchori-Fein Einat, Inbar Moshe, Gottlieb Yuval, Adachi-Hagimori Tetsuya, Kelly Suzanne E, Asplen Mark K, Hunter Martha S
Department of Evolutionary and Environmental Biology, University of Haifa, Haifa, Israel.
PLoS One. 2009;4(3):e4767. doi: 10.1371/journal.pone.0004767. Epub 2009 Mar 10.
Many intracellular microbial symbionts of arthropods are strictly vertically transmitted and manipulate their host's reproduction in ways that enhance their own transmission. Rare horizontal transmission events are nonetheless necessary for symbiont spread to novel host lineages. Horizontal transmission has been mostly inferred from phylogenetic studies but the mechanisms of spread are still largely a mystery. Here, we investigated transmission of two distantly related bacterial symbionts--Rickettsia and Hamiltonella--from their host, the sweet potato whitefly, Bemisia tabaci, to three species of whitefly parasitoids: Eretmocerus emiratus, Eretmocerus eremicus and Encarsia pergandiella. We also examined the potential for vertical transmission of these whitefly symbionts between parasitoid generations. Using florescence in situ hybridization (FISH) and transmission electron microscopy we found that Rickettsia invades Eretmocerus larvae during development in a Rickettsia-infected host, persists in adults and in females, reaches the ovaries. However, Rickettsia does not appear to penetrate the oocytes, but instead is localized in the follicular epithelial cells only. Consequently, Rickettsia is not vertically transmitted in Eretmocerus wasps, a result supported by diagnostic polymerase chain reaction (PCR). In contrast, Rickettsia proved to be merely transient in the digestive tract of Encarsia and was excreted with the meconia before wasp pupation. Adults of all three parasitoid species frequently acquired Rickettsia via contact with infected whiteflies, most likely by feeding on the host hemolymph (host feeding), but the rate of infection declined sharply within a few days of wasps being removed from infected whiteflies. In contrast with Rickettsia, Hamiltonella did not establish in any of the parasitoids tested, and none of the parasitoids acquired Hamiltonella by host feeding. This study demonstrates potential routes and barriers to horizontal transmission of symbionts across trophic levels. The possible mechanisms that lead to the differences in transmission of species of symbionts among species of hosts are discussed.
许多节肢动物的细胞内微生物共生体通过严格的垂直传播方式，并以增强自身传播的方式操控宿主的繁殖。然而，罕见的水平传播事件对于共生体传播到新的宿主谱系来说是必要的。水平传播大多是从系统发育研究中推断出来的，但传播机制在很大程度上仍是个谜。在这里，我们研究了两种远缘细菌共生体——立克次氏体和汉密尔顿菌——从它们的宿主烟粉虱传播到三种粉虱寄生蜂：埃氏啮小蜂、荒漠啮小蜂和佩氏恩蚜小蜂的情况。我们还研究了这些粉虱共生体在寄生蜂代际间垂直传播的可能性。通过荧光原位杂交（FISH）和透射电子显微镜，我们发现立克次氏体在受立克次氏体感染的宿主发育过程中侵入埃氏啮小蜂幼虫，在成虫和雌虫体内持续存在，并到达卵巢。然而，立克次氏体似乎并未穿透卵母细胞，而是仅定位在滤泡上皮细胞中。因此，立克次氏体不会在埃氏啮小蜂中垂直传播，诊断性聚合酶链反应（PCR）也证实了这一结果。相比之下，立克次氏体在恩蚜小蜂的消化道中只是短暂存在，并在蜂化蛹前随蜕皮排出。所有三种寄生蜂的成虫经常通过与受感染的粉虱接触而感染立克次氏体，最有可能是通过取食宿主血淋巴（宿主取食），但在将黄蜂从受感染的粉虱中移出后的几天内，感染率急剧下降。与立克次氏体不同，汉密尔顿菌在任何测试的寄生蜂中都未定植，且没有寄生蜂通过宿主取食获得汉密尔顿菌。这项研究证明了共生体跨营养级水平水平传播的潜在途径和障碍。文中还讨论了导致共生体物种在宿主物种间传播存在差异的可能机制。
