Genetic Diversity of the Invasive Gall Wasp Leptocybe invasa (Hymenoptera: Eulophidae) and of its Rickettsia Endosymbiont, and Associated Sex-Ratio Differences

The blue-gum chalcid Leptocybe invasa Fisher & LaSalle (Hymenoptera: Eulophidae) is a gall wasp pest of Eucalyptus species, likely native to Australia. Over the past 15 years it has invaded 39 countries on all continents where eucalypts are grown. The worldwide invasion of the blue gum chalcid was attributed to a single thelytokous morphospecies formally described in 2004. Subsequently, however, males have been recorded in several countries and the sex ratio of field populations has been found to be highly variable in different areas. In order to find an explanation for such sex ratio differences, populations of L. invasa from a broad geographical area were screened for the symbionts currently known as reproductive manipulators, and both wasps and symbionts were genetically characterized using multiple genes. Molecular analyses suggested that L. invasa is in fact a complex of two cryptic species involved in the rapid and efficient spread of the wasp, the first recovered from the Mediterranean region and South America, the latter from China. All screened specimens were infected by endosymbiotic bacteria belonging to the genus Rickettsia. Two closely related Rickettsia strains were found, each infecting one of the two putative cryptic species of L. invasa and associated with different average sex ratios. Rickettsia were found to be localized in the female reproductive tissues and transovarially transmitted, suggesting a possible role of Rickettsia as the causal agent of thelytokous parthenogenesis in L. invasa. Implications for the variation of sex ratio and for the management of L. invasa are discussed.

蓝桉广肩小蜂（blue-gum chalcid）Leptocybe invasa Fisher & LaSalle隶属于膜翅目（Hymenoptera）姬小蜂科（Eulophidae），是桉属植物的瘿蜂类害虫，推测原产于澳大利亚。过去15年间，该物种已入侵所有种植桉树的大陆上的39个国家。此前全球范围的入侵事件被认为由2004年正式描述的单一产雌孤雌生殖（thelytokous）形态种所致。然而后续在多个国家均记录到雄虫，且野外种群的性比在不同区域呈现高度变异。
为解释此类性比差异，研究人员对广泛地理区域内的L. invasa种群开展了针对目前已知生殖调控共生菌的筛选，并通过多基因位点对该小蜂及共生菌进行遗传鉴定。分子分析结果显示，L. invasa实则为两个隐存种（cryptic species）构成的复合群，二者共同推动了该瘿蜂的快速高效扩散：其中一个隐存种采自地中海地区与南美洲，另一个则采自中国。所有被筛选的标本均感染了立克次体属（Rickettsia）的内共生细菌。研究发现了两种亲缘关系紧密的立克次体菌株，分别侵染L. invasa的两个推定隐存种，并与不同的平均性比相关联。进一步观察显示，立克次体定位于雌虫生殖组织内，并可经卵传递，这表明立克次体可能是L. invasa中产雌孤雌生殖的诱因。本文最后讨论了该研究结果对解释性比变异及L. invasa防控的启示。