摘要
榕树(桑科:榕属)和榕小蜂(昆虫纲:膜翅目:小蜂总科)是研究适应、雄性多型等进化问题的理想材料。榕小蜂由于长期生活在榕果(榕树的果状隐头花序)里,它们的外部形态特征和生活方式等都发生了很多适应性的变化。传粉小蜂和部分非传粉小蜂的雌虫需要穿过榕小孔进行产卵,其头部发生了显著的特化;雄虫由于生活在黑暗潮湿的榕果内,其形态结构也发生了明显的特化。
在这些变化里面,最引人注目的两大特征就是极端雌雄异型和雄性多型现象。在榕小蜂的生活史中,成熟的雌虫出果并飞到合适的产卵地点进行产卵,它们都具有发达的复眼和翅;而大部分的雄虫由于终生生活在黯淡无光的榕果里,所以它们相比较同种的雌蜂来说,个小、体光滑、无翅、且复眼退化。由于生殖策略的分化,榕小蜂的雄虫还出现了种内的异型现象。有些个体有翅且复眼发达,有些却无翅且复眼退化,甚至无翅雄虫还会出现个体大小、形态和行为上的多态性。
雌雄异型现象在生物界中广泛存在,而大多数榕小蜂拥有的雌雄异型现象可能是动物界中最极端的事例之一。同一榕果内一般都有几种以上的榕小蜂生活在其中,有的甚至多达几十种。小小的榕果里出现这么多小蜂物种本来就给鉴定工作带来了很大的困难,而普遍存在的极端异型和雄性多型现象又使得问题大大复杂化。传统的形态鉴定很难将同一榕果内的所有物种明确区分并将同种的雌雄虫正确归类到一起。本研究试图找到最适合于对榕小蜂进行身份识别的分子标记,从而推进榕小蜂的鉴定工作。我们选取了垂叶榕上的Walkerella属2种共计40个个体的雌雄榕小蜂,比较了两个DNA序列片段(COI及ITS2 )在榕小蜂物种鉴定、雌雄虫归类的适用情况。结果表明: ITS2序列能够将同种的个体高支持率地聚成一枝,它相比于COI分子标记更合适用于物种鉴定的一个。我们的结果还暗示,COI序列虽然也能将大部分个体很好的聚类,但可能会由于Numts或内共生菌的影响而产生错误信号。
Fig wasps (Insecta: Hymenoptera: Chalcidoidea) and their host figs (Moraceae: Ficus) is an excellent model for the study of such evolutionary topics as adaptation, male polymorphism, etc. Fig wasps have many adaptive evolution on their morphological characters and lifestyles due to their long-time hosted life in the syconium of fig. The head of pollinators and some non-pollinators females are extraordinarily specialized for crawling through the ostiole; the diversity of ovipositing places and styles resulted in the diversity of the ovipositor length and the timing of ovipositing; males have peculiar respiratory organs and other morphological changes in relation with their habitat in the enclosed fig cavity.
The two most outstanding characteristics of these adaptive changes are distinct sexual dimorphism (SD) and male polymorphism. Mature females emerge from the syconium and fly to appropriate ovipositing places, so they have developed compound eyes and wings. But the life cycle of most males is restricted to the cavity of the fig, so they are wingless and have vestigial eyes. Different male species may have different morphs and behaviors since they adopt different strategies. There can also be two or more male phenotypes intra-species with different phenotype having different mating tactics.
Sexual dimorphism is common among various species, and some fig wasps are perhaps among the most extreme cases of sexual dimorphism in the animal kingdom. Each fig shelters a very diverse assemblage of fig wasps species in TIS enclosed inflorescence- syconium, which makes the taxonomy of fig wasps a difficult task. What’s more, the prevalent phenomena of sexual dimorphism and male polymorphism bring much intricacy to the problem. So the species identification of fig wasps based on traditional morphology is not only difficult but also fuzzy and false-prone. Assistance from other measure besides morphology is requisite for well fulfilling the species identification in the complex assemblage of fig wasps. We here want to compare the practicability of several gene fragments with different evolution rates in species identification of fig wasps. We amplified four fragments of COI and ITS2 from 40 individuals of 2 species of Walkerella associated with Ficus benjamini (Ficus, Moraceae) and analyzed their utilization in species identification. Our results showed that ITS2 can cluster all the individuals of the same species into a highly supported clade, so it is the best marker used in species classification of these fig wasps. COI could also act well but with a few unordinary exceptions, which indicates TIS deficient use in DNA barcoding due to the possible effect of Numts or symbionts.
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