高榕榕果内Eupristina属两种榕小蜂的遗传进化关系

对生长在福州地区的高榕进行长期追踪观察,发现高榕榕果内仅生活着Eupristina altissima和Eupristina sp.榕小蜂,前者为高榕的传粉小蜂,后者无传粉行为,两者雌蜂之间在体色、触角、花粉袋和花粉刷等部位存在细微的差异,而两者雄蜂之间无形态差异。通过克隆福建地区5个样地的高榕榕果内收集到的E. altissima和Eupristina sp.榕小蜂,以及细叶榕的传粉小蜂Eupristina verticillata(外群)的Cytb及COI基因,并进行碱基组成及遗传距离分析,用邻接法构建系统发育树,分析两榕小蜂群体之间的遗传进化关系,结果显示:(1)榕小蜂COI及Cytb序列碱基组成中A+T的含量(Cytb序列中A+T=75.3%,COI序列中A+T=75.5%)显著高于G+C,符合膜翅目昆虫线粒体基因碱基组成特征。(2)对两群体小蜂进行遗传距离分析显示,Cytb序列中E. altissima和Eupristina sp. 群体内各样本之间的平均遗传距离分别为0.0092和0.0030,而E. altissima与Eupristina sp. 群体间的平均距离为0.1588;COI序列中E. altissima 与Eupristina sp. 群体内各样本之间的平均遗传距离分别为0.0065和0.0205,而二者群体间的平均遗传距离为0.1043,表明两者群体间的遗传距离明显大于各自群体内各样本间的遗传距离。统计GenBank中下载的6个属34种榕小蜂Cytb序列的种间遗传距离为0.0811-0.1723,6个属28种榕小蜂COI序列的种间遗传距离为0.0939-0.1986。由此认为E. altissima和Eupristina sp.之间的遗传距离差异已经达到了种间水平,即E. altissima与Eupristina sp.为两个不同的种。(3)在形态上,两种小蜂的雌蜂之间有微小差异,而二者雄蜂之间无差异,但Cytb与COI序列分析结果一致表明:E. altissima与Eupristina sp.雄蜂之间,以及二者雌蜂之间的遗传距离均差异显著,表明形态变异滞后于基因变异。雌蜂在表型上进化快于雄蜂,可能是由于雌蜂羽化后从榕果出飞,受到外界环境因素的影响较大,且两种雌蜂在传粉功能上存在差异,故二者之间的形态差异较大,而雄蜂寿命短,又终生生活在黑暗封闭、环境变化相对恒定的榕果内,两种雄蜂在行为上不存在差异,故二者表型变异较为缓慢。E. altissima和Eupristina sp.小蜂对宿主的专一性不强,在榕-蜂协同进化过程中,可能发生过宿主转移事件。

The genetic evolutionary relationships of two Eupristina species on Ficus altissima

We observed the habits of two Eupristina species living in Ficus altissima. Only two fig wasp species, Eupristina altissima and Eupristina sp., are found to be associated with the syconia of Ficus altissima around the Fuzhou area, China. The former wasp is the Ficus altissima pollinator, while the latter wasp does not have pollination behavior. Some subtle differences are found in females of both wasp species regarding body color, antennae, pollen pockets, comb of setae on the fore coax. So far, no clear characters have been found to be used for distinguishing the males of these two wasp species. To better understand the genetic relationship between E. altissima and Eupristina sp., we cloned the mitochondrial Cytb and COI genes derived from both E. altissima and Eupristina sp. collected from five different locations in Fuzhou. Eupristina verticillata, the pollinator of Ficus microcarpa, was used as an outgroup control. We analyzed the base composition and genetic distance of the gene sequences and constructed a neighbor-joining phylogenetic tree to investigate the genetic evolutionary relationships. The data are summarized as follows: (1) The base composition of Cytb and COI show a high adenine (A) and thymine (T) bias (Cytb, A+T=75.3%; COI, A+T=75.5%), which pattern is consistent with the typical characteristics of mitochondrial gene sequences of hymenopteran. (2) Genetic distance analyses of two wasps groups show that the average distance of the Cytb sequence within each group of E. altissima and Eupristina sp. are 0.0092 and 0.0030, respectively, far beyond the average genetic distance between E. altissima and Eupristina sp. that is 0.1588; COI sequence analyses show that the average genetic distances within each group of E. altissima and Eupristina sp. are 0.0065 and 0.0205, respectively, although the average genetic distance between E. altissima and Eupristina sp. is 0.1043. Thus, the genetic distance between the two groups is significantly greater than that found within each respective group. Statistic analyses of the Cytb sequences derived from 6 genera and 34 species of wasps from GenBank show that the interspecies genetic distances of the Cytb sequence are between 0.0811 and 0.1723, and the interspecies genetic distances in the COI sequences derived from 6 genera and 28 species of wasps are between 0.0939 and 0.1986. These data suggest that the genetic distance between E. altissima and Eupristina sp. has reached to an interspecies difference level. Therefore, our molecular marker studies support the morphological characterization, namely E. altissima and Eupristina sp. are indeed two different species. (3) Only a slight difference is found in females between the two species regarding the morphological characteristics, and no tangible difference is found in the males. However, the data of genetic distances derived from Cytb and COI sequence analyses consistently show the significant differences between the two species regardless their genders. It is likely that the morphological differentiation lags behind the genetic differentiation and the female wasps may evolve faster than the male wasps in the phenotypic features. The females fly out from figs after the emergence, which may be significantly affected by environmental factors resulting in the morphological differentiation between the female wasps of the two species. In contrast, the male wasps of the two species live in the closed darkness syconia with constantly environment lifelong, thus, no behavior differentiation may occur along with the extremely slow events of phenotypic mutating. The host specificity of E. altissima and Eupristina sp. is not as stringent as previously assumed. The host switching events may have occurred during the fig-wasps co-evolution process.