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生殖是昆虫种群延续和繁荣的重要手段,主要分为有性生殖(Sexual reproduction)和孤雌生殖(Parthenogenesis)两大类,特别是前者,其两性配子的结合不仅直接决定昆虫种群内驱动力,而且对于“整合”害虫的各项治理措施具有重要的参考意义[1-2]。云南切梢小蠹(Tomicus yunnanensis)隶属于鞘翅目( Coleoptera)、象虫科(Curculionidae)、小蠹亚科(Scolytinae),是我国西南地区云南松松林的毁灭性害虫[3]。自上世纪80年代爆发危害以来,该蠹虫迄今已导致大于15万公顷的云南松林死亡,且持续向四川、贵州、西藏等地区蔓延扩散[4]。与其它树皮小蠹相比[3, 5],云南切梢小蠹具有两个典型的危害特征,即蛀梢期和蛀干期。前者是新羽化的成虫离开繁殖树(Natal tree),重新寻找健康的松树,并蛀入嫩梢取食,借以补充营养,完成生殖系统的发育成熟;后者是性成熟的个体,则会选择树势衰弱的云南松,按照自上至下的方式蛀入树干产卵、繁殖后代,此时会阻断树木水分和营养物质输送,导致树木死亡[3, 6-7]。
如何使云南切梢小蠹有虫不成灾,基于害虫种群区域性生态调控的系统策略是关键,这就需要研究影响云南切梢小蠹种群波动和灾变的内在因子,而这与云南切梢小蠹的生殖系统、繁殖行为及其制约因素直接相关。云南切梢小蠹是一夫一妻制(Monogamy)的婚配模式,一生仅交配一次(少数姊妹种例外),可产卵60~80粒[3, 5, 7],考虑到高效的种内信息通讯模式[8-9]和独特的隐蔽式生活[3],加之与真菌、细菌以及病毒的协同共生[5],使其能够有效地克服树木的多元抗性,侵入寄主树木取食、繁殖,从而维持其种群密度的相对稳定。当前,在明确云南切梢小蠹卵巢和发育进度的条件下,有必要对其雄性生殖系统的形态、组成及其精子结构进行详细的研究。另外,目前已报道的小蠹亚科昆虫有6 056种[5],但仅有山松大小蠹(Dendroctonus monticolae)[10]、咖啡果小蠹(Hypothenemus hampei)[11]、华山松大小蠹(D. armandi)[12]、克里角梢小蠹(Trypophloeus klimeschi)[13]和长林小蠹(Hylurgus ligniperda)[14-15]、法吉枝小蠹(Ernoporus fagi)和常春藤吻道小蠹(Kissophagus hederae)[16]的雄性生殖系统和精子进行过研究,而不同物种之间又存在着广泛的变异[14, 16]。如雄虫生殖系统一般由精巢、储精囊、附腺、输精管和射精管组成,但咖啡果小蠹没有附腺[11];再如精子结构,一般包括细胞核、9 + 9 + 2式轴丝和一个似壶嘴状的管胞[16],但华山松大小蠹存在一个特有的海绵体[12]。因此,本研究以云南切梢小蠹雄虫为研究对象,应用光学显微镜和透射电镜技术观察雄性内生殖器形态与精子超微结构,以期为深入理解云南切梢小蠹的种群动力与害虫生态调控提供依据。
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云南切梢小蠹雄性内生殖系统位于第4腹板下方,由1对精巢(Testes)、2对管状附腺(Tubular accessory glands)、1对多管状腺体(Multilobulated accessory glands)、1对储精囊(Seminal vesicles)、1对输精管(Deferent ducts)和1根射精管(Ejaculatory duct)组成,其中精巢和多管状腺体为椭球形,其它器官为长管状;射精管末端开口于第8腹节的末端,即与外生殖器相连接,而各器官表面均分布有大量的气管和微气管(图1A)。在内生殖器发育过程中(图1B~D;表1),新羽化1 d的成虫,即性未发育者,其精巢及其附腺均为无色透明状,各管状体体纤细,几乎不膨大,未发现有精子(图1B);待发育至10月,此时卵巢和多管状腺体的表面呈现淡黄色,各管状体为乳白色,管壁明显膨大、增厚,最典型特征是储精囊内有大量精子存在,预示着性器官已基本成熟(图1A、C),占比约为总体的36.7%(N=15);12月份时,最大的变化是精巢和多管状腺体为橘黄色,储精囊内亦出现大量的精子,而其它器官仅是略微的膨大(图1D),这些个体占比约为88.9%(N=45)。
图 1 云南切梢小蠹雄性生殖系统及其发育动态
Figure 1. Male reproductive system and its deveopmental dynamics in the Tomicus yunnanensis adults
表 1 云南切梢小蠹雄虫的内生殖器组成、形态及发育变化
Table 1. Shape and size of parts of the male genital organs of Tomicus yunnanensis adults.
器官
Organ形态
Shape3月 March 10月 October 12月 December 颜色
Color长度
Length/um*颜色
Color长度
Length/um颜色
Color长度
Length/um精巢
Sperm椭球形 无色透明 188.6 ± 32.8 淡黄色 227.7 ± 18.6 橘黄色 364.3 ± 86.9 储精囊
Seminal vesicles椭球形 无色透明 56.6 ± 3.8 乳白色 157.5 ± 13.8 乳白色 192.9 ± 46.0 管状腺体
Multilobulated accessory glands长管状 无色透明 500.2 ± 54.8 乳白色 584.6 ± 58.8 乳白色 614.5 ± 67.5 多管状腺体
Tubular accessory glands椭球形 无色透明 51.2 ± 4.0 淡黄色 109.3 ± 14.3 橘黄色 144.1 ± 12.1 输精管
Deferent ducts长管状 无色透明 125.8 ± 10.8 淡黄色 127.9 ± 7.5 乳白色 126.1 ± 10.7 射精管
Ejaculatory duct长管状 无色透明 278.5 ± 32.3 淡黄色 309.1 ± 14.7 乳白色 316.5 ± 14.6 注:*:均值 ± 标准误,椭球形为长径与短径的均值
Notes: *: Mean ± standard error, ellipsoid shape is the mean of the long and short diameters -
每一个精巢均由单层结构的精囊膜(Scrotal membrane)包裹,内部含有20根精巢小管(Testiculartubules),精巢小管外形呈梨形,横切面显示其有一个大型的细胞核,可计数的精子数量为403 ± 20个(图2),说明每个精巢小管可育512个(29)精细胞。
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成熟精子的外形呈线状,长度为170.1 ± 39.1 μm(N=16),不聚集成束,呈游离状,包括头部和尾部两部分(图1A、图3)。其中,头部为针状,平均长度23.5 ± 3.4 μm,中部宽1.6 ± 0.4 μm;尾部细长,略微卷曲,末端处突然收缩变细,平均长度为127.9 ± 23.1 μm,中部宽1.4 ± 0.2 μm,占整个长度的75.2%。
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从超薄切片上看,精子头部可明显区分为顶体复合体和细胞核两部分(图4A、B)。顶体复合体外形似一个帽子,长度仅有1 μm,由3层非均质物质组成:顶体前腔基质、顶体腔和顶体(图4C~E)。顶体前腔电子密度非常低,近乎透明状,包裹在顶体腔前端的外延物周围;顶体腔具有较高的电子密度,外形为h-型,腔内含有一个丝状的顶体;顶体的电子密度与顶体腔类似,其直径约为77.0 ± 9.0 nm,几乎充满整个顶体腔。
图 4 云南切梢小蠹精子头部的超微结构
Figure 4. Ultrastructural images of sperm head in the Tomicus yunnanensis adults
细胞核是电子密度最深的区域,为圆柱形,被一层薄的核膜包围,是头部最长的区域(约为总长度95.8%)。在核的前端,其外形似椭圆形,与顶体紧密相连,此处是核半径最大的区域,约为0.77 ± 0.14 μm;在末端处,其逐渐变细,呈现出一个内陷的凹面,凹面内部含有中心粒,中心粒末端则以颗粒状嵌入细胞核内,而其它区域则为同质的电子致密区,中心粒基部位于大线粒体衍生物和小线粒体衍生物端部之间,并与轴丝相连接(图4B、F~G)。
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从精子的横切面和纵切面来看(图5A、B),云南切梢小蠹精子尾部由1根9 + 9 + 2式微管、2个线粒体衍生物、1对管胞、1个海绵体以及1个中心体衍生物组成。其中,微管是由位于远端中心粒的末端发出,包括位于中心的2条平行的中心微管和外围的9条纤维组成,而9条纤维又可分为附属微管和微管双联体两类,附属微管位于中间,微管双联体则位于最外围,并呈双环状环绕中心微管,各微管的直径相似(18.47~27.66 nm,p=0.79),微管与微管之间则以微管连接物加以连接,进而形成了典型的9 + 9 + 2式微管结构(图5C、D)。线粒体衍生物则有大线粒体衍生物和小线粒体衍生物之分,大线粒体衍生物起源于细胞核凹面的顶点处,直径约为0.59 ± 0.26 μm;小线粒体衍生物则源于轴丝与中心粒后端的顶点处(图4B),直径约为0.28 ± 0.04 μm,两者以螺旋形式围绕着轴丝(图5B、D),且大、小线粒体内壁均有发达的嵴,嵴之间的间距约为47.6 nm,预示着两者是精子运动最重要的供能器官。管胞是由一类嗜锇物质组成,位于轴丝两侧,且与之大致平行,外形呈角状或三角形,而在左侧管胞下方,则与中心体衍生物相连。中心体衍生物外形类似壶嘴状,也被称为壶嘴状衍生物(like-puff expansions),其构成物质与大、小线粒体衍生物类似(图5D)。另外,在大、小线粒体螺旋交界处,特别是小线粒体上方,通常会观察到海绵体和囊泡(图5B、C、E)。
图 5 云南切梢小蠹精子尾部的超微结构
Figure 5. Ultrastructural images of sperm tail in the Tomicus yunnanensis adults
从精子尾部的起点至末尾,构成尾部的各细胞器均会逐渐减小,呈不对称状,主要贡献者是中心体衍生物、大线粒体衍生物以及小线粒体衍生物。另外,这些细胞器向尾部末端方向则会逐渐消失(图5E~H),如尾部2/3处,小线粒体衍生物会首先消失,接着是右侧管胞和左侧管胞(图5E),至尾部3/4处时,仅剩下大线粒体衍生物(图5F);到达尾部4/5处时,大、小线粒体衍生物和1对管胞均消失不见,而微管连接物也会开始消融(图5G);待到达尾部尾端,微管双连体首先消失,接着是附属微管和微管双联体,直至剩下一层薄薄的核膜(图5H)。
云南切梢小蠹雄性内生殖器形态与精子超微结构
Male Reproductive System and Sperm Ultrastructure of the Yunnan Pine Shoot Beetle, Tomicus yunnanensis (Coleoptera: Curculionidae: Scolytinae)
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摘要:
目的 昆虫生殖是种群维持与繁荣的重要手段,研究云南切梢小蠹雄性生殖系统及其精子结构,对于探究云南切梢小蠹的持续危害规律及其灾变机理具有重要的理论和现实意义。 方法 以云南切梢小蠹新羽化成虫(性未成熟)和蛀干前期成虫(性成熟)为研究对象,用光学显微镜和透射电子显微镜观察雄性内生殖器形态与精子超微结构。 结果 云南切梢小蠹雄性内生殖器由1对精巢、2对管状附腺、1对多管状腺体、1对储精囊、1对输精管和1根射精管组成,各组成器官在初孵时管壁纤薄,无色透明状;待性成熟后,管壁则明显增厚,精巢和多管状腺体呈橙黄色,而其它部分则为乳白色,储精囊内亦出现大量的精子。每个精子外形为线状,全长170.1 ± 39.1 μm,分为头部和尾部两部分。头部似针状,长23.5 ± 3.4 μm,包括1个3层的顶体复合体和1个圆柱状的细胞核;尾部细长,由2个大小不等的线粒体衍生物、2个等大的管胞、1个壶嘴状的中心体衍生物和1个典型的9 + 9 + 2式轴丝组成,但各细胞器的形态高度分化,不对称,管胞、小线粒体衍生物、管胞、大线粒体衍生物向尾部末端方向则逐渐消失不见。 结论 云南切梢小蠹雄性内生殖器的形态、组成以及精子超微结构与象虫科昆虫类似,特别是3层的顶体复合体、9 + 9 + 2式轴丝以及高度分化的尾部细胞器,研究结果不仅可丰富小蠹亚科的昆虫分类与系统发育研究,而且可为深入理解云南切梢小蠹的受精生理提供参考。 Abstract:Objective Insect reproduction is of vast significance in population processing for the growths and declines in natural or management habitats. Based on the theoretical and practical implications of pest population management, it is essential to gain the male reproductive system and sperm structure of Tomicus yunnanensis (Coleoptera: Curculionidae: Scolytinae), which are the most damaged insects in the Yunnan pine forest in southwest China. Method The male genital system and sperms from newly emerged adults (sexual immaturity) and the precocity of trunk-boring attacks (sexual maturity) of this species are examined structurally using light and transmission electron microscopy. Result The male genital system contains a pair of testes, two twins of tubular accessory glands, a couple of multilobulated accessory glands and seminal vesicles, and deferent ducts connected with a single ejaculatory duct. After sexual immaturity, all genital cysts were only thin transparent tubules, which were milky and gradually thicker after sexual maturity. However, there was orange for the testes and multilobulated accessory glands. Mature sperms were numerous in seminal vesicles currently. Each sperm was filiform in shape, 170.1 ± 39.1 μm in length, and consisted of the head and flagellum. The head was needle-like, with 23.5 ± 3.4 μm in length, and comprised a three-layer acrosomal complex and an inferior cylindrical nucleus. The long flagellum included two mitochondrial derivatives of different sizes, two accessory bodies with a sizeable puff-like expansion, and a classical 9 + 9 + 2 axoneme microtubule pattern. More importantly, the sperm tail showd a high degree of differentiation in the shape of the asymmetrical organelles. Some tail organelles, including puff-like expansion, minor mitochondrial derivatives, accessory bodies, and major mitochondrial derivatives, would gradually disappear toward the end of the sperm. Conclusion . The overall genital characteristics and sperm ultrastructure of the male adults of T. yunnanensis are like those of Curculionidae, which conserve the primitive characters such as three-layer acrosomal complex, 9 + 9 + 2 axoneme and high differentiation of asymmetrical tail organelles. Current results not only enrich the taxonomic and phylogenetic studies of the subfamily Scolytinae, but also help us further understand the physiological attributes in insemination specificity of Tomicus yunnanensis. -
Key words:
- Insect reproduction
- / Tomicus yunnanensis
- / Testes
- / Sperm ultrastructure
- / Electron microscopy
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表 1 云南切梢小蠹雄虫的内生殖器组成、形态及发育变化
Table 1. Shape and size of parts of the male genital organs of Tomicus yunnanensis adults.
器官
Organ形态
Shape3月 March 10月 October 12月 December 颜色
Color长度
Length/um*颜色
Color长度
Length/um颜色
Color长度
Length/um精巢
Sperm椭球形 无色透明 188.6 ± 32.8 淡黄色 227.7 ± 18.6 橘黄色 364.3 ± 86.9 储精囊
Seminal vesicles椭球形 无色透明 56.6 ± 3.8 乳白色 157.5 ± 13.8 乳白色 192.9 ± 46.0 管状腺体
Multilobulated accessory glands长管状 无色透明 500.2 ± 54.8 乳白色 584.6 ± 58.8 乳白色 614.5 ± 67.5 多管状腺体
Tubular accessory glands椭球形 无色透明 51.2 ± 4.0 淡黄色 109.3 ± 14.3 橘黄色 144.1 ± 12.1 输精管
Deferent ducts长管状 无色透明 125.8 ± 10.8 淡黄色 127.9 ± 7.5 乳白色 126.1 ± 10.7 射精管
Ejaculatory duct长管状 无色透明 278.5 ± 32.3 淡黄色 309.1 ± 14.7 乳白色 316.5 ± 14.6 注:*:均值 ± 标准误,椭球形为长径与短径的均值
Notes: *: Mean ± standard error, ellipsoid shape is the mean of the long and short diameters -
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