Asymmetries are a pervading phenomenon in otherwise bilaterally symmetric organisms and recent studies have highlighted their potential impact on our understanding of fundamental evolutionary processes like the evolution of development and the selection for morphological novelties caused by behavioural changes. One character system that is particularly promising in this respect is animal genitalia because (1) asymmetries in genitalia have evolved many times convergently, and (2) the taxonomic literature provides a tremendous amount of comparative data on these organs. This review is an attempt to focus attention on this promising but neglected topic by summarizing what we know about insect genital asymmetries, and by contrasting this with the situation in spiders, a group in which genital asymmetries are rare. In spiders, only four independent origins of genital asymmetry are known, two in Theridiidae (Tidarren/Echinotheridion, Asygyna) and two in Pholcidae (Metagonia, Kaliana). In insects, on the other hand, genital asymmetry is a widespread and common phenomenon. In some insect orders or superorders, genital asymmetry is in the groundplan (e.g. Dictyoptera, Embiidina, Phasmatodea), in others it has evolved multiple times convergently (e.g. Coleoptera, Diptera, Heteroptera, Lepidoptera). Surprisingly, the huge but widely scattered information has not been reviewed for over 70 years. We combine data from studies on taxonomy, mating behaviour, genital mechanics, and phylogeny, to explain why genital asymmetry is so common in insects but so rare in spiders. We identify further fundamental differences between spider and insect genital asymmetries: (1) in most spiders, the direction of asymmetry is random, in most insects it is fixed; (2) in most spiders, asymmetry evolved first (or only) in the female while in insects genital asymmetry is overwhelmingly limited to the male. We thus propose that sexual selection has played a crucial role in the evolution of insect genital asymmetry, via a route that is accessible to insects but not to spiders. The centerpiece in this insect route to asymmetry is changes in mating position. Available evidence strongly suggests that the plesiomorphic neopteran mating position is a female-above position. Changes to male-dominated positions have occurred frequently, and some of the resulting positions require abdominal twisting, flexing, and asymmetric contact between male and female genitalia. Insects with their median unpaired sperm transfer organ may adopt a one-sided asymmetric position and still transfer the whole amount of sperm. Spiders with their paired sperm transfer organs can only mate in symmetrical or alternating two-sided positions without foregoing transfer of half of their sperm. We propose several hypotheses regarding the evolution of genital asymmetry. One explains morphological asymmetry as a mechanical compensation for evolutionary and behavioural changes of mating position. The morphological asymmetry per se is not advantageous, but rather the newly adopted mating position is. The second hypothesis predicts a split of functions between right and left sides. In contrast to the previous hypothesis, morphological asymmetry per se is advantageous. A third hypothesis evokes internal space constraints that favour asymmetric placement and morphology of internal organs and may secondarily affect the genitalia. Further hypotheses appear supported by a few exceptional cases only.

译文

:不对称现象在其他情况下是双向对称的生物中普遍存在,最近的研究强调了它们对我们对基本进化过程的理解的潜在影响,这些进化过程包括发展的演变以及行为改变引起的形态学新颖性的选择。在这方面特别有希望的一种字符系统是动物生殖器,因为(1)生殖器中的不对称性已经收敛了许多次,并且(2)分类文献提供了有关这些器官的大量比较数据。这篇综述试图通过总结我们对昆虫生殖器不对称性的了解,并将其与蜘蛛(生殖器不对称性很少见的人群)的情况进行对比来集中精力于这个有前途但被忽视的话题。在蜘蛛中,只有四个独立的生殖器不对称起源是已知的,两个在Theridiidae(Tidarren / Echinotheridion,Asygyna)和两个在Polcidae(Metagonia,Kaliana)。另一方面,在昆虫中,生殖器不对称是普遍且普遍的现象。在某些昆虫纲或超纲动物中,生殖器不对称性存在于平面图中(例如鳞翅目,Embiidina,Phasmatodea),而在另一些昆虫中,它已经收敛了多次(例如鞘翅目,双翅目,异翅目,鳞翅目)。令人惊讶的是,巨大却分散的信息已经有70多年没有被审查了。我们结合分类学,交配行为,生殖器力学和系统发育研究的数据,以解释为什么生殖器非对称性在昆虫中如此普遍而在蜘蛛中如此罕见的原因。我们确定蜘蛛和昆虫生殖器不对称之间的进一步根本区别:(1)在大多数蜘蛛中,不对称的方向是随机的,在大多数昆虫中,它是固定的; (2)在大多数蜘蛛中,不对称首先在雌性中发展(或仅在雌性中发展),而在昆虫中,生殖器不对称主要限于雄性。因此,我们提出,性选择通过昆虫可接触但蜘蛛不可接触的途径在昆虫生殖器官不对称性的进化中起着至关重要的作用。这种不对称昆虫路径的核心是交配位置的变化。现有证据有力地表明,拟异形opter蝶交配位置是雌性以上的位置。男性主导的位置经常发生变化,某些位置需要腹部扭转,弯曲以及男性和女性生殖器之间的不对称接触。具有不成对的精子转移器官的昆虫可能会采用单侧不对称的位置,但仍会转移整个精子。具有配对的精子转移器官的蜘蛛只能在对称或交替的两侧位置交配,而不会事先转移一半的精子。我们提出了关于生殖器不对称进化的几种假设。人们将形态不对称解释为对交配位置的进化和行为变化的机械补偿。形态上的不对称本身不是有利的,而是新采用的配合位置是有利的。第二个假设预测右侧和左侧之间的功能分离。与先前的假设相反,形态不对称本身是有利的。第三个假设引起了内部空间的约束,这种约束有利于内部器官的不对称放置和形态,进而可能影响生殖器。进一步的假设似乎仅受少数例外情况的支持。

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