Weiss Katharina, Herzner Gudrun, Strohm Erhard
Evolutionary Ecology Group, Institute of Zoology, University of Regensburg, Universitätsstr. 31, 93053, Regensburg, Germany.
BMC Evol Biol. 2017 Jun 6;17(1):128. doi: 10.1186/s12862-017-0963-6.
Sexual selection is thought to promote evolutionary changes and diversification. However, the impact of sexual selection in relation to other selective forces is difficult to evaluate. Male digger wasps of the tribe Philanthini (Hymenoptera, Philanthinae) scent mark territories to attract receptive females. Consequently, the organs for production and storage of the marking secretion, the mandibular gland (MG) and the postpharyngeal gland (PPG), are subject to sexual selection. In female Philanthini, these glands are most likely solely subject to natural selection and show very little morphological diversity. According to the hypothesis that sexual selection drives interspecific diversity, we predicted that the MG and PPG show higher interspecific variation in males than in females. Using histological methods, 3D-reconstructions, and multivariate statistical analysis of morphological characters, we conducted a comparative analysis of the MG and the PPG in males of 30 species of Philanthini and three species of the Cercerini and Aphilanthopsini, two related tribes within the Philanthinae.
We found substantial interspecific diversity in gland morphology with regard to gland incidence, size, shape and the type of associated secretory cells. Overall there was a phylogenetic trend: Ensuing from the large MGs and small PPGs of male Cercerini and Aphilanthopsini, the size and complexity of the MG was reduced in male Philanthini, while their PPG became considerably enlarged, substantially more complex, and associated with an apparently novel type of secretory cells. In some clades of the Philanthini the MG was even lost and entirely replaced by the PPG. However, several species showed reversals of and exceptions from this trend. Head gland morphology was significantly more diverse among male than among female Philanthinae.
Our results show considerable variation in male head glands including the loss of an entire gland system and the evolution of a novel kind of secretory cells, confirming the prediction that interspecific diversity in head gland morphology is higher in male than in female Philanthini. We discuss possible causes for the remarkable evolutionary changes in males and we conclude that this high diversity has been caused by sexual selection.
性选择被认为会促进进化变化和物种多样化。然而，性选择相对于其他选择力量的影响难以评估。食蜂虻族（膜翅目，食蜂虻亚科）的雄性掘土蜂会用气味标记领地以吸引处于发情期的雌性。因此，用于产生和储存标记分泌物的器官，即下颌腺（MG）和咽后腺（PPG），受到性选择的影响。在雌性食蜂虻中，这些腺体很可能仅受自然选择影响，形态多样性很小。根据性选择驱动种间多样性的假说，我们预测MG和PPG在雄性中的种间变异比在雌性中更高。我们使用组织学方法、三维重建以及形态特征的多变量统计分析，对食蜂虻族30个物种以及食蜂虻亚科内两个相关族——突眼食蜂虻族和无垫食蜂虻族的三个物种的雄性MG和PPG进行了比较分析。
我们发现腺体形态在腺体发生率、大小、形状以及相关分泌细胞类型方面存在显著的种间多样性。总体上存在系统发育趋势：从突眼食蜂虻族和无垫食蜂虻族雄性的大型MG和小型PPG开始，食蜂虻族雄性的MG大小和复杂性降低，而它们的PPG变得相当大、复杂得多，并且与一种明显新类型的分泌细胞相关。在食蜂虻族的一些分支中，MG甚至消失并完全被PPG取代。然而，有几个物种表现出这种趋势的逆转和例外情况。食蜂虻亚科雄性的头部腺体形态比雌性的显著更多样化。
我们的结果表明雄性头部腺体存在相当大的变异，包括整个腺体系统的丧失和一种新型分泌细胞的进化，证实了食蜂虻族雄性头部腺体形态的种间多样性高于雌性的预测。我们讨论了雄性显著进化变化的可能原因，并得出结论，这种高度多样性是由性选择引起的。
