Function and development of male nuptial pads in frogs: from the better-grip-theory to the transfer of sex pheromones

雄性第二性征在许多动物的繁殖过程中起着重要的作用。在青蛙中，最常见的特征之一，只能在处于繁殖状态的雄性青蛙中找到，这就是所谓的婚垫。这些垫可以在手的第一个手指上找到（因此也称为拇指垫），并且通常覆盖着深色，皮质，多刺的表皮突起（EP）。由于这些EP，经常有人认为，婚垫的发展，使男性更好地抓住女性时，抱抱-特别是在水中交配。然而，许多物种不发展这样的EP，但有光滑，轻的婚垫。虽然表皮结构（即EP与光滑）在物种之间有很大差异，但底层真皮结构总是包含腺体，类似于两栖动物专门的繁殖腺体，参与蛋白质信息素或粘附蛋白的生产。这表明，繁殖腺可能参与化学通讯或在男性在抱性粘附。因此，在无尾两栖类的繁殖过程中，婚垫的腺体和EP可能具有不同的功能。在拟议的研究中，我们的目标是调查这两个，腺体及其分泌的内容，以及EP和青蛙的繁殖生态之间的关联的潜在作用。为此，我们建议三个工作包：WP 1是一个大规模的理论方法，比较了广泛的无尾两栖类物种的婚垫，并系统地回顾和分析它们的外部形态和它们的生态和生活史特征之间的关系，交配和繁殖。有了这个WP，我们的目标是更多地了解生态和进化的原因，关于大，暗，多刺的婚垫的发展。在WP 2中，我们更多地关注婚垫中信息素和粘附蛋白的潜在产生，并旨在找到物种之间的潜在模式。为此，我们将对40多个青蛙物种（来自不同的科，在温带和热带地区）进行采样，并分析和聚类其婚垫的基因表达模式。最后，在WP 3中，我们将使用开发方法。使用来自温带的一组较小的物种，我们将比较婚垫如何在形态上变化，以及它们的基因表达和共表达网络如何在繁殖季节与非繁殖季节之间变化。有了这个，我们打算进一步了解哪些结构和基因对生殖特别重要，例如，参与深色着色、腺体组织的维持或信息素和粘合剂的产生。总而言之，我们相信我们的研究将大大提高我们对外婚垫形态以及繁殖腺和此处产生的蛋白质和分泌物的生殖意义的理解。

Male secondary sexual characters play an important role during reproduction in many animal species. In frogs, one of the most common characteristics that can only be found in males in breeding condition are so-called nuptial pads. These pads can be found on the first digits of the hand (therefore also called thumb pads) and are often covered in dark, cornified, spiny epidermal projections (EPs). Because of these EPs, it is frequently suggested that the nuptial pads develop to give the male a better grip when clutching the female during amplexus – especially when mating in water. However, many species do not develop such EPs, but have smooth, light nuptial pads. While the epidermal structures (i.e. EPs vs. smooth) vary substantially between species, the underlying dermal structures always contain glands, that resemble amphibian specialized breeding glands shown to be involved in protein pheromone or adhesive protein production. This suggests that the breeding glands may be involved in chemical communication or in adhesion of the male during amplexus. Glands and EPs of the nuptial pads may therefore have different functions during anuran reproduction. In the proposed study we aim to investigate both, the potential roles of the glands and their secreted content, as well as the association between the EPs and the breeding ecology of the frogs. For this we suggest three work packages: WP1 is a large-scale theoretical approach that compares the nuptial pads of a phylogenetically wide range of anuran species, and systematically reviews and analyses the relationship between their outer morphology and their ecological and life history traits related to mating and reproduction. With this WP we aim to learn more about the ecological and evolutionary causes regarding the development of large, dark, and spiny nuptial pads. In WP2 we focus more on the potential production of pheromones and adhesive proteins in the nuptial pads and aim to find potential patterns among species. For this, we will sample over 40 frog species (from different families, in temperate and tropical zones) and analyse and cluster the gene expression patterns of their nuptial pads. Finally, in WP3, we will use a developmental approach. Using a smaller set of species from temperate zones, we will compare how the nuptial pads change morphologically as well as how their gene expression and co-expression networks vary between breeding versus non-breeding season. With this, we intend to advance our understanding of which structures and genes are especially important for reproduction, e.g., being involved in the dark colouration, the maintenance of glandular tissue or the production of pheromones and adhesives. Altogether, we are confident that our study will drastically improve our understanding of the reproductive significance of both, the outer nuptial pad morphology as well as the breeding glands and the herein produced proteins and secretions.