KU Leuven, Zoological Institute, Naamsestraat 59, Box 2466, 3000 Leuven, Belgium.
Sorbonne Université, CNRS, Institut d'Écologie et des Sciences de l'Environnement, 75005 Paris, France.
Arthropod Struct Dev. 2020 Nov;59:100979. doi: 10.1016/j.asd.2020.100979. Epub 2020 Aug 17.
As in other Hymenoptera, adult ants cannot secrete silk, unlike the larvae that spin a cocoon prior to metamorphosis. Fisher and Robertson (1999) first showed the existence of a silk gland in the head of adult Melissotarsus beccarii workers, and we confirm this with detailed histology and ultrastructural comparisons of both queens and workers. This African genus exhibits extreme morphological adaptations (legs, head shape and mandibular muscles) for tunnelling behaviour inside living trees, that underlie an obligate mutualism with scale insects. Rhopalomastix is its sister genus distributed across Asia, and we show that queens and workers also have a silk gland. This lineage of minute workers relies on silk to secure their network of tunnels against other arboreal ants. We show striking differences between these genera in the anatomy and ultrastructure of the cells that secrete silk, especially numerous vacuoles and an unexpectedly branched end apparatus in Melissotarsus. Moreover, the legs of Melissotarsus are much more specialized for tunnelling, and this includes highly expanded basitarsi. The latter house the novel 'Delage-Darchen gland', and we document its anatomy and ultrastructure, suggesting a proteinaceous secretion to harden roofs made of silk combined with wood fragments. The restriction of the Delage-Darchen gland to Melissotarsus, combined with a modified silk gland (an almost three-fold increase in the number of secretory cells, and ultrastructural differences suggestive of higher secretory activity), are evidence of an outstanding evolutionary divergence relative to Rhopalomastix. Synthesis of silk by adults is a significant innovation among ants, but its augmented production in Melissotarsus makes them better adapted for the hazards of arboreal life.
与其他膜翅目昆虫一样，成年蚂蚁不能分泌丝，而幼虫在变态前会吐丝结茧。Fisher 和 Robertson（1999）首次在成年 Melissotarsus beccarii 工蚁的头部发现了一个丝腺，我们通过对女王蚁和工蚁的详细组织学和超微结构比较证实了这一点。这个非洲属表现出极端的形态适应（腿、头部形状和下颌肌肉），用于在活树内挖掘行为，这是与介壳虫的强制性互利共生。Rhopalomastix 是其分布在亚洲的姐妹属，我们发现女王蚁和工蚁也有丝腺。这些微小的工蚁依赖丝来固定它们的隧道网络，以防止其他树栖蚂蚁入侵。我们展示了这两个属在分泌丝的细胞的解剖结构和超微结构上存在显著差异，尤其是在 Melissotarsus 中存在大量的空泡和出乎意料的分支末端装置。此外，Melissotarsus 的腿更专门用于挖掘，包括高度扩展的基跗节。后者容纳了新颖的“Delage-Darchen 腺”，我们记录了它的解剖结构和超微结构，表明这是一种蛋白质分泌物，可将由丝和木屑组成的屋顶变硬。Delage-Darchen 腺仅局限于 Melissotarsus，结合改良的丝腺（分泌细胞数量增加近三倍，超微结构差异表明分泌活性更高），这表明与 Rhopalomastix 相比，发生了显著的进化分歧。成年蚂蚁合成丝是一个重大的创新，但在 Melissotarsus 中增加了丝的产量，使它们更适应树栖生活的危险。
