Abstract
Spiders have evolved a unique male copulatory organ, the pedipalp bulb. The morphology of the bulb is species specific and plays an important role in species recognition and prezygotic reproductive isolation. Despite its importance for spider biodiversity, the mechanisms that control bulb development are virtually unknown. We have used confocal laser scanning microscopy (CLSM) and diffusible iodine-based contrast-enhanced micro computed tomography (dice-µCT) to study bulb development in the spider Parasteatoda tepidariorum. These imaging technologies enabled us to study bulb development in situ, without the use of destructive procedures for the first time. We show here that the inflated pedipalp tip in the subadult stage is filled with haemolymph that rapidly coagulates. Coagulation indicates histolytic processes that disintegrate tibia and tarsus, similar to histolytic processes during metamorphosis in holometabolous insects. The coagulated material contains cell inclusions that likely represent the cell source for the re-establishment of tarsus and tibia after histolysis, comparable to the histoblasts in insect metamorphosis. The shape of the coagulated mass prefigures the shape of the adult tarsus (cymbium) like a blueprint for the histoblasts. This suggests a unique role for controlled coagulation after histolysis in the metamorphosis-like morphogenesis of the male pedipalp.
Highlights
With over 1 million described species, arthropods are by far the most speciose group of animals[1]
The primordium of the male pedipalp bulb is first established in the pre-subadult stage, but the majority of developmental processes occur during the subadult stage, i.e. between the penultimate moult and the ultimate moult
The morphology of the copulatory bulb of the male spider pedipalp is essential for the correct fitting inside the female genital opening and for the safe transfer of the sperm
Summary
With over 1 million described species, arthropods are by far the most speciose group of animals[1]. Under terrestrial conditions a more direct way of gamete delivery is required that protects the sperm from dehydration For this purpose, true spiders (Araneae) have evolved a unique copulatory organ in the male, the so-called pedipalp bulb. In its simplest form the bulb is a soft, sac-shaped protrusion, but in the majority of species the bulb is equipped with a complex set of strongly sclerotised sclerites[5,6,7] In these species, the bulb is used for sperm storage and transfer, but the sclerites ensure a safe locking of the bulb inside of the female genital system to prevent a premature separation and sperm loss. These imaging technologies enabled us to visualise the forming bulb primordium in situ, and to describe its morphogenesis during postembryonic development of P. tepidariorum without the use of destructive procedures
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