The embryonic origin of the ampullate silk glands of the spider Cupiennius salei

•We describe the (post-)embryonic development of the ampullate silk glands of Cupiennius salei.•We trace their embryonic origin back to ectodermal invaginations of the spinnerets.•We describe the morphogenesis of the spinneret limb buds in relation to silk gland invaginations.•We show that the gene CsASH2 is expressed at ampullate silk gland invagination sites.•We discuss how these results might further our understanding of the evolution of spider silk glands.

Silk production in spiders is considered a key innovation, and to have been vital for the diversification of the clade. The evolutionary origin of the organs involved in spider silk production, however, and in particular of the silk glands, is poorly understood. Homologies have been proposed between these and other glands found in arachnids, but lacking knowledge of the embryonic development of spider silk glands hampers an evaluation of hypotheses. This study focuses on the embryonic origin of the largest silk glands of the spider Cupiennius salei, the major and minor ampullate glands. We show how the ampullate glands originate from ectodermal invaginations on the embryonic spinneret limb buds, in relation to morphogenesis of these buds. Moreover, we visualize the subsequent growth of the ampullate glands in sections of the early postembryonic stages. The invaginations are shown to correlate with expression of the proneural gene CsASH2, which is remarkable since it has been proposed that spider silk glands and their nozzles originate from sensory bristles. Hence, by confirming the ectodermal origin of spider silk glands, and by describing the (post-)embryonic morphogenesis of the ampullate glands, this work provides a starting point for further investigating into the genetic program that underlies their development.