Eggs to long-legs: embryonic staging of the harvestman Phalangium opilio (Opiliones), an emerging model arachnid

Abstract

Background The comparative embryology of Chelicerata has greatly advanced in recent years with the integration of classical studies and genetics, prominently spearheaded by developmental genetic works in spiders. Nonetheless, the understanding of the evolution of development and polarization of embryological characters in Chelicerata is presently limited, as few non-spider species have been well studied. A promising focal species for chelicerate evo-devo is the daddy-long-legs (harvestman) Phalangium opilio, a member of the order Opiliones. Phalangium opilio, breeds prolifically and is easily accessible in many parts of the world, as well as tractable in a laboratory setting. Resources for this species include developmental transcriptomes, a draft genome, and protocols for RNA interference, but a modern staging system is critically missing for this emerging model system. Results We present a staging system of P. opilio embryogenesis that spans the most important morphogenetic events with respect to segment formation, appendage elongation and head development. Using time-lapse imaging, confocal microscopy, colorimetric in situ hybridization, and immunohistochemistry, we tracked the development of synchronous clutches from egg laying to adulthood. We describe key events in segmentation, myogenesis, neurogenesis, and germ cell formation. Conclusion Considering the phylogenetic position of Opiliones and the unduplicated condition of its genome (in contrast to groups like spiders and scorpions), this species is poised to serve as a linchpin for comparative studies in arthropod development and genome evolution. The staging system presented herein provides a valuable reference for P. opilio that we anticipate being useful to the arthropod evo-devo community, with the goal of revitalizing research in the comparative development of non-spider arachnids.

Evolution of the spider homeobox gene repertoire by tandem and whole genome duplication

“…Panel B shows a magnification of the posterior region of a flat-mounted embryo. Panels Á-Ĺ, Ó and P′ show SYBR green stained embryos as shown in panels A-L, O and P. Developmental stages of P. tepidariorum (after Mittmann and Wolff (2012)) and P. opilio (after Gainett et al (2022)) are indicated in the upper right corner of each panel with a bright field image. Arrows in panels A, C-J, O and P mark expression of NK3 and NK4 in the developing heart of P. tepidariorum , and expression of NK4 in the developing heart of P. opilio .…”

“…The dissected legs and pedipalps present lateral views, dorsal to the right. Panels Á-É show SYBR green stained embryos as shown in panels A-E. Developmental stages after Gainett et al (2022) are indicated in the upper right corner of each panel with a bright field image. Arrowheads in panels A and B mark the sharp anterior border of Emx1 expression anterior to the cheliceral segment.…”

“…Panels Á-F, I-K′, Ń, Ó, T′-Ý, b′-d′, and ǵ-í show SYBR green stained embryos as shown in panels with corresponding bright field panels. Developmental stages of P. tepidariorum (after Mittmann and Wolff (2012)) and P. opilio (after Gainett et al (2022) are indicated in the upper right corner of each panel with a bright field image. Arrowheads in panels B, D, and E mark dorsal expression in the opisthosoma.…”

“…tepidariorum, and Gainett et al (2022) for P. opilio. We investigated all developmental stages of P. tepidariorum and P. opilio from the formation of the early germ band to dorsal closure.…”

Dual functions oflabialresolve the Hox logic of chelicerate head segments

“…Embryos of the horseshoe crab Limulus polyphemus were collected in Woodshole-MA, USA in June 2022. Embryonic staging nomenclature follows Gainett et al (Gainett et al 2022) and Sekiguchi et al (Sekiguchi et al 1982). Details of collection, maintenance and fixation of embryos are described in the Electronic Supplementary Methods.…”