A comparative developmental analysis of gene expression in spiders and related arachnids has challenged a widely held assumption regarding how arthropod bodies establish their segments. In the fruit fly model system Drosophila melanogaster, body axis segmentation proceeds under the direction of an established genetic hierarchy where segment-polarity genes, notably engrailed and wingless, delineate parasegmental boundaries at cellular interfaces. Morphological indentations known as parasegmental grooves form transiently along these boundaries anterior to the domain of engrailed expression in the fly embryo. For decades, developmental biologists have debated whether these visible grooves represent an ancestral morphological feature shared across all arthropod lineages. The present investigation, published in Development Genes and Evolution by Ralf Janssen, Natascha Turetzek, and Matthias Pechmann, directly examines engrailed expression and embryonic morphology across several spider groups and an outgroup harvestman, finding no evidence that parasegmental grooves form in these chelicerates.
The concept that parasegmental grooves represent an evolutionarily conserved feature across the phylum Arthropoda largely originated from findings in Drosophila and an earlier study on the wandering spider Cupiennius salei, alongside tentative reports in the brine shrimp Artemia franciscana. In Drosophila, segment-polarity genes establish both segmental boundaries, which manifest as permanent grooves posterior to engrailed expression, and transient parasegmental grooves that appear anterior to engrailed. Because previous literature described parasegmental grooves forming anterior to engrailed in Cupiennius salei, several comparative evolutionary models posited that such visible grooves were an ancestral trait of arthropod developmental architecture. However, many researchers examining segmentation across diverse arthropod taxa noted an absence of visible parasegmental grooves in their experimental systems, raising unresolved questions about whether the morphology reported in Cupiennius was truly typical of chelicerates or reflected an analytical misinterpretation.
To rigorously test whether parasegmental grooves occur across chelicerate lineages, the researchers analyzed the expression of the segment-polarity gene engrailed across multiple spider species representing the primary evolutionary branches of the group. Their investigation included the common house spider Parasteatoda tepidariorum, representing the Entelegynae lineage; the cellar spider Pholcus phalangioides, representing the Haplogynae lineage; and the tarantula Acanthoscurria geniculata, representing the Mygalomorphae. By comparing these divergent taxa, the team sought to establish whether transverse surface indentations appear anterior to the newly initiated bands of engrailed expression during early embryonic germ band development.
In each of the examined spider species, gene expression assays revealed that engrailed-1 is consistently expressed in segmentally repeated transverse stripes across the developing germ band. However, morphological observations demonstrated that transverse grooves never form anterior to these expression stripes. Instead, morphological grooves were found exclusively at the posterior margin of the engrailed-1 expression domains, matching the precise position of canonical segmental boundaries seen across arthropods. In Acanthoscurria, Parasteatoda, and Pholcus, early engrailed-1 expression in newly developing segments emerged without any visible anterior surface groove, directly contradicting the structural patterns predicted by the conserved parasegmental groove hypothesis.
Given the striking contrast between their observations in these three spider species and the published literature on Cupiennius salei, the authors re-evaluated embryonic development in Cupiennius itself. Re-examining the expression of engrailed-1 in Cupiennius embryos alongside Sybr-green nuclear staining revealed that morphological grooves align strictly with the posterior border of engrailed-1 expression stripes. Morphological grooves anterior to engrailed-1 were entirely absent. Based on these findings, the authors concluded that the earlier report attributing parasegmental grooves to Cupiennius had misidentified standard segmental boundaries as parasegmental grooves, removing the primary empirical chelicerate pillar supporting the conserved groove model.
To rule out the possibility that minute or shallow parasegmental grooves escaped detection under conventional dissection microscopy, the researchers applied high-resolution confocal laser scanning microscopy to Parasteatoda tepidariorum embryos. Embryos were stained with fluorescent FastRed via in situ hybridization to map engrailed-1 transcripts and counterstained with the nuclear marker DAPI. Optical sections scanned at increments of one micrometer through the germ band allowed three-dimensional evaluation of cell surface contours and nuclear positioning before and during the onset of engrailed-1 transcription. Neither nuclear orientation nor cell surface topographies revealed any trace of an anterior indentation or groove preceding engrailed-1 expression, confirming that even subtle morphological boundaries are absent at this interface.
The study also examined the genetic composition and molecular evolution of engrailed across chelicerates, confirming that all examined arachnopulmonates—including spiders, scorpions, and whip scorpions—possess two engrailed ohnologs, termed engrailed-1 and engrailed-2. This gene duplication stems from an ancient whole genome duplication event in the arachnopulmonate stem lineage. Phylogenetic Bayesian analysis of complete protein sequences separated these genes into distinct, monophyletic engrailed-1 and engrailed-2 clades. Protein sequence comparisons revealed significant structural divergences between the paralogs: while Engrailed-1 proteins retain a serine-rich domain and canonical C-terminal aspartic acid and glutamic acid domains, Engrailed-2 lacks the C-terminal acidic region.
Expression mapping of engrailed-2 revealed that the duplicated gene has undergone both sub-functionalization and neo-functionalization in spiders. Unlike engrailed-1, which is expressed in precise stripes with segmental grooves forming in the posterior sector, engrailed-2 displays broader segmental domains that span both anteriorly and posteriorly across the segmental groove. Furthermore, spider engrailed-2 is expressed in the embryonic stomodaeum and occupies a distinct dorsal and anterior position in developing appendages, yet it is completely absent from the ventral nervous system. In contrast, the harvestman Phalangium opilio, which belongs to an outgroup lineage that did not undergo the whole genome duplication, possesses only a single engrailed gene that faithfully mirrors the expression pattern of spider engrailed-1.
The absence of parasegmental grooves across these chelicerate taxa carries notable implications for broader evolutionary hypotheses regarding bilaterian segmentation. In annelids such as the nereid Platynereis dumerilii, morphological segmental boundaries form at the cellular interface between engrailed- and wingless-expressing cells, an arrangement that mirrors the parasegmental boundaries of Drosophila. Previous evolutionary models had proposed that conserved parasegmental grooves in arthropods were directly homologous to the segmental grooves of annelids, supporting the hypothesis of an ancestral segmented protostome. By demonstrating that parasegmental groove formation is not a conserved ancestral characteristic of arthropods, these findings weaken the developmental link between annelid and arthropod groove formation and prompt a critical reassessment of early animal body patterning evolution.
Ultimately, while segment-polarity gene networks establish molecular parasegmental compartments that remain essential for maintaining segment polarity throughout arthropods, the morphological realization of these borders as physical grooves appears highly restricted. The current evidence indicates that visible parasegmental grooves represent a specialized developmental feature of certain insects such as Drosophila rather than an ancestral ground plan shared across the arthropod phylum. Future investigations into the functional role of the arachnopulmonate-specific engrailed-2 paralog may help uncover what developmental novelties arose alongside whole genome duplication in spiders, while the general mechanism of physical boundary formation in chelicerate embryos remains an active area of research.
Methodological workflows for identifying and validating engrailed sequences relied on comprehensive genomic and transcriptomic datasets across chelicerates. The research team mined embryonic transcriptomes from diverse taxa, including the jumping spider Marpissa muscosa and the whip spiders Charinus acosta and Euphrynichus bacillifer, alongside the sequenced genome of Parasteatoda tepidariorum. Potential orthologs were established through reciprocal tBLASTn searches against the solitary engrailed gene of the velvet worm onychophoran Euperipatoides kanangrensis. To obtain physical clones for expression analyses, researchers extracted total ribonucleic acid across various embryonic stages, reverse-transcribed the material into complementary DNA, and amplified targeted gene fragments using gene-specific primers via reverse transcription polymerase chain reaction. These amplified segments were subsequently cloned into pCR-II or pJet1.2 CloneJet vectors and sequenced on an ABI3730XL automated analyzer using BigDye dye-terminator chemistry.
Embryological visualization combined standardized histochemical assays with optical microscopy to track gene expression relative to surface topography. Whole-mount in situ hybridization utilized FastRed staining to yield fluorescent signals suitable for inverted confocal laser scanning microscopy on a Leica SP8 system, where embryos were immobilized in one percent low-melting agarose. Cellular architecture was mapped by incubating embryos in phosphate-buffered saline containing 0.1 percent Tween-20 supplemented with either Sybr-green at a 1:10,000 dilution or DAPI at one microgram per milliliter for twenty to thirty minutes. Epifluorescence and brightfield observations were recorded with a Leica dissection microscope coupled to a Leica DC100 digital camera, followed by linear contrast adjustments in ImageJ and Adobe Photoshop. These imaging routines ensured that tissue contours could be assessed across consecutive one-micrometer optical planes without distorting delicate germ band surfaces.
Spatial mapping further elucidated fine-scale differences between paralogs across specific embryonic structures, including the segment addition zone and head architecture. Segmental stripes of engrailed-2 extended into the ocular and precheliceral territories, exhibiting prominent expression in the stomodaeum as well as dorsal patches situated above the opisthosomal limb buds. Within developing pedipalps and walking legs, engrailed-2 transcripts localized centrally rather than along the posterior margin seen for engrailed-1 and ancestral harvestman stripes. Moreover, re-evaluating Cupiennius accounted for phylogenetic diversity within entelegyne spiders, as Cupiennius represents the distinct retrolateral tibial apophysis clade, whereas Parasteatoda occupies a separate branch. Despite resolving these detailed spatial distributions, the authors emphasized that functional genetic perturbation assays for engrailed-2 are currently unavailable in spiders, leaving the exact developmental role of this duplicated gene open to future experimental investigation.
Subject of Research: Arthropod developmental biology and gene expression during embryonic segmentation
Article Title: Lack of evidence for conserved parasegmental grooves in arthropods
Article References: Janssen, R., Turetzek, N., & Pechmann, M. (2022). Lack of evidence for conserved parasegmental grooves in arthropods. Development Genes and Evolution, 232(1), 27-37. https://doi.org/10.1007/s00427-022-00684-5
Image Credits: AI Generated
DOI: 10.1007/s00427-022-00684-5
Keywords: arthropod segmentation, developmental biology, engrailed, spider development, chelicerates, parasegmental grooves, segment-polarity genes, Parasteatoda tepidariorum, Cupiennius salei, Pholcus phalangioides, Acanthoscurria geniculata, Phalangium opilio, whole genome duplication, confocal microscopy, evolutionary developmental biology, paralog subfunctionalization
News Source: Juliet Wilcox. (November 12, 2026). Spider Gene Expression Study Challenges Conserved Parasegmental Grooves in Arthropod Evolution. Scienmag.
