Homoplastic evolution of proximal podomere patterning across Arthropoda
- 1University of Wisconsin-Madison, Department of Biology, Madison 53706, Wisconsin, United States
Abstract
The jointed arthropod leg is often credited as a key innovation spurring the evolutionary success of the phylum. Across lineages, each territory of the appendage has served as the substrate for myriad adaptations and structural innovations. While much of the available data has supported a conserved suite of transcription factors responsible for the initiation and patterning of the arthropod appendage, recent works have begun to highlight instances of lineage-specific shifts in expression dynamics, particularly amongst the chelicerates, correlated both with the emergence of novel structures and developmental system drift in the specification of plesiomorphic structures. These works confound efforts to align or homologize the leg segments across arthropod lineages, an effort already challenged by ground plan differences in segment number between the subphyla. An even more unusual architecture of leg segments occurs in sea spiders (Pycnogonida), which alone among Arthropoda are considered by systematists to bear three coxae (proximal segments), in contrast to the single coxa of the remaining arthropods. Here, we examined proximal podomere patterning in the developing appendages of a sea spider and an arachnid. We find that a particular spatial arrangement of Lim1 and SoxNeuro characterizes the developing coxa and trochanter of the harvestman Phalangium opilio (Opiliones) throughout appendage formation, with RNAi depletions of Lim1 yielding deletions of coxa and trochanter. Comparison of expression patterns across taxa shows that Lim1 and SoxNeuro have a similar spatial arrangement in the harvestman and sea spider, but these suggest conflicting interpretations of leg segment homology. We show that this is attributable to homoplastic patterns of evolution in SoxNeuro across Arthropoda. Expanding our surveys of prominent PD axis-patterning transcription factors supports additional instances of homoplasy in the domains of Sp6-9 and Irx2. The pervasiveness of homoplasy in gene expression patterns demands measured skepticism of homology statements and careful comparisons of multiple patterning genes, toward achieving falsifiability in the homology of structures.
Related
- Predator Becomes Prey: Records of Intraguild Predation Between Velvet Worms (Onychophora) and Arthropods (Arthropoda) With an Expanded Review of Predation on Velvet Worms
- Spatially restricted bmp16 in an extra-embryonic syncytium promotes dorsoventral patterning
- Canonical insulin-like receptor DAF-2 signaling-independent patterning and role for FoxO/DAF-16 in early embryos
- Mechanoscribed Xolography for Multiscale Mechanical Patterning of Cell-laden Hydrogels
- Autism genes converge on three functional programs organized by neuronal subclass, developmental timing, and cortical patterning
BibTeX
@misc{klementz2026homoplastic,
title = {{Homoplastic evolution of proximal podomere patterning across Arthropoda}},
author = {B. C. Klementz and M. Hämmerle and E. M. Laumer and S. M. Neu and M. M. Koebrick and N. Atla and P. Panda and N. Verma and G. Brenneis and P. P. Sharma},
howpublished = {bioRxiv},
year = {2026},
month = oct,
doi = {10.64898/2026.10.30.763126},
url = {https://www.biorxiv.org/content/10.64898/2026.10.30.763126v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Homoplastic evolution of proximal podomere patterning across Arthropoda
AU - B. C. Klementz
AU - M. Hämmerle
AU - E. M. Laumer
AU - S. M. Neu
AU - M. M. Koebrick
AU - N. Atla
AU - P. Panda
AU - N. Verma
AU - G. Brenneis
AU - P. P. Sharma
PY - 2026
DA - 2026/10/30
PB - bioRxiv
DO - 10.64898/2026.10.30.763126
UR - https://www.biorxiv.org/content/10.64898/2026.10.30.763126v1
AB - The jointed arthropod leg is often credited as a key innovation spurring the evolutionary success of the phylum. Across lineages, each territory of the appendage has served as the substrate for myriad adaptations and structural innovations. While much of the available data has supported a conserved suite of transcription factors responsible for the initiation and patterning of the arthropod appendage, recent works have begun to highlight instances of lineage-specific shifts in expression dynamics, particularly amongst the chelicerates, correlated both with the emergence of novel structures and developmental system drift in the specification of plesiomorphic structures. These works confound efforts to align or homologize the leg segments across arthropod lineages, an effort already challenged by ground plan differences in segment number between the subphyla. An even more unusual architecture of leg segments occurs in sea spiders (Pycnogonida), which alone among Arthropoda are considered by systematists to bear three coxae (proximal segments), in contrast to the single coxa of the remaining arthropods. Here, we examined proximal podomere patterning in the developing appendages of a sea spider and an arachnid. We find that a particular spatial arrangement of Lim1 and SoxNeuro characterizes the developing coxa and trochanter of the harvestman Phalangium opilio (Opiliones) throughout appendage formation, with RNAi depletions of Lim1 yielding deletions of coxa and trochanter. Comparison of expression patterns across taxa shows that Lim1 and SoxNeuro have a similar spatial arrangement in the harvestman and sea spider, but these suggest conflicting interpretations of leg segment homology. We show that this is attributable to homoplastic patterns of evolution in SoxNeuro across Arthropoda. Expanding our surveys of prominent PD axis-patterning transcription factors supports additional instances of homoplasy in the domains of Sp6-9 and Irx2. The pervasiveness of homoplasy in gene expression patterns demands measured skepticism of homology statements and careful comparisons of multiple patterning genes, toward achieving falsifiability in the homology of structures.
ER -