New Insights from Sea Spider Genome Reveal Evolutionary Secrets

An international collaboration led by the University of Vienna and the University of Wisconsin-Madison has produced the first high-quality genome assembly of the sea spider *Pycnogonum litorale*, offering significant insights into the evolutionary history of chelicerates. This groundbreaking research, published in the journal BMC Biology on July 4, 2025, addresses the unique anatomical features of sea spiders, which are marine arthropods characterized by their narrow trunks and elongated legs, with internal organs extending into their limbs.

The research aimed to understand the genetic factors that contribute to the distinctive body plan of sea spiders and their evolutionary relationships within the chelicerate group, which includes spiders, scorpions, and horseshoe crabs. According to Dr. Nikolaos Papadopoulos, the study's first author and a researcher at the Department of Evolutionary Biology at the University of Vienna, the genome assembly is a significant advancement in understanding these unique organisms. "The genomes of many non-canonical laboratory organisms are challenging to assemble, and *Pycnogonum* is no exception. Only the combination of modern high-throughput data sources made a high-quality genome possible," he stated.

The researchers utilized a two-pronged approach to generate the genome assembly. Initially, long-read sequencing technology was employed to capture extensive stretches of DNA from a single individual. Subsequently, the spatial organization of the genome was analyzed from a second individual, allowing the team to accurately piece together the genomic regions. This methodology resulted in the assembly of 57 pseudochromosomes, covering nearly the entirety of the sea spider genome with unprecedented resolution. The genome also included new datasets reflecting gene activity at various developmental stages of *P. litorale*.

A particularly intriguing finding from the study was the absence of the abdominal-A (Abd-A) gene, a part of the Hox gene cluster typically associated with the development and specification of the posterior body regions. Dr. Andreas Wanninger, another project lead from the University of Vienna, explained, "In arthropods, Hox genes are essential 'master controllers' during body plan development. The absence of Abd-A in *P. litorale* may be linked to its extremely reduced abdomen, echoing similar conditions observed in other arthropods with reduced posterior features such as certain mites and barnacles."

The research not only sheds light on the evolutionary adaptations of sea spiders but also contributes to the broader understanding of arthropod biology. Unlike other chelicerates, such as spiders and scorpions, which exhibit signs of ancient whole-genome duplications, the genome of *P. litorale* shows no such traces. This observation suggests that the genome of the common ancestor of chelicerates did not possess these duplications, indicating that such events occurred later in the evolutionary timeline for specific sub-groups of chelicerates.

The assembly of this new reference genome positions *P. litorale* as a critical subject for future comparative studies. As Dr. Georg Brenneis, a senior researcher involved in the study, noted, "From an evolutionary developmental perspective, sea spiders are very interesting. Their mode of development may be ancestral for arthropods, but they also possess multiple unique body plan innovations. With the genome now available, we can systematically study these aspects on a molecular level."

The implications of this research extend beyond the biological insights it provides; it also highlights the potential for further investigations into gene regulation, developmental processes, and the remarkable regenerative abilities of sea spiders. As researchers continue to explore these avenues, the findings may offer profound insights into the evolutionary success and diversity of this intriguing group of marine arthropods.

The complete study is available in BMC Biology, with the full citation as follows: Papadopoulos, N., Kulkarni, S. S., Baranyi, C., Fromm, B., Setton, E. V. W., Sharma, P. P., Wanninger, A., & Brenneis, G. (2025). The genome of a sea spider corroborates a shared Hox cluster motif in arthropods with a reduced posterior tagma. BMC Biology. DOI: https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-025-02276-x