Unveiling the Secrets of Sea Spiders

Sea spiders (Pycnogonida) are marine arthropods characterized by their unique anatomical structure. They have a narrow, short trunk, with many of their internal organs extending into their long legs, while their abdomen is so reduced that it is almost unrecognizable. Sea spiders belong to the arachnid group, which also includes well-known creatures like spiders, scorpions, ticks, and horseshoe crabs. The peculiar body structure of these creatures raises intriguing questions about the genetic factors behind their formation and what they can reveal about the evolutionary history of arachnids. The answers lie in their genome.

High-Resolution Genomic Structure

Researchers assembled the genome using complementary sequencing techniques. First, genetic material from a single P. litorale specimen was obtained using “long-read sequencing,” which captures long DNA segments, facilitating the assembly of repetitive or complex genomic regions. Then, the spatial organization of the genome in another specimen was analyzed to uncover adjacent genetic segments in the cell nucleus. This combination of data resulted in the assembly of 57 virtual chromosomes, representing almost the entire sea spider genome with unprecedented accuracy. New datasets on gene activity during different developmental stages of P. litorale were added.

Nikolaos Papadopoulos, the study’s lead author from the Department of Evolutionary Biology at the University of Vienna, states, “Assembling genomes of non-model organisms is challenging, and Pycnogonum is no exception. Only the combination of modern high-throughput data sources made it possible to obtain a high-quality genome.”

Missing Genes and Their Visible Effects

The research team focused particularly on the Hox cluster, an evolutionarily conserved gene family across the animal kingdom. Andreas Wanninger, one of the project leaders, explains that these genes play a crucial role in determining different body parts in arthropods and are also important as master regulators in the development of body plans in many other animal groups.

The intriguing aspect of P. litorale is that part of the Hox cluster is entirely missing from the genome, specifically the abdominal-A (Abd-A) gene, which plays a role in defining and developing the rear part of the body. Its absence may be related to the significant reduction of the sea spider’s abdomen. Similar conditions have been observed in other arthropods with reduced rear parts, such as some ticks and barnacles. Sea spiders provide another example of the documented evolutionary relationship between the loss of Hox genes and physical reduction.

Broader Evolutionary Patterns

The genome also offers insights into broader evolutionary patterns. Unlike spiders and scorpions, whose genomes show clear signs of ancient genome duplications, no such traces exist in the P. litorale genome. Since sea spiders are considered the sister group to all arachnids, this suggests that the ancestral arachnid genome did not contain these duplications; rather, they must have occurred later in the evolution of some arachnid subgroups.

Conclusion

The newly assembled high-quality genome serves as a starting point for further comparative studies, making P. litorale a valuable new reference for questions related to arachnid relationships and the evolution of their body plans, as well as the genetic mechanisms underlying arthropod diversity. From an evolutionary perspective, sea spiders are fascinating: their evolutionary pattern may be fundamental to arthropods, yet they exhibit unique innovations in body plans. George Brenneis, the study’s senior author, explains that the genome and comprehensive data on gene activity during development enable the study of all these aspects at the molecular level.

Researchers will use the new reference genome for further studies on gene regulation, evolution, and regeneration in arachnids, aiming for a better understanding of the processes behind the evolutionary success of this group.