Joyce, I. | Search bioRxiv

bioRxiv2025

The Expression of Pax6 Genes in an Eyeless Arachnid Suggests Their Ancestral Role in Arachnid Head Development

BackgroundMany animal lineages utilize Pax6 transcription factors during eye development. Within Arthropoda, evidence suggests that Pax6 genes are necessary for the specification of eyes in myriapods, crustaceans, and insects. However, recent data have argued that Pax6 genes lack a role in the development of the eyes in Chelicerata (=arachnids, horseshoe crabs, and sea spiders). An alternative hypothesis argues that the absence of Pax6 expression in developing chelicerate eyes could be explained by an earlier role for these genes in patterning eye precursor cells. The arachnid mite Archegozetes longisetosus lacks eyes, however it retains two Pax6 paralogs in its genome. By leveraging these aspects of A. longisetosus, we tested the hypothesis that ancestrally chelicerates did not use Pax6 genes to pattern their eyes but rather used them to pattern the central nervous system. We reasoned that if we observed comparable expression patterns of Pax6 genes in A. longisetosus in comparison to those in arachnids that have retained eyes, then this would support the hypothesis that Pax6 genes were not ancestrally used for eye specification in chelicerates. ResultsWe followed the expression of canonical arthropod retinal determination genes to confirm that A. longisetosus does not develop vestigial eyes. We found that the expression of the Pax6 paralogs was consistent with their roles in the development of the ocular region and central nervous system. By co- staining for these genes simultaneously with the conserved head patterning gene orthodenticle, we also observed early expression patterns of these genes in the protocerebrum of early A. longisetosus embryos that are comparable to those arachnids with embryonic eyes. ConclusionsOur data provide support for the hypothesis that Pax6 genes were not ancestrally used to pattern chelicerate eyes. The expression patterns of Pax6 genes in A. longisetosus were comparable to those of other arachnids that have eyes. This suggests that the retention of Pax6 genes in A. longisetosus is due to their ancestral, non-eye patterning roles. Further supporting this hypothesis is our observation that A. longisetosus does not pattern vestigial eyes. Lastly, our data suggests that the Pax6 genes, with orthodenticle, acted to specify the ancestral arachnid protocerebrum.

developmental biology↗

bioRxiv2024

A novel expression domain of extradenticle underlies the evolutionary developmental origin of the chelicerate patella

Neofunctionalization of duplicated gene copies is thought to be an important process underlying the origin of evolutionary novelty and provides an elegant mechanism for the origin of new phenotypic traits. One putative case where a new gene copy has been linked to a novel morphological trait is the origin of the arachnid patella, a taxonomically restricted leg segment. In spiders, the origin of this segment has been linked to the origin of the paralog dachshund-2, suggesting that a new gene facilitated the expression of a new trait. However, various arachnid groups that possess patellae do not have a copy of dachshund-2, disfavoring the direct link between gene origin and trait origin. We investigated the developmental genetic basis for patellar patterning in the harvestman Phalangium opilio, which lacks dachshund-2. Here, we show that the harvestman patella is established by a novel expression domain of the transcription factor extradenticle. Leveraging this definition of patellar identity, we surveyed targeted groups across chelicerate phylogeny to assess when this trait evolved. We show that a patellar homolog is present in Pycnogonida (sea spiders) and various arachnid orders, suggesting a single origin of the patella in the ancestor of Chelicerata. A potential loss of the patella is observed in Ixodida. Our results suggest that the modification of an ancient gene, rather than the neofunctionalization of a new gene copy, underlies the origin of the patella. Broadly, this work underscores the value of comparative data and broad taxonomic sampling when testing hypotheses in evolutionary developmental biology.

evolutionary biology↗