Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Evolutionary Conservation of Five Pax6 Homologs under Strong Purifying Selection in the Horseshoe Crab Species Cluster

Version 1 : Received: 2 September 2023 / Approved: 4 September 2023 / Online: 5 September 2023 (09:16:05 CEST)

A peer-reviewed article of this Preprint also exists.

Dakarapu, T.; Friedrich, M. Evolutionary Conservation and Diversification of Five Pax6 Homologs in the Horseshoe Crab Species Cluster. Arthropoda 2024, 2, 85-98. Dakarapu, T.; Friedrich, M. Evolutionary Conservation and Diversification of Five Pax6 Homologs in the Horseshoe Crab Species Cluster. Arthropoda 2024, 2, 85-98.

Abstract

Horseshoe crabs represent the most ancestral chelicerate lineage characterized by marine ecology and the possession of lateral compound eyes. While considered living fossils, recent studies reported an unusual number of Pax6 transcription factor genes in the Atlantic horseshoe crab Limulus polyphemus. Pax genes encode a family of ancient metazoan transcription factors, which comprise seven subfamilies. Among these, the members of the Pax6 subfamily confer critical functions in the development of the head, the visual system, and further body plan components. Arthropods are characterized by two Pax6 subfamily homologs that were discovered in Drosophila and named eyeless (ey) and twin of eyeless (toy). However, whole genome sequence searches uncovered three homologs of ey and two homologs of toy in L. polyphemus. These numbers are explained by the occurrence of three whole genome duplications (WGD) in the lineage to the last common ancestor of L. polyphemus and its closely related three additional members of the extant horseshoe crab species cluster. Here we report comparative sequence evidence for the functional conservation of the five L. polyphemus Pax6 transcription factor homologs. Our analyses reveal that all paralogs are conserved in the approximately 135 million-year-old horseshoe crab species cluster and that they evolve under strong purifying selection. These findings identify subfunctionalization as the likeliest post-WGD outcome for the five Pax6 homologs. While awaiting confirmation by gene expression studies, this scenario would reconcile the discrepancy between the WGD-expanded gene repertoires and phenotypic stasis in the horseshoe crabs.

Keywords

Pax6; Xiphosura; eye evolution; chelicerates; visual system; horseshoe crab; subfuctionalization; gene family evolution; gene duplication; gene regulatory network evolution; whole genome duplication

Subject

Biology and Life Sciences, Ecology, Evolution, Behavior and Systematics

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