Conservation and variation in Ubx expression among chelicerates.

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Arthropod Genomics

Background:

• Chelicerates, an ancient arthropod group, possess a distinct body plan with prosoma and opisthosoma.
• The Hox gene Ultrabithorax (Ubx) plays a crucial role in segment identity during development.
• Previous studies in spiders showed Ubx expression beginning in the second opisthosomal segment (O2).

Purpose of the Study:

• To investigate the ancestral expression pattern of Hox genes Ultrabithorax and abdominal-A (UbdA) in basal chelicerates.
• To test the hypothesis that the spider Ubx expression pattern is secondarily modified due to a reduced first opisthosomal segment (O1).
• To correlate UbdA expression boundaries with morphological variations in the anterior opisthosoma of scorpions and xiphosurans.

Main Methods:

• Examined UbdA gene expression patterns in two basal chelicerate lineages: scorpions (Paruroctonus mesaensis) and xiphosurans (Limulus polyphemus).
• Utilized in situ hybridization techniques to visualize gene expression during embryonic development.
• Compared UbdA expression boundaries across different chelicerate species with varying anterior opisthosomal morphologies.

Main Results:

• In scorpions, early UbdA expression initiates in the posterior O2 segment, later expanding to include O1.
• In xiphosurans, early UbdA expression starts in the O2 segment, with a later shift to the O1 segment.
• The earliest expression boundary of UbdA was consistently observed within the O2 segment across all examined chelicerates.

Conclusions:

• The UbdA expression pattern in spiders, initiating in O2, likely represents the ancestral condition for chelicerates.
• Observed variations in anterior opisthosomal morphology do not appear to be directly associated with changes in the primary UbdA expression boundary.
• Secondary modifications to UbdA expression, if they occur, are likely associated with later developmental stages.