Meeting Abstract
We would like to understand how animal body plans arose. Panarthropods (arthropods, onychophorans, and tardigrades) have modular, segmented bodies, facilitating comparisons between body plans. How the body plan of tardigrades, which consists of a head and four leg-bearing segments, relates to that of other panarthropods, and how the tardigrade body plan evolved, are unclear. To address these issues, we developed in situ hybridization methods for tardigrades and investigated the expression patterns of Hox genes and several other genes using the tardigrade Hypsibius dujardini. Consistent with anterior Hox expression patterns in other panarthropods, we found that labial is expressed in the developing pharyngeal bulb; Hox3 is expressed in leg-bearing segments 2 and 3; and Deformed is expressed in the posterior of leg-bearing segment 2 through segment 3. Unlike in other panarthropods, fushi tarazu and the genital segment marker Abdominal-B share anterior expression boundaries in the same segment, leg-bearing segment 4. Consistent with conserved panarthropod antererior and posterior body regions, we found that the head gap gene orthodenticle is expressed across the head, while the gap gene caudal is expressed in the posterior of leg-bearing segment 4. Our results suggest that the body plan of H. dujardini is primarily composed of segments homologous to arthropod head segments and that this species lacks segments homologous to arthropod trunk segments. Hox genes that specify trunk segments in arthropods are reconstructed as ancestral for Panarthropoda, but are missing in the H. dujardini genome. Our results suggest that there has been a gap mutant-like loss of a region homologous to the arthropod trunk in the tardigrade lineage.
