Asymmetric Segregation of Maternal mRNAs and Germline-related Determinants in Cephalochordate Embryos: Implications for the Evolution of Early Patterning Events in Chordates.

This article focuses on the developmental mechanisms underlying primordial germ cell (PGC) specification and asymmetric gonad formation in cephalochordates, specifically the amphioxus species Asymmetron lucayanum. Using conserved germline markers vasa and nanos, the study demonstrates that A. lucayanum employs a preformation/maternal inheritance mechanism for PGC specification, similar to that observed in Branchiostoma species. Unlike Branchiostoma, which develops bilateral gonads, A. lucayanum initially forms PGCs on both body sides during early development but subsequently eliminates the left-side PGCs during late larval stages or after metamorphosis, resulting in right-sided gonads. The authors discuss the potential involvement of conserved left-right patterning pathways, such as Nodal-Pitx2 signaling, in this asymmetric PGC elimination and suggest that this process may represent an ancestral trait within cephalochordates with implications for understanding the evolution of chordate developmental mechanisms.