Spatially and temporally regulated expression of the LIM class homeobox gene Hrlim suggests multiple distinct functions in development of the ascidian, Halocynthia roretzi

Hrlim is a LIM class homeobox gene that was first isolated from the ascidian Halocynthia roretzi. To assess its roles in early development of the ascidian, spatial and temporal expression of Hrlim was examined by whole mount in situ hybridization. This revealed that transcription of Hrlim is activated at the 32-cell stage specifically in the endoderm lineage. Hrlim is also transiently expressed in all notochord precursor cells. Expression in the endoderm lineage continues through to the middle of gastrulation. After gastrulation, Hrlim is expressed in certain lineages that give rise to subsets of cells in the brain and spinal cord. Based on these observations, it is suggested that Hrlim plays multiple distinct roles in ascidian embryogenesis.