Expression analysis of amphioxus orthologues of genes expressed in vertebrate lateral plate or pharyngeal mesoderm

The lateral plate mesoderm of vertebrates, which borders the other mesodermal territories, develops during embryogenesis into a variety of tissues and organs such as blood, heart, vasculature, kidney and smooth muscles. This mesoderm compartment, as well as the unsegmented pharyngeal mesoderm which gives rise to head muscles and part of the heart, have been proposed as vertebrate innovations. Indeed, in the two other chordate clades, the tunicates and the cephalochordates, no such mesoderm regions are formed during development. However, in ascidians, the most studied tunicate group, some cells in the larva which participate to siphon muscles and heart formation are thought to be homologous to the cardiopharyngeal field of vertebrates. Moreover, in the cephalochordate amphioxus, lateral plate and pharyngeal mesoderm marker genes were shown to be expressed in different regions of the fully segmented paraxial mesoderm. In this work, we decided to look at the embryonic expression in amphioxus of several of these mesoderm marker genes, that could give new insights into the putative homology between cephalochordate somite regions and vertebrates' mesoderm compartments. Here, we describe the expression pattern of Erg/Fli1a, Erg/Fli1b, Lmo2, Mesp, Npas4/4l, Osr1/2a, Osr1/2b, Tcf21/Msc and Tcf21/Mscb. Our results highlight the presence of a putative hematopoietic field in the first somite pair as previously proposed, and suggest that some genes were probably specifically recruited during vertebrate evolution for the development of pharyngeal or lateral plate mesoderm derivatives.