Ladybird homeobox (Lbx) transcription factors have crucial functions in muscle 
and nervous system development in many animals. Amniotes have two Lbx genes, but 
only Lbx1 is expressed in spinal cord. In contrast, teleosts have three lbx 
genes and we show here that zebrafish lbx1a, lbx1b, and lbx2 are expressed by 
distinct spinal cell types, and that lbx1a is expressed in dI4, dI5, and dI6 
interneurons, as in amniotes. Our data examining lbx expression in Scyliorhinus 
canicula and Xenopus tropicalis suggest that the spinal interneuron expression 
of zebrafish lbx1a is ancestral, whereas lbx1b has acquired a new expression 
pattern in spinal cord progenitor cells. lbx2 spinal expression was probably 
acquired in the ray-finned lineage, as this gene is not expressed in the spinal 
cords of either amniotes or S. canicula. We also show that the spinal function 
of zebrafish lbx1a is conserved with mouse Lbx1. In zebrafish lbx1a mutants, 
there is a reduction in the number of inhibitory spinal interneurons and an 
increase in the number of excitatory spinal interneurons, similar to mouse Lbx1 
mutants. Interestingly, the number of inhibitory spinal interneurons is also 
reduced in lbx1b mutants, although in this case the number of excitatory 
interneurons is not increased. lbx1a;lbx1b double mutants have a similar spinal 
interneuron phenotype to lbx1a single mutants. Taken together these data suggest 
that lbx1b and lbx1a may be required in succession for correct specification of 
dI4 and dI6 spinal interneurons, although only lbx1a is required for suppression 
of excitatory fates in these cells.