Postembryonic development of the posterior lateral line in zebrafish.

We examine how the posterior lateral line of the zebrafish grows and evolves from the simple midbody line present at the end of embryogenesis into the complex adult pattern. Our results suggest that secondary neuromasts do not form through budding from the embryonic line, but rather new waves of neuromasts are added anteroposteriorly. We propose that the developmental module that builds the embryonic pattern of neuromasts is used repeatedly during postembryonic development and that additional (secondary) primordia generate the additional neuromasts. We show that differentiated neuromasts migrate ventrally, and eventually generate "stitches" by successive bisections. We also examine the repatterning of the terminal neuromasts, which anticipates the up-bending of the tail leading to the highly asymmetrical caudal fin of the adult (which develops exclusively from the ventral part of the tail). Because terminal repatterning affects all aspects of tail formation, including its sensory development, we speculate that terminal axis bending may have become intimately associated with the terminal Hox genes before the appearance of the tetrapod lineage.