DAVIDSON, Brad; Univ. of California, Berkeley: Ascidian mesoderm specification and the emergence of the chordate body plan

Ascidians occupy a crucial evolutionary position at the base of the chordates. Intensive research into early ascidian development has begun to illuminate the evolutionary relationship between mesoderm development in ascidians and higher chordates. It appears that aspects of mesoderm specification and patterning are highly conserved but include modifications related to a rapid embryogenesis. In particular, early maternal determination of ascidian tail muscle, through the cytoplasmic determinant Macho-1, overlays a more regulative and ancestral function of T-box mediated specification. However comparisons are complicated due to the ascidianís bi-phasic life history (rapid embryogenesis of a larval form followed by prolonged differentiation of adult structures after metamorphosis). Tail muscle is the only mesodermal tissue that rapidly differentiates during embryogenesis. The remaining mesodermal tissues, including blood, heart, gonad and adult musculature, develop over a much longer time-span, through metamorphosis. This review will provide an in-depth comparison between mesoderm development in ascidians and higher chordates taking into account the distortions caused by major shifts in life-history patterns.