EVOLUTIONARY MODIFICATIONS OF THE SPIRALIAN DEVELOPMENTAL PROGRAM

The Spiralia, an assemblage of phyla united by their stereotypic patte rn of early embryonic cell divisions (spiral cleavage), is an interest ing group in which to investigate the evolution of development, This p aper examines modifications of developmental mechanisms within the Spi ralia with emphasis on the basally-branching forms, Although demonstra ting a notable degree of evolutionary conservation, the equal quartet cleavage pattern, which appears to be the ancestral condition, nonethe less exhibits modifications within the various spiralian groups, such as unequal cleavage, changes in cell size and rate of division, format ion of two rather than four quadrants (duet spiral cleavage), and in e xtreme cases the loss of any trace of the spiral pattern, While the ce ll lineages of spiralians are remarkably conserved, one can discern ev olutionary changes, for example in the cells that give rise to mesoder m, Studies of blastomere specification in many spiralian groups and an alyses of axis determination indicate that embryos with equal versus u nequal cleavage typically use different determinative mechanisms to es tablish cell fates and the dorsoventral axis, These properties are est ablished early in species exhibiting unequal cleavage, While previous experiments suggested that equal cleavage was associated with late spe cification, there is now evidence of precocious specification of quadr ant fates in some equal-cleaving species, such as the nemerteans and t he polyclad turbellarians.