Evolutionary Modifications of the Spiralian Developmental Program

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Abstract
SYNOPSIS. The Spiralia, an assemblage of phyla united by their stereotypic pattern of early embryonic cell divisions (spiral cleavage), is an interesting group in which to investigate the evolution of development. This paper examines modifications of developmental mechanisms within the Spiralia with emphasis on the basallybranching forms. Although demonstrating a notable degree of evolutionary conservation, the equal quartet cleavage pattern, which appears to be the ancestral condition, nonetheless exhibits modifications within the various spiralian groups, such as unequal cleavage, changes in cell size and rate of division, formation of two rather than four quadrants (duet spiral cleavage), and in extreme cases the loss of any trace of the spiral pattern. While the cell lineages of spiralians are remarkably conserved, one can discern evolutionary changes, for example in the cells that give rise to mesodenn. Studies of blastomere specification in many spiralian groups and analyses of axis determination indicate that embryos with equal versus unequal cleavage typically use different determinative mechanisms to establish cell fates and the dorsoventral axis. These properties are established early in species exhibiting unequal cleavage. While previous experiments suggested that equal cleavage was associated with late specification, there is now evidence of precocious specification of quadrant fates in some equal-cleaving species, such as the nemerteans and the polyclad turbellarians

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