Asexual propagation in the cheilostomes Metrarabdotos and Coscinopleura: effects on genetic variation and larval productivity

Although separated by 30 million years, Metrarabdotos and Coscinopleura share an arborescent growth habit potentially capable of propagating asexuaily by fragmentation and regeneration, and a relative paucity of (ovicelled) zooids modified for production of larvae. This combination of characters suggests a tradeoff between vegetative growth and sexual productivity; yet the potential for asexual propagation is abundantly realized in Coscinopleura and only rarely so in Metrarabdotos . The evolutionary significance of such a trade off is the extent to which a swatch to asexual propagation depresses genetic variability. Estimated from the ratio between among-colonies and within-colonies variance in zooid morphology, however, genetic variation does not differ significantly between species of Metrarabdotos showing asexual propagation and those lacking it. or between Metrarabdotos and Coscinopleura . Moreover, total numbers of ovicelled zooids are at least as great in Coscinopleura as in coeval cheilostomes in which the percentage of such zooids is much higher. Thus, vegetative proliferation appears to be an added capability, perhaps related to growth in areas of nutrient enrichment, and not a trade off diverting resources from the maintenance of genetic diversity.