JACOBS, MW*; DEGNAN, BM; Univ. of Washington; Univ. of Queensland: Delay of Metamorphosis in Solitary Ascidians: Genetics, Morphology, and Ecology

Metamorphosis is both an ecological and a developmental genetic transition that an organism undergoes as a normal part of ontogeny. Many organisms have the ability to delay metamorphosis when conditions are unsuitable. This strategy carries obvious benefits, but may also result in severe consequences for larvae that run low on time and energy. Some solitary ascidians exhibit extensive precocious differentiation of juvenile structures when the tadpole larval phase of the life cycle is prolonged. We used microarrays to investigate the genetic consequences of delay of metamorphosis in larvae of Herdmania curvata, a solitary ascidian, which displays a conservative amount of precocious morphological differentiation. We did not detect any significant genetic change between competent and aged larvae, although there was a trend for expression levels of many genes, such as lectins, which are normally up-regulated during metamorphosis, to drop over prolonged larval life. We could not find any significant differences in gene expression levels between juveniles from delayed tadpoles and juveniles from tadpoles which metamorphosed as soon as they were competent, suggesting any changes in RNA levels that occur during an extended planktonic period are compensated for during early metamorphosis. These data suggest that (i) despite some morphological change over extended larval life, Herdmania are essentially in genetic stasis as larvae, and (ii) molecular mechanisms exist during early metamorphosis to ensure that postlarvae derived from different aged larvae are morphogenetically equal. We will discuss our results in the context of developmental patterns in other ascidians (both solitary and colonial), microarray studies of metamorphosis in other invertebrates, and ecological consequences of delay of metamorphosis.