BUCHHOLZ, D.*; HAYES, T.: Evolution of Diversity in Anuran Tadpoles: Accelerated metamorphosis in spadefoot toads

Spadefoot toads exhibit a large diversity in larval period and size at metamorphosis. We used growth and development data to examine evolutionary, physiological, and morphological aspects of this diversity. To obtain comparable data across taxa, we reared tadpoles under laboratory conditions from all spadefoot toads and one parsley frog. Differences in growth and development suggested physiology, rather than phenotypic plasticity, explained larval period diversity. Mapping larval data onto a molecular phylogeny indicated that rapid metamorphosis originated within the spadefoot toad family. To understand the physiological basis for accelerated metamorphosis, we used radioimmunoassay to estimate thyroid hormone levels throughout prometamorphosis and climax in Scaphiopus couchii, Spea multiplicata, and Pelobates syriacus. We also used an in vitro tail tip assay to measure tissue sensitivity to thyroid hormone. The higher thyroid hormone levels and the higher sensitivity of the tail in vitro in Sc. couchii correlated with its shorter larval period length. We examined the relationship between larval period length and gonad, spade, and thyroid develoement. Gonads and spades differentiated earlier in stage in Pelobates compared to Spea and Scaphiopus. In contrast, thyroid gland differentiation correlated with larval size rather than larval period length. In conclusion, the differences in thyroid physiology across taxa may be endocrinological mechanisms underlying accelerated metamorphosis. Also, because morphological development during metamorphosis is affected by thyoid hormone, physiological evolution to shorten larval period in New World spadefoot toads may have had heterochronic side-effects in spadefoot development.