Evolution and endocrinology of accelerated metamorphosis in spadefoot toads (Anura: Pelobatidae)

The larvae of spadefoot toads exhibit a large diversity in time to forelimb emergence and body size at metamorphosis. The desert-adapted New World spadefoot (Scaphiopus and Spea) tadpoles have accelerated metamorphosis, the most rapid known among anurans. The closest relatives to the New World spadefoot toads are the Old World spadefoot toads (Pelobates) which have much longer larval periods than Scaphiopus and Spea. I examined the phylogenetic pattern of diversity of the larval period differences between taxa and pursued possible endocrine mechanisms for these differences. I examined three potential endocrine mechanisms to explain the differences in larval period length among spadefoot toads: differences in thyroid hormone levels in the tail and liver during prometamorphosis and climax, in vitro tissue sensitivity, and in vitro tissue responsivity to thyroid hormone.; I compared the larval period of Scaphiopus couchii with that of a sympatric relative, Spea multiplicata, under identical laboratory conditions. Sc. couchii developed 1.2--1.5 times faster and metamorphosed 4--6 days earlier than Sp. multiplicata , but Sc. couchii grew 2--3-fold slower in body and tail length and 10--20-fold slower in mass and metamorphosed at half the length and about fourteen percent the mass compared to Sp. multiplicata.; I obtained larval life history data to study the evolutionary origin of accelerated metamorphosis, the relationships between breeding habitat and larval period, and the evolutionary pattern of growth and development. Taxa within genera were similar to each other and were different from taxa in other genera for eight response variables measured at 24, 28, and 32°C.; Rates of development and growth correlated with times to forelimb emergence and sizes at tail resorption across genera. Mapping these tadpole characters onto a phylogeny revealed that rapid metamorphosis originated in the common ancestor of the New World toads. Analysis using phylogenetically independent contrasts showed that larval growth and development correlated with phylogenetic groupings and not with breeding habitat, and that there was no relationship between larval growth and development.; I estimated thyroid hormone levels by measuring T3 and T4 using radioimmunoassay in whole bodies, liver, and tail throughout prometamorphosis and climax in Scaphiopus couchii, Spea multiplicata, and Pelobates syriacus . The levels of T3 and T4 in Sc. couchii and Sp. multiplicata tails were about 3.5 and 3 times greater, respectively, than in Pb. syriacus. In the liver, there was no significant difference between taxa in T3 and T4 at forelimb emergence as well as the entire climax for T3. However, for T4, Pb. syriacus had 1.5--2.5 more than in the New World taxa.; I used an in vitro tail tip assay to measure tissue sensitivity and responsivity to the thyroid hormones T3 and T4 at three developmental stages in Scaphiopus couchii, Spea multiplicata, and Pelobates syriacus. The sensitivity and responsivity differences between taxa correlate with their larval period lengths; Scaphiopus with the shortest larval period is the most sensitive and responsive to T3 in vitro, whereas the converse is true for Pb. syriacus. (Abstract shortened by UMI.)...