ADAPTIVE PLASTICITY IN DEVELOPMENT OF SCAPHIOPUS COUCHII TADPOLES IN DESERT PONDS.

Abstract

Couch's spadefoot toads (Scaphiopus couchii) breed in ephemeral desert ponds that are highly variable in duration. Rapid development is expected to be advantageous in short-duration ponds, but slower development, allowing more time for growth, may be advantageous in ponds of longer duration. Previous experiments have revealed both genetic variation in development time and phenotypic plasticity in response to pond drying. In this paper, I examine the norms of reaction of five sibships of tadpoles to see whether there is genetic variation in the effect of pond duration, i.e., in phenotypic plasticity. Several important results emerged. 1) Differences among sibships in development time that were seen in the lab were also seen in the field. 2) There was no evidence for genetic variation in plasticity of development; all sibships exhibited faster development and decreased larval period in ponds of short duration. Plasticity in development appears to be adaptive, as size at metamorphosis was correlated with duration of larval period. The slowest developing sibship, however, suffered higher mortality compared to other sibships in short duration ponds. 3) Sibships did not differ in growth or size at metamorphosis in short-duration ponds, but the slowest developing sibship metamorphosed at the largest size in long duration ponds, resulting in a significant genotype x environment interaction for size at metamorphosis. Thus, although only one of the five sibships responded differently, there appears to be genetic variation for plasticity in growth, and a genetically determined trade-off between fitness in short-duration ponds (via rapid development) and fitness in long duration ponds (via large size at metamorphosis). This may explain the existence of both phenotypic plasticity and genetic variation in development. A single genotype, although capable of adaptive plasticity, is not sufficiently flexible to have equally high fitness in both long- and short-duration ponds.