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Podolsky Lab Research

Physiological ecology of development in highly variable environments

Fig. 1.  Melanochlamys diomedea adult (black), egg mass (cream) with long sand-mucous tether (pink arrow points to end), and embryos suspended in gel (inset) in tide pool at False Bay on San Juan Island, WA.

Extreme environments, such as polar oceans and hydrothermal events, are especially useful for the study of how biological systems respond to physical challenges.  Among marine habitats, intertidal areas are extreme in their variability: tidal exchanges can lead to rapid and severe fluctuations in a suite of physical conditions.  Early stages of development are especially sensitive to such perturbations and often represent a life-history bottleneck.  Members of at least three major taxa--molluscs, polychaetes, and crustaceans--reproduce and develop in intertidal areas, providing an opportunity to study the ecology of development in response to extremes of environmental variability.

This project addresses how temperature variability and stress influence development, reproductive success, and life-history evolution in the mollusc Melanochlamys diomedea.  Adults deposit and tether egg masses in shallow, soft-bottom tidal pools (Fig. 1), where temperatures can fluctuate more than 23ēC over a single exchange and regularly exceed thresholds for the expession of heat shock proteins (HSP-70 and -90; Fig.
Fig. 2.  Tidepool temperature fluctuation over 1 mo and 12-h tide exchange (inset) at  False Bay, WA.  Dashed line is 26° threshold for HSP expression.
2).  To understand how selective forces on the embryonic and adult life-history stages interact to determine reproductive success in the field, this project integrates information on cellular defense mechanisms, whole-organism developmental performance, and population-specific ecological conditions and reproductive patterns.

A conceptual framework for these functional relationships is shown below. Development temperatures depend on the interaction of weather, tidal patterns, and spawning time. Temperature alters development directly through Q10 effects and indirectly through energetic costs of thermal variability and stress. Effects of stress can be separated into periods before and after embryos gain the capacity for production of HSP transcripts. In turn, these processes alter development rate, growth, and survival. Temporal variation in offspring fitness could select on adult spawning time (dashed line) if adults can make use of environmental information that predicts future conditions for developing embryos (dotted line). Finally, larger-scale spatial variation among sites (e.g., False Bay, WA) in sediment type, drainage, and wind exposure result in between-population variation in thermal risks for embryos.  In a recent paper I outlined an approach to translating this framework into a quantitative model, and I have begun to collect data to address each of these functional relationships.

Research funded by NSF over the next few years will develop several novel approaches to understanding the ecological and evolutionary dynamics of this system.  In particular, field and laboratory work will establish (1) seasonal changes in temperature thresholds and the developmental onset of HSP expression, (2) thermotolerance of embryos and the ecological relevance of HSP expression as a protective mechanism for developmental processes, (3) measures of developmental performance (development rate, hatching size, and survival) in response to four different components of temperature variation--mean, variance, stress level, and rate of change--that can be partitioned in laboratory culture, (4) developmental consequences of temperature variation during field exposures, using multivariate models that estimate the interactive effects of maximum tidal temperature (Tmax) and age of exposure, and (5) associations between adult reproduction and environmental conditions on preceding tides--as evidence of the use of environmental signals that could help to predict future conditions for embryos--and experimental tests of these associations.  Because embryos lack HSP transcription during their first tide and Tmax is correlated across tides, short term predictive information could be especially important in timing reproduction to benefit embryos.  Field and laboratory analyses will be carried out at a set of field sites that vary markedly in physical parameters, and coupled with estimates of genetic differentiation among these populations using molecular markers.

Other research directions are possible and I welcome student interest and involvement in this project.

Robert D. Podolsky Home | Research Overview | Teaching | Publications
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