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Podolsky Lab Research

Ecological and evolutionary physiology of fertilization along a latitudinal gradient

Fig. 1.  Ranges of sea urchin species (S. purpuratus, hatched; S. droebachiensis, solid)  showing approximate northern and southern limits and spawning season temperatures.

Broadcast spawning is the typical and ancestral mode of reproduction in marine invertebrates.  Fertilization success of broadcast spawners is highly variable and can be a key determinant of reproductive success.  Once released, eggs and sperm are subject to environmental conditions that affect the rates at which they make contact and achieve fertilization.  Selection is therefore likely to favor characteristics of gametes that overcome environmental challenges to fertilization.

Environmental temperature has pervasive effects on the biology of ectotherms.  Because temperature varies predictably with latitude, latitudinal comparisons offer the opportunity to make and test strong predictions about spatial patterns of temperature adaptation.  For gametes of diverse taxa, upper lethal temperature is correlated with environmental temperature.  Little is known, however, about the effects of temperature on sub-lethal measures of fertilization performance, about processes that underlie variation in performance, or about responses of gamete traits through genetic change or acclimation.  I am addressing these issues in the sea urchin genus Strongylocentrotus, which includes species with

Fig. 2.  Fertilization as a function of temperature and sperm concentration (increasing in half log-10 steps from bottom (1000 ml-1) to top (gametes of the sand dollar Dendraster excentricus).

broad ranges that partially overlap and together span latitudes from warm-temperate Mexico to arctic Alaska (Fig. 1).

As "model organisms," sea urchin gametes offer several advantages over complex, long-lived ectotherms for the study of evolutionary physiology: sperm physiology is simple and directed at maximizing energy use for locomotion during a brief lifetime; sperm rely entirely on aerobic metabolism and endogenous energy stores; gametes are active for a restricted season, minimizing the need to integrate function over annual variation in conditions; and their performance has a direct relation to reproductive success and fitness.

This project integrates three levels of analysis for populations at the extremes of each species range: (1) patterns of performance are being assessed by measuring the temperature dependence of fertilization kinetics (see Fig. 2); (2) two temperature dependent processes--sperm swimming speed and metabolic rate--that underlie those patterns are

Fig. 3.  Sperm swimming velocity as a function of temperature (sperm of the sand dollar Dendraster excentricus).

being measured, using motion analysis (see Fig. 3) and measures of oxygen consumption at different temperatures; (3) characteristics of a key biochemical mechanism involved in these processes--the thermostability and catalytic efficiency of dynein, an ATPase that influences the rate of force production by the sperm flagellum--are being measured.  Thus, this research is an integrative study of temperature adaptation, from ecological performance through physical processes and biochemical correlates.

The thermal biology of locomotion at this scale is of special interest because, as I showed previously, temperature influences motility both through physiology and through mechanical effects of changes in water viscosity.  I am therefore using manipulations of seawater viscosity to separate these two effects of temperature on sperm motility and sperm energetics. 

[This project was postponed after a large collection of tissue samples was destroyed in shipment.  I welcome student interest in reviving this project.]

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