How common is countergradient variation in reptile development?

I recently published my first solo author paper in the Frontiers in Physiology research topic: Coping with Environmental Fluctuations: Ecological and Evolutionary Perspectives.

Ever since reading classic papers by Conover and Schultz 1995 and Levins 1969, I have been fascinated by patterns of countergradient variation in traits, whereby genetic variation opposes environmental variation across ecological gradients. I have even observed these patterns in our wall lizard experiments – under a common temperature, embryos from high altitude (cold) populations develop faster than those from low altitude (warm) environments.

Is this a general pattern across all reptiles? How and why might this happen?

For egg laying species, temperature experienced during embryonic development can have important fitness consequences. Incubation temperature can affect hatching success (survival), alter size at hatching, growth rate and the quantity and quality of offspring. Temperature poses a strong influence on physiological rates underlying energy acquisition and use. For example, relative to warm environments, low nest temperatures often increase development time (time from fertilization to hatching) and decrease metabolic rate (rate of energy expenditure), yet in cold climates it is crucial that embryos complete development and commence feeding and growth before the onset of winter. Countergradient variation can enable populations to compensate for the direct effects of temperature on physiological rates, to ensure persistence of populations under extreme climatic regimes.

Another reason why reptiles may show countergradient variation in development is to increase the efficiency use of their finite energy reserves. The thermal sensitivities of developmental and metabolic rates determine how energy use during development (fertilization until nutritional independence) scales with temperature (see article here and blog post here). Increasing either development time, or metabolic rate will increase the costs of development, and therefore reduce the amount of residual energy at hatching. The recently proposed Development Cost Theory explains how the relative temperature sensitivities of development time and metabolic rate determine the amount of energy expended at any given temperature. At cooler developmental temperatures, development time is often extended more than metabolic rate decreases, so cold environments generally increase total energy use, which reduces energy available upon hatching. Developmental Cost Theory can provide a useful framework for detecting local adaptation by providing a potentially general mechanism to explain the temperature sensitivity of development time and metabolic rate and fitness across environmental thermal gradients.

I compiled data from the literature on common garden and reciprocal transplant studies to test for evidence of countergradient variation in development time and metabolic rate across cold- and warm-adapted populations of reptiles. I found that most studies (17/22) show evidence for countergradient variation between development time and environmental temperature, supporting the generality of countergradient variation in reptile development. Development under cool conditions necessitates a countergradient adaptive response for faster development and earlier hatching time, enabling embryos to hatch before winter while resources are still available and ensuring residual energy at hatching. However, I found little support to suggest that countergradient variation is common for metabolic rate – overall, reptile embryos from locally-adapted cooler climates did not maintain higher metabolic rates compared with populations from warmer climates.

Effect sizes (Hedges’ g) for differences in the thermal sensitivity of development time (time from oviposition until hatching) and metabolic (heart) rate across cold and warm-adapted populations for 15 species of reptiles across 8 families (± variance). For development time (green data points and variance bars), negative values of indicate countergradient variation. For metabolic rates (orange data points and variance bars), positive values of indicate negative countergradient variation.

You can find the full article open access here: https://www.frontiersin.org/articles/10.3389/fphys.2020.00547/full or the PDF is available on my publications page.

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