Genetic divergence for physiological response to temperature between populations of a C3-C4 intermediate annual

Premise of research. Local adaptation ultimately arises as a result of adaptive divergence in phenotypic characters. Within a species, a large portion of the phenotypic traits that confer local adaptation must be physiological characters. Yet few studies consider adaptive divergence in physiological characters or estimate genetic variation within populations for physiological traits that may confer local adaptation. Here I test the hypothesis that there is genetic variation within and between populations of a C3-C4 intermediate for the CO2 compensation point (Γ) and other photosynthetic traits. In addition to local adaptation, population-level variation in photosynthetic characters of C3-C4 intermediates reflects the potential for evolution of more C4-like photosynthesis.

Methodology. I chose two populations of Mollugo verticillata, one from a relatively cool climate and one from a warm climate. I quantified variation within and between populations by measuring photosynthetic physiology at 24° and 35°C, temperatures that match the average maximums during the growing season at the populations’ native sites.

Pivotal results. I detected little evidence of genetic variation within populations for Γ and carboxylation efficiency, but there was temperature-dependent population variation for Γ, where the population from the cool climate had a lower average Γ at 24°C. Apparent chlorophyll concentration was also greater in this population at both temperatures.

Conclusions. The results demonstrate that there has been divergence among populations in Γ. Interestingly, the population from the cooler climate had lower Γ at 24°C. There is little evidence of adaptive evolutionary potential within populations; estimates of broad-sense heritability were not significant even when pooling samples from both populations to increase the overall sample size. Adaptation of more C4-like photosynthesis may proceed through the evolution of other physiological characters not directly related to concentrating carbon.

研究背景：局部适应本质上源于表型性状的适应性分化。在同一物种内，赋予局部适应的表型性状中有很大一部分属于生理性状。然而，鲜有研究关注生理性状的适应性分化，或是针对可能赋予局部适应的生理性状估算种群内的遗传变异。本文提出假说：C3-C4中间型（C3-C4 intermediate）物种的种群内部及种群间，存在CO2补偿点（CO2 compensation point，Γ）及其他光合性状的遗传变异。除局部适应外，C3-C4中间型光合性状的种群水平变异，还反映了其向更接近C4型光合途径演化的潜力。
研究方法：本研究选取了两个覆瓦状粟米草（Mollugo verticillata）种群，一个来自相对凉爽的气候区，另一个来自温暖气候区。通过在24℃与35℃下测定光合生理指标，量化种群内与种群间的性状变异——这两个温度分别对应两个种群原生栖息地生长季的平均最高温。
关键结果：本研究未在种群内检测到Γ与羧化效率的显著遗传变异，但Γ存在温度依赖性的种群间差异：来自凉爽气候区的种群在24℃下的平均Γ更低。此外，该凉爽种群在两个温度下的表观叶绿素浓度均更高。
研究结论：研究结果表明，不同种群间的Γ已发生分化。值得注意的是，凉爽气候种群在24℃下的Γ水平更低。种群内几乎未体现出适应性演化潜力：即使合并两个种群的样本以扩大总样本量，广义遗传力（broad-sense heritability）的估算值仍未达到显著水平。向更接近C4型光合途径的适应性演化，或可通过其他与碳浓缩机制无直接关联的生理性状的演化来实现。