Caruso Christina M, Maherali Hafiz, Mikulyuk Alison, Carlson Kjarstin, Jackson Robert B
Department of Biology and Mathematics, Grinnell College, Grinnell, Iowa 50112, USA.
Evolution. 2005 Apr;59(4):826-37.
Physiological traits that control the uptake of carbon dioxide and loss of water are key determinants of plant growth and reproduction. Variation in these traits is often correlated with environmental gradients of water, light, and nutrients, suggesting that natural selection is the primary evolutionary mechanism responsible for physiological diversification. Responses to selection, however, can be constrained by the amount of standing genetic variation for physiological traits and genetic correlations between these traits. To examine the potential for constraint on adaptive evolution, we estimated the quantitative genetic basis of physiological trait variation in one population of each of two closely related species (Lobelia siphilitica and L. cardinalis). Restricted maximum likelihood analyses of greenhouse-grown half-sib families were used to estimate genetic variances and covariances for seven traits associated with carbon and water relations. We detected significant genetic variation for all traits in L. siphilitica, suggesting that carbon-gain and water-use traits could evolve in response to natural selection in this population. In particular, narrow-sense heritabilities for photosynthetic rate (A), stomatal conductance (gs), and water-use efficiency (WUE) in our L. siphilitica population were high relative to previous studies in other species. Although there was significant narrow-sense heritability for A in L. cardinalis, we detected little genetic variation for traits associated with water use (gs and WUE), suggesting that our population of this species may be unable to adapt to drier environments. Despite being tightly linked functionally, the genetic correlation between A and gs was not strong and significant in either population. Therefore, our L. siphilitica population would not be genetically constrained from evolving high A (and thus fixing more carbon for growth and reproduction) while also decreasing gs to limit water loss. However, a significant negative genetic correlation existed between WUE and plant size in L. siphilitica, suggesting that high WUE may be negatively associated with high fecundity. In contrast, our results suggest that any constraints on the evolution of photosynthetic and stomatal traits of L. cardinalis are caused primarily by a lack of genetic variation, rather than by genetic correlations between these functionally related traits.
控制二氧化碳吸收和水分流失的生理特性是植物生长和繁殖的关键决定因素。这些特性的变异通常与水、光和养分的环境梯度相关，这表明自然选择是导致生理多样性的主要进化机制。然而，对选择的响应可能会受到生理特性的现有遗传变异量以及这些特性之间的遗传相关性的限制。为了研究适应性进化受到限制的可能性，我们估计了两个近缘物种（北美山梗菜和红花山梗菜）各自一个种群中生理特性变异的数量遗传基础。利用对温室种植的半同胞家系进行的限制最大似然分析，来估计与碳和水分关系相关的七个性状的遗传方差和协方差。我们在北美山梗菜中检测到所有性状都存在显著的遗传变异，这表明该种群中碳获取和水分利用性状可能会因自然选择而进化。特别是，相对于之前对其他物种的研究，我们北美山梗菜种群中光合速率（A）、气孔导度（gs）和水分利用效率（WUE）的狭义遗传力较高。虽然红花山梗菜中A存在显著的狭义遗传力，但我们检测到与水分利用相关的性状（gs和WUE）几乎没有遗传变异，这表明该物种的这个种群可能无法适应更干燥的环境。尽管A和gs在功能上紧密相关，但在两个种群中，它们之间的遗传相关性都不强且不显著。因此，我们的北美山梗菜种群在进化出高光合速率（从而为生长和繁殖固定更多碳）的同时降低gs以限制水分流失，在遗传上不会受到限制。然而，北美山梗菜中WUE与植株大小之间存在显著的负遗传相关性，这表明高水分利用效率可能与高繁殖力呈负相关。相比之下，我们的结果表明，对红花山梗菜光合和气孔性状进化的任何限制主要是由于缺乏遗传变异，而不是由于这些功能相关性状之间的遗传相关性。
