DISSERTATION RESEARCH: Genetic Basis and Adaptive Significance of Key Developmental Traits Underlying Ecological Differences Between Sympatric Sister Species

大多数物种是在地理屏障将种群长时间隔离的情况下形成的。为了使物种在彼此接触后保持独特性，它们必须通过生态分化进化出生殖隔离。在没有生殖隔离的情况下，早期的物种会杂交并融合成一个单一的物种。如果没有生态分化，激烈的竞争往往会导致一个物种在当地灭绝。目前共存的物种可能在物理上分离的情况下进化出了生态差异，或者它们可能在同一地区通过选择来减少杂交或竞争而进化出了差异。对于允许物种在同一地区共存的生态性状差异或导致物种之间生态差异的遗传位点，人们知之甚少。本研究将确定两种北美西部野花物种之间共存的生态差异的特征和遗传位点。在一些地区，分布广泛的古土云雀（Mimulus guttatus）和其相对罕见的姊妹种裸土云雀（Mimulus nudatus）共同出现，但它们生活在不同的微生境中，在夏季干旱的开始时间不同。Mimulus nudatus生长在春天早期干涸的蛇形斜坡上，而M. guttatus栖息在短暂的溪流中，干旱造成的死亡比M. nudatus栖息地晚。这种罕见的特有植物具有一些可能适应早期干旱的特征，包括较短的诱导开花的临界光周期，导致开花芽（抽苔）的产生更早，开花时间比更常见的M. guttatus更早。了解稀有特有物种如何与更广泛的物种维持在一起，对保护和保护生物多样性具有重要意义。确定使物种共存的潜在遗传变化将揭示有助于维持物种生物多样性的遗传、生态和进化过程之间的相互作用。该项目将维持和发展针对初中、高中和本科阶段未被充分代表的群体的新的指导机会。拟议的研究结果将通过讲座和出版物向公众和科学界广泛传播。该项目将明确测试维持两个近缘种间空间、表型和遗传差异的生态和进化力量。同域物种之间的生态相似性可能导致竞争排斥。因此，生态分化是稳定共存的必要前提。据推测，常见的guttatus和罕见的特有的M. nudatus在开花时间上的差异，使这些物种能够通过居住在不同的微栖息地中，在夏季干旱的开始时间上有所不同，从而在同域发生。三个数量性状位点（QTL）对关键光周期和抽苔的差异有深远的影响，并已被定位，但我们不知道这些位点在自然界中如何促进微生境分化和物种相互作用。本项目将利用QTL定位、互惠移植和竞争实验来测试：(1)田间开花时间差异与室内光期和抽苔是否具有相同或不同的遗传基础；(2)光期和抽苔位点是否具有适应性，并通过促进生态位分化实现共存；(3)两种密切相关的同域物种的开花时间差异是否由生态性状或生殖性状位移造成。

Most species are formed when geographic barriers physically separate populations for long periods of time. For species to remain distinct once they are in contact with one another they must evolve reproductive isolation often through ecological differentiation. Without reproductive isolation incipient species will hybridize and fuse back together into a single species. Without ecological differentiation intense competition will often cause one species to go locally extinct. Species that presently co-occur may have evolved ecological differences while physically separated, or they may have evolved differences in the same area through selection to minimize hybridization or competition. Little is known about the ecological trait differences that allow species to coexist in the same area or the genetic loci responsible for ecological differences between species. This study will identify the traits and genetic loci underlying ecological differences that enable coexistence between two western North American wildflower species. The broadly distributed Mimulus guttatus, and its relatively rare sister species Mimulus nudatus co-occur in some areas but live in distinct microhabitats that differ in the onset of summer drought. Mimulus nudatus grows on serpentine slopes that dry out early in the spring, while M. guttatus inhabits ephemeral streams where drought imposed death is later than in the M. nudatus habitat. The rare endemic possesses several traits that are likely adaptations to earlier drought, including a shorter critical photoperiod to induce flowering resulting in an earlier production of a flowering shoot (bolting) and an earlier flowering time than the more common M. guttatus. Understanding how rare endemic species are maintained with more widespread species has important implications for conservation and the preservation of biodiversity. Determining the underlying genetic changes to enable species coexistence will shed light on the interaction among genetic, ecological, and evolutionary processes that contribute to the maintenance of species biodiversity. This project will maintain and develop new mentoring opportunities targeting underrepresented groups at the middle school, high school, and undergraduate level. The results of the proposed research will be broadly disseminated though talks and publications to the public and scientific community.This project will explicitly test the ecological and evolutionary forces maintaining spatial, phenotypic and genetic differences between two closely related species of Mimulus. Ecological similarity between sympatric species may lead to competitive exclusion. Consequently, ecological divergence is a necessary prerequisite for stable coexistence. Differences in flowering time between the common M. guttatus and the rare endemic M. nudatus are hypothesized to allow these species to occur in sympatry by inhabiting distinct microhabitats that differ in the onset of summer drought. Three quantitative trait loci (QTL) have been identified that profoundly influence differences in critical photoperiod and bolting and have already been genetically mapped, yet we do not know how these loci contribute to microhabitat differentiation and species interactions in nature. This project will utilize QTL mapping, reciprocal transplant and competition experiments to test: (1) whether flowering time differences in the field have the same or different genetic basis from photoperiod and bolting in the lab, (2) whether photoperiod and bolting loci are adaptive and enable coexistence by contributing to niche differentiation, and (3) whether ecological or reproductive character displacement are responsible for flowering time differences in the two closely related sympatric species.