Population increase in Deschampsia Antarctica: genotypic variation or phenotypic plasticity?

PROBLEM DEFINITION

Genetic exchange between local populations of Deschampsia Antarctica (Poaceae), one of the two phanerogams occurring in Antarctica, might be difficult. Dispersal of the species within and between vegetated areas is believed to be via rain splash and via birds, collecting tufts of the species for nesting material, thus facilitating both vegetative and generative dispersal, but this is probably of local importance only. Its long distance dispersion is unclear. And yet, the species has a large distribution area, ranging from the frigid Antarctic to oceanic and alpine habitats. A marked increase in numbers of individual plants in established populations, as well as a vigorous colonization of recently deglaciated areas over the last four decades has been reported. Recent publications report also vigorous flowering, as well as abundant seed set and presence of seedlings throughout its distribution area, often attributed to parameters of global change, especially temperature increase. The fact that the species becomes more reproductive with increasing temperature could be either a plastic response, or selection for more reproductive genotypes under a changing temperature regime, or both. The role of genetic variation and phenotypic plasticity in the adaptation of Deschampsia Antarctica to increasing temperatures and its recent colonization of deglaciated areas is the aim of this study.

APPROACH

Using molecular techniques, the genotypic variation of D. Antarctica will be established in living samples and samples taken from herbarium specimens. From herbarium material only chlorophyll DNA can be successfully analyzed. Fresh material will be collected and preserved for the collection of nuclear DNA. Within and between population variability will be studied at the genotypic level using the overall AFLP fingerprint similarity as well as at the gene level, assuming Hardy Weinberg equilibriums (Schneider et al. 2000). Chloroplast DNA variation will be investigated using universal primers and digestion with restriction enzymes. This will provide phylogenetic analysis of the regions (phylogeography) and possibly allow a reconstruction of historical colonization patterns. The amount of nuclear DNA will be determined by flow cytometry of fresh material in different populations as this may affect the duration of the life cycle (Bennett et al, 1982).

Genotypic variation and phenotypic plasticity in reproductive traits will be determined by growing cloned plant-genotypes from different populations under a set of environmental (temperature, moisture) circumstances and measure generative traits. Differences in the response of genotypes or populations to these environmental conditions would indicate within- or among population variation in reproductive traits.

问题定义

南极发草（Deschampsia Antarctica）是南极洲仅有的两种显花植物（禾本科Poaceae）之一，其本地种群间的基因交流可能存在障碍。学界普遍认为，该物种在植被区内及植被区间的传播依赖雨水溅射，以及鸟类收集其草簇作为筑巢材料——这一途径可同时促进营养繁殖与有性繁殖，但这类传播大概率仅局限于局域尺度。目前其长距离扩散机制仍不明确。然而，该物种的分布范围却极为广泛，涵盖寒冷的南极地区、海洋性生境与高山生境。据报道，在过去四十年间，已建立的种群中个体数量显著增长，同时近期冰川消退后的区域的旺盛定植现象也被多次记录。近期研究还显示，该物种在整个分布区内均表现出旺盛的开花、充足的结实以及幼苗的广泛存在，这类现象常被归因于全球变化相关参数，尤其是气温升高。随着温度升高，该物种的繁殖能力增强，这一现象可能源于表型可塑性响应，或是在温度变化环境下对高繁殖力基因型的选择，亦或是两者共同作用。本研究旨在探究遗传变异与表型可塑性在南极发草适应气温升高过程中，以及其近期对冰川消退后区域定植过程中所发挥的作用。

研究方法

本研究将借助分子技术，对活体样本与标本馆馆藏样本中的南极发草基因型变异进行解析。仅能从标本馆馆藏材料中成功分析叶绿素DNA（chlorophyll DNA）；针对核DNA（nuclear DNA）的提取，则需要采集新鲜样本并妥善保存。研究将从基因型层面开展种群内与种群间的变异分析，一方面基于扩增片段长度多态性（AFLP）指纹相似度，另一方面结合哈迪-温伯格平衡（Hardy Weinberg equilibriums）假设（Schneider等，2000）进行基因层面的分析。叶绿体DNA（chloroplast DNA）变异将通过通用引物扩增结合限制性酶切消化进行检测，以此实现相关区域的系统发育分析（系统地理学phylogeography），并有望重建历史定植模式。此外，针对不同种群的新鲜样本，将通过流式细胞术（flow cytometry）测定其核DNA含量，因为核DNA含量可能影响生命周期时长（Bennett等，1982）。

本研究还将通过将不同种群来源的克隆植物基因型置于一系列环境（温度、湿度）条件下培养，测定其有性繁殖相关性状，以此明确繁殖性状的基因型变异与表型可塑性。若不同基因型或种群对上述环境条件的响应存在差异，则可说明繁殖性状在种群内或种群间存在变异。