Genomics and biology of sex determination in poplars, willows and relatives (Salicaceae)

植物从雌雄同体祖先中分离两性的起源是生物学上的一个主要问题。与雌雄同体相比，单性个体（雌雄异株）的植物相对较少，并且具有完全测序的基因组的例子相对较少。之前关于这个问题的大部分工作都是在Silene （campion）上进行的，它的基因组还没有完全测序。我的实验室（Hefer, Geraldes et al., in prep.）最近与UBC的其他人合作，确定了杨树（Populus trichocarpa）基因组的性别决定区域，这是一种具有优秀基因组资源的雌雄异株树。**本项目将全面表征杨树的性别决定区，以确定这些树木性别分化的基础。我们还将确定柳树是否使用相同的系统。目前的理论认为，染色体上的性别决定区域应该迅速扩大（经过几百万年），形成非配对性染色体。矛盾的是，尽管杨树的性别决定区看起来很古老，但它仍然很小，只占其染色体的一小部分，染色体仍然沿着其大部分长度成对。本研究项目旨在通过分析众多杨树物种性别连锁序列的进化动态，发现性别决定区扩展有限的原因。**柳树和杨树（水杨科）构成了加拿大景观的生态和经济重要组成部分，从北方森林到山区和草原。人们种植杨树是为了获取木材和纸浆，作为生物质作物和农场防风林。柳树越来越多地被种植为短期轮作的生物质作物，用于环境稳定（斜坡、河岸）和植物修复。雌雄异株是这些植物生物学的一个重要方面，性别的划分影响着它们的生态、进化、生长生理和商业育种。我们已经开始开发基因分析，在验证测试中可以利用是否存在性别特异性标记来正确确定性别。这为科学探索开辟了许多途径。**一个例子是性别比例测定。从理论上讲，1:1的性别比例应该成为标准是有原因的。然而，在许多柳树和一些杨树中，根据生殖个体的计数记录了显著的偏差（通常为F2:1M）。我们将使用新的性别测定工具来确定这些偏差是在生命周期的哪个阶段建立的，是在花粉期、胚胎期、幼苗期还是成虫期。了解性别比例确定的生命周期阶段是让我们建立机制的第一步。此外，这项研究将使我们对这些重要的加拿大景观植物的基因流动和种群遗传学产生新的见解。非重组、单系遗传的性染色体为物种鉴定（“条形码”）、树祖先测定和杂交动力学提供了一个非常合适的遗传标记。例如，它可以用来帮助确定种植的外来杨树是否和在多大程度上可能与本地杨树杂交。**这项工作的一个重要实际成果将是为作为群落资源的所有具有经济和生态重要性的北美水杨科物种开发可靠的性别测定方法。快速准确地鉴定杨树和柳属植物的性别有许多用途。例如，它将允许加速育种材料的选择，并允许准确确定野生种群的性别比例和性别分布。

The origin of separate sexes in plants from hermaphrodite ancestors is a major problem in biology. Plants with unisexual individuals (dioecy) are relatively rare compared to hermaphrodites and there are as yet comparatively few examples with fully sequenced genomes. Much of the previous work on this problem has been carried out on Silene (campion) which does not yet have a fully sequenced genome. Recent work in my laboratory (Hefer, Geraldes et al., in prep.) and in collaboration with others at UBC, has located the gender-determining region of the genome of poplar (Populus trichocarpa), a dioecious tree with excellent genomic resources.**This project will fully characterize the sex-determining region in poplar in order to determine the basis of gender differentiation in these trees. We will also determine whether or not willow uses the same system. Current theory suggests that a gender-determining region on a chromosome should quickly expand (over a few million years) to form a non-pairing sex chromosome. Paradoxically, although the poplar gender-determining region appears to be old it is still small, occupying only a small part of its chromosome, which still pairs along most of its length. This research program seeks to discover the reason for the limited expansion of the sex-determining region by analyzing the evolutionary dynamics of sex-linked sequences across numerous poplar species. **Willows and poplars (Salicaceae) form an ecologically and economically important component of the Canadian landscape, from the boreal to mountains and prairie. Poplars are grown for timber and pulp, as biomass crops and for farm windbreaks. Willows are increasingly grown as short rotation biomass crops and for environmental stabilization (slopes, riverbanks) and phytoremediation. Dioecy is an important aspect of the biology of these plants, and the division of sexes affects their ecology, evolution, growth physiology and commercial breeding. We have begun to develop genetic assays which in validation tests can correctly determine gender using the presence or absence of gender-specific markers. This opens numerous avenues of scientific exploration.**An example is sex-ratio determination. There are theoretical reasons why sex-ratios of 1:1 should be the norm. However in numerous willows and some poplars significant deviations have been recorded (usually F2:1M) based on counts of reproductive individuals. We will use novel gender-assay tools to determine at what stage of the life-cycle these deviations are established, whether at pollen, embryo, seedling or adult stage. Knowing the life-cycle stage at which gender ratios are established is the first step towards allowing us to establish a mechanism. Furthermore this research will allow us to generate new insights into gene flow and population genetics of these important Canadian landscape plants. The non-recombining, uniparentally inherited sex chromosome provides a highly suitable genetic marker for species identification ("barcoding"), tree ancestry determination and hybridization dynamics. It could be used, for example to help establish whether, and to what extent, planted exotic poplars may be hybridizing with native poplars.**An important practical outcome of this work will be the development of reliable gender assays for all economically and ecologically important N. American Salicaceae species as a community resource. Rapid and accurate identification of gender in Populus and Salix has many uses. For instance it will permit accelerated selection of material for breeding and allow accurate determination of sex-ratio and sex distribution in wild populations.