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

点击翻译按钮获取中文摘要

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.