Soza Valerie L, Le Huynh Vietnam, Di Stilio Verónica S
Department of Biology, University of Washington, Box 351800, Seattle, WA 98195-1800 USA.
Evodevo. 2014 Nov 12;5:42. doi: 10.1186/2041-9139-5-42. eCollection 2014.
Lepidium sisymbrioides, a polyploid New Zealand endemic, is the sole dioecious species in Brassicaceae and therefore the closest dioecious relative of the model plant Arabidopsis thaliana. The attractiveness of developing this system for future studies on the genetics of sex determination prompted us to investigate historical and developmental factors surrounding the evolution of its unisexual flowers. Our goal was to determine the evolutionary pattern of polyploidization of L. sisymbrioides and the timing and process of flower reproductive organ abortion. To that end, we used a combination of phylogenetics to place this species within the complex history of polyploidization events in Lepidium and histology to compare its floral ontogeny to that of its closest hermaphroditic relatives and to A. thaliana.
Using a nuclear locus (PISTILLATA), we reconstructed the gene tree among Lepidium taxa and applied a phylogenetic network analysis to identify ancestral genomes that contributed to the evolution of L. sisymbrioides. Combining this phylogenetic framework with cytological and genome size data, we estimated L. sisymbrioides as an allo-octoploid resulting from three hybridization events. Our investigations of flower development showed that unisexual flowers appear to abort reproductive organs by programmed cell death in female flowers and by developmental arrest in male flowers. This selective abortion occurs at the same floral developmental stage in both males and females, corresponding to Arabidopsis stage nine.
Dioecy in Brassicaceae evolved once in L. sisymbrioides following several allopolyploidization events, by a process of selective abortion of reproductive organs at intermediate stages of flower development. Different developmental processes, but similar timing of abortions, affect male versus female flower development. An increased understanding of how and when reproductive organs abort in this species, combined with our estimates of ancestral genome contributions, ploidy and genome size, lay the foundation for future efforts to examine the genetic mechanisms involved in the evolution of unisexual flowers in the closest dioecious relative of the best studied model plant.
新西兰特有多倍体植物绵果独行菜是十字花科中唯一的雌雄异株物种，因此是模式植物拟南芥最近的雌雄异株近缘种。开发该系统用于未来性别决定遗传学研究的吸引力促使我们研究围绕其单性花进化的历史和发育因素。我们的目标是确定绵果独行菜多倍体化的进化模式以及花生殖器官败育的时间和过程。为此，我们结合系统发育学将该物种置于独行菜属多倍体化事件的复杂历史中，并运用组织学将其花的个体发育与最接近的雌雄同体近缘种以及拟南芥进行比较。
利用一个核基因座（PISTILLATA），我们重建了独行菜属类群间的基因树，并应用系统发育网络分析来确定对绵果独行菜进化有贡献的祖先基因组。将这个系统发育框架与细胞学和基因组大小数据相结合，我们估计绵果独行菜是由三次杂交事件产生的异源八倍体。我们对花发育的研究表明，单性花似乎通过雌花中的程序性细胞死亡和雄花中的发育停滞来使生殖器官败育。这种选择性败育在雄性和雌性中发生在相同的花发育阶段，对应于拟南芥的第九阶段。
十字花科的雌雄异株现象在绵果独行菜中经过几次异源多倍体化事件后进化而来，是通过花发育中期生殖器官的选择性败育过程实现的。不同的发育过程，但败育时间相似，影响雄花和雌花的发育。对该物种生殖器官如何以及何时败育的进一步了解，再加上我们对祖先基因组贡献、倍性和基因组大小的估计，为未来研究这个研究最深入的模式植物最近的雌雄异株近缘种中单性花进化所涉及的遗传机制奠定了基础。
