New York Botanical Garden, Bronx, NY, USA.
The Graduate Center, City University of New York, New York, NY, USA.
Ann Bot. 2021 Jul 30;128(2):217-230. doi: 10.1093/aob/mcab059.
The ovule is a synapomorphy of all seed plants (gymnosperms and angiosperms); however, there are some striking differences in ovules among the major seed plant lineages, such as the number of integuments or the orientation of the ovule. The genetics involved in ovule development have been well studied in the model species Arabidopsis thaliana, which has two integuments and anatropous orientation. This study is approached from what is known in arabidopsis, focusing on the expression patterns of homologues of four genes known to be key for the proper development of the integuments in arabidopsis: AINTEGUMENTA (ANT), BELL1, (BEL1), KANADIs (KANs) and UNICORN (UCN).
We used histology to describe the morphoanatomical development from ovules to seeds in Gnetum gnemon. We carried out spatiotemporal expression analyses in G. gnemon, a gymnosperm, which has a unique ovule morphology with an integument covering the nucellus, two additional envelopes where the outermost becomes fleshy as the seed matures, and an orthotropous orientation.
Our anatomical and developmental descriptions provide a framework for expression analyses in the ovule of G. gnemon. Our expression results show that although ANT, KAN and UCN homologues are expressed in the inner integument, their spatiotemporal patterns differ from those found in angiosperms. Furthermore, all homologues studied here are expressed in the nucellus, revealing major differences in seed plants. Finally, no expression of the studied homologues was detected in the outer envelopes.
Altogether, these analyses provide significant comparative data that allows us to better understand the functional evolution of these gene lineages, providing a compelling framework for evolutionary and developmental studies of seeds. Our findings suggest that these genes were most likely recruited from the sporangium development network and became restricted to the integuments of angiosperm ovules.
胚珠是所有种子植物（裸子植物和被子植物）的一个同源特征；然而，在主要的种子植物谱系中，胚珠在数量、珠被的取向等方面存在显著差异。拟南芥作为模式物种，其胚珠具有两层珠被和倒生胚珠的特性，对其胚珠发育相关的遗传学研究较为深入。本研究基于对拟南芥中已知基因的研究，重点研究了四个已知对拟南芥珠被正常发育起关键作用的基因（ANT、BEL1、KANs 和 UCN）的同源物的表达模式。
我们使用组织学方法描述了买麻藤胚珠到种子的形态解剖发育过程。我们对买麻藤进行了时空表达分析，这是一种具有独特胚珠形态的裸子植物，其珠被覆盖珠心，有两个额外的包被，最外层在种子成熟时肉质化，胚珠为直生胚珠。
我们的解剖学和发育描述为买麻藤胚珠的表达分析提供了一个框架。我们的表达结果表明，尽管 ANT、KAN 和 UCN 的同源物在珠被内表达，但它们的时空模式与被子植物不同。此外，我们研究的所有同源物都在珠心表达，这揭示了种子植物的主要差异。最后，在外部包被中没有检测到研究同源物的表达。
总的来说，这些分析提供了重要的比较数据，使我们能够更好地理解这些基因谱系的功能进化，并为种子的进化和发育研究提供了一个引人注目的框架。我们的发现表明，这些基因很可能是从孢子体发育网络中招募而来，并局限于被子植物胚珠的珠被中。
