Floyd Sandra K, Bowman John L
University of California, Davis, Section of Plant Biology, One Shields Avenue, Davis, California 95616, USA.
Curr Biol. 2006 Oct 10;16(19):1911-7. doi: 10.1016/j.cub.2006.07.067.
Vascular plants diverged more than 400 million years ago into two lineages, the lycophytes and the euphyllophytes . Leaf-like organs evolved independently in these two groups . Microphylls in lycophytes are hypothesized to have originated as lateral outgrowths of tissue that later became vascularized (the enation theory) or through the sterilization of sporangia (the sterilization hypothesis) . Megaphylls in euphyllophytes are thought to represent modified lateral branches . The fossil record also indicates that the seed plant megaphyll evolved uniquely in the ancestor of seed plants, independent of megaphylls in ferns, because seed plants evolved from leafless progymnosperm ancestors . Surprisingly, a recent study of KNOX and ARP gene expression in a lycophyte was reported to indicate recruitment of a similar mechanism for determinacy in both types of leaves . We examined the expression of Class III HD-Zip genes in the lycophyte Selaginella kraussiana and in two gymnosperms, Ginkgo and Pseudotsuga. Our data indicate that mechanisms promoting leaf initiation, vascularization, and polarity are quite different in lycophytes and seed plants, consistent with the hypotheses that megaphylls originated as lateral branches whereas microphylls originated as tissue outgrowths.
维管植物在4亿多年前分化为两个谱系，即石松类植物和真叶植物。叶状器官在这两个类群中独立进化。石松类植物的小型叶被认为起源于后来形成维管组织的组织侧生突出物（叶起源学说），或者是通过孢子囊的不育化（不育假说）。真叶植物的大型叶被认为是由侧生枝演化而来。化石记录还表明，种子植物的大型叶在种子植物的祖先中独立于蕨类植物的大型叶独特地演化而来，因为种子植物是从无叶的前裸子植物祖先演化而来。令人惊讶的是，最近一项关于石松类植物中KNOX和ARP基因表达的研究报告指出，两种类型的叶在决定叶型方面采用了类似的机制。我们研究了石松类植物小翠云以及两种裸子植物银杏和花旗松中III类HD-Zip基因的表达。我们的数据表明，促进叶起始、维管化和极性的机制在石松类植物和种子植物中差异很大，这与大型叶起源于侧生枝而小型叶起源于组织突出物的假说一致。
