Distinct developmental mechanisms reflect the independent origins of leaves in vascular plants

Vascular plants diverged more than 400 million years ago into two lineages, the lycophytes and the euphyllophytes [1-4]. Leaf-like organs evolved independently in these two groups [1-3, 5, 6]. Microphylls in lycophytes are hypothesized to have originated as lateral outgrowths of tissue that later became vascularized (the enation theory) [2, 3, 5-8] or through the sterilization of sporangia (the sterilization hypothesis) [3]. Megaphylls in euphyllophytes are thought to represent modified lateral branches [7, 9, 10]. 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 [3, 8, 11-13]. 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 [14]. We examined the expression of Class III HD-Zip genes in the lycophyte Seiaginella 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.