title: Independent recruitment of a conserved developmental mechanism during leaf evolution.
abstract: Vascular plants evolved in the Middle to Late Silurian period, about 420 million years ago. The fossil record indicates that these primitive plants had branched stems with sporangia but no leaves. Leaf-like lateral outgrowths subsequently evolved on at least two independent occasions. In extant plants, these events are represented by microphyllous leaves in lycophytes (clubmosses, spikemosses and quillworts) and megaphyllous leaves in euphyllophytes (ferns, gymnosperms and angiosperms). Our current understanding of how leaves develop is restricted to processes that operate during megaphyll formation. Because microphylls and megaphylls evolved independently, different mechanisms might be required for leaf formation. Here we show that this is not so. Gene expression data from a microphyllous lycophyte, phylogenetic analyses, and a cross-species complementation experiment all show that a common developmental mechanism can underpin both microphyll and megaphyll formation. We propose that this mechanism might have operated originally in the context of primitive plant apices to facilitate bifurcation. Recruitment of this pathway to form leaves occurred independently and in parallel in different plant lineages.
authors.affiliation: Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.
authors.forename: C Jill
authors.initials: CJ
authors.lastname: Harrison
authors.forename: Susie B
authors.initials: SB
authors.lastname: Corley
authors.forename: Elizabeth C
authors.initials: EC
authors.lastname: Moylan
authors.forename: Debbie L
authors.initials: DL
authors.lastname: Alexander
authors.forename: Robert W
authors.initials: RW
authors.lastname: Scotland
authors.forename: Jane A
authors.initials: JA
authors.lastname: Langdale
mesh_terms: D032161:Antirrhinum / Q000235:genetics; D017360:Arabidopsis / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics; D005075:Biological Evolution; D005580:Fossils; D018628:Gene Dosage; D018506:Gene Expression Regulation, Plant / Q000235:genetics; D017343:Genes, Plant / Q000235:genetics; D005816:Genetic Complementation Test; D018519:Meristem / Q000254:growth & development; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010802:Phylogeny; D018515:Plant Leaves / Q000235:genetics / Q000254:growth & development* / Q000502:physiology; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000254:growth & development; D011485:Protein Binding; D018749:RNA, Plant / Q000032:analysis / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics; D003313:Zea mays / Q000235:genetics
chemical_list: C456239:ASYMMETRIC LEAVES1 protein, Arabidopsis; D029681:Arabidopsis Proteins; D010940:Plant Proteins; D018749:RNA, Plant; D014157:Transcription Factors