Comparative development of simple and compound leaves in the genus Cecropia

Abstract

Plants develop leaves that range from simple to compound in shape. The evolutionary divergence of simple and compound leaves has spurred research into identifying the cellular and molecular processes involved in determining leaf shape. The roles of various genes and signalling pathways have been characterized in specifying leaf shape; however, few studies have investigated leaf primordium structure and shoot apex organization throughout the development of both simple and compound leaves. Using Cecropia obtusa Trécul and Cecropia sciadophylla Martius, two putatively closely related species bearing simple palmate and palmately-compound leaves, respectively, we compared the morphogenesis of leaves of both species at the shoot apex. Analysis of shoot apices using scanning electron microscopy yielded a nonsignificant difference in leaf primordium divergence angles and plastochron ratios, suggesting that divergence of the two leaf types occurred independently of primordium organization and growth rate at the shoot apex. Qualitative analysis of primordium initiation and morphogenesis revealed that both species share highly homologous development, as primordium structure and lobe/leaflet initiation sites are complementary in both leaf types. Our observations suggest a high degree of conserved ontogeny in the developmental pathways underlying the morphogenesis of simple palmate and palmately-compound leaves in these two species.