Simple and Divided Leaves in Ferns: Exploring the Genetic Basis for Leaf Morphology Differences in the GenusElaphoglossum(Dryopteridaceae)

Despite the implications leaves have for life, their origin and development remain debated. Analyses across ferns and seed plants are fundamental to address the conservation or independent origins of megaphyllous leaf developmental mechanisms.Class I KNOXexpression studies have been used to understand leaf development and, in ferns, have only been conducted in species with divided leaves. We performed expression analyses of theClass I KNOXandHistone H4genes throughout the development of leaf primordia in two simple-leaved and one divided-leaved fern taxa. We foundClass I KNOXare expressed (1) throughout young and early developing leaves of simple and divided-leaved ferns, (2) later into leaf development of divided-leaved species compared to simple-leaved species, and (3) at the leaf primordium apex and margins.H4expression is similar in young leaf primordia of simple and divided leaves. PersistentClass I KNOXexpression at the margins of divided leaf primordia compared with simple leaf primordia indicates that temporal and spatial patterns ofClass I KNOXexpression correlate with different fern leaf morphologies. However, our results also indicate thatClass I KNOXexpression alone is not sufficient to promote divided leaf development in ferns.Class I KNOXpatterns of expression in fern leaves support the conservation of an independently recruited developmental mechanism for leaf dissection in megaphylls, the shoot-like nature of fern leaves compared with seed plant leaves, and the critical role marginal meristems play in fern leaf development.