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

Alejandra Vasco,Barbara A Ambrose

doi:10.3390/ijms21155180

Abstract

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 KNOX expression 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 the Class I KNOX and Histone H4 genes throughout the development of leaf primordia in two simple-leaved and one divided-leaved fern taxa. We found Class I KNOX are 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. H4 expression is similar in young leaf primordia of simple and divided leaves. Persistent Class I KNOX expression at the margins of divided leaf primordia compared with simple leaf primordia indicates that temporal and spatial patterns of Class I KNOX expression correlate with different fern leaf morphologies. However, our results also indicate that Class I KNOX expression alone is not sufficient to promote divided leaf development in ferns. Class I KNOX patterns 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.

Highlights

Leaves are the dominant organ in most extant vascular plants and their evolutionary origin, likely in the early Devonian, fundamentally changed life on earth, and the basic Bauplan of vascular plants [1,2]
To gain a more detailed evolutionary history of Class I KNOX in ferns and to discover the putative Elaphoglossum Class I KNOX gene copies for our expression studies, we isolated putative homologs from selected species spanning the phylogeny of ferns, and all three orders of lycophytes by PCR and database mining (Appendix A)
Within vascular plants, Class I KNOX lycophyte sequences are recovered in four different clades, successively sister to euphyllophytes, and not reciprocally monophyletic (Figure 2)

Summary

Introduction

Leaves are the dominant organ in most extant vascular plants and their evolutionary origin, likely in the early Devonian, fundamentally changed life on earth, and the basic Bauplan of vascular plants [1,2]. Despite this profound importance, the number of times leaves have evolved in vascular plants is still debated, and it is mainly within Euphyllophytes (ferns and seed plants) that the number of times leaves have evolved is still not settled. Comparative approaches to understand the genetic pathway affecting megaphyll shape outside of angiosperms have mainly focused on the Class I KNOTTED1-like HOMEOBOX (Class I KNOX) genes [9,10,11]

Methods

Results

Conclusion