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

Unparticle

“All science is either physics or stamp collecting.” So claimed Ernest Rutherford, the British physicist who discovered the atomic nucleus in 1910, touting the explanatory power of physics over the busywork of classifying elements or planets or animals. One hundred years later, the endless variety of matter postulated by physics—within the nucleus and throughout the universe—has far surpassed the inventories of the periodic table and solar system, leading particle physicists to refer to their domain as a bestiary and one textbook to be aptly titled A Tour of the Subatomic Zoo. There are electrons and protons and neutrons, as well as quarks and positrons and neutrinos. There are also gluons and muons—the unexpected discovery of which, in 1936, led the physicist Isidor Rabi to quip, “Who ordered that?”—and potentially axions and saxions and saxinos. In this menagerie it’s not easy for a new particle, especially a hypothetical one, to get attention. The unparticle, first proposed by American physicist Howard Georgi in 2007, is therefore remarkable for garnering worldwide media attention and spurring more than a hundred scholarly papers, especially considering that there’s no experimental evidence for it, nor is it called for mathematically by any prior theory. What an unparticle is, exactly, remains vague. The strange form of matter first arose on paper when Georgi asked himself what properties a “scale-invariant” particle might have and how it might interact with the observable universe. Scale invariance is a quality of fractals, such as snowflakes and fern leaves, that makes them look essentially the same at any magnification. Georgi’s analogous idea was to imagine particles that would interact with the same force regardless of the distance between them. What he found was that such particles would have no definite mass, which would, for example, exempt them from obeying special relativity. “It’s very difficult to even find the words to describe what unparticles are,” Georgi confessed to the magazine New Scientist in 2008, “because they are so unlike what we are familiar with.” For those unprepared to follow his mathematics, the name evokes their essential foreignness.