On the evolutionary history of Ephedra: Cretaceous fossils and extant molecules

Abstract

Gnetales comprise three unusual genera of seed plants, Ephedra, Gnetum, and Welwitschia. Their extraordinary morphological diversity suggests that they are survivors of an ancient, more diverse group. Gnetalean antiquity is also supported by fossil data. Dispersed "ephedroid" (polyplicate) pollen first appeared in the Permian >250 million years ago (Myr), and a few megafossils document the presence of gnetalean features in the early Cretaceous. The Cretaceous welwitschioid seedling Cratonia cotyledon dates the split between Gnetum and Welwitschia to before 110 Myr. Ages and character evolution of modern diversity are, however, controversial, and, based on molecular data, it has recently been suggested that Ephedra is very young, only 8-32 Myr. Here, we present data on the evolutionary history of Ephedra. Fossil seeds from Buarcos, Portugal, unequivocally link one type of Cretaceous polyplicate pollen to Ephedra and document that plants with unique characters, including the peculiar naked male gametophyte, were established already in the Early Cretaceous. Clades in our molecular phylogeny of extant species correspond to geographical regions, with African species in a basal grade/clade. The study demonstrates extremely low divergence in both molecular and morphological characters in Ephedra. Features observed in the fossils are present in all major extant clades, showing that modern species have retained unique reproductive characters for >110 million years. A recent origin of modern species of Ephedra would imply that the Cretaceous Ephedra fossils discussed here were members of widespread, now extinct sister lineage(s), and that no morphological innovations characterized the second diversification.

Fig. 1.

Early Cretaceous Ephedra seeds from Buarcos, Portugal. (A) Overview of one of the fossil seeds (S-107680). (B) Details of papillae on the inner surface of the outer envelope (S-107685). (C) Pollen grains in the micropylar region. Note the inner circular integument and the outer squared envelope (S-107680). (D) Rugulate surface of the pollen grains (S-107680). (E) Macerated seed, exposing two shed pollen exines inside the micropyle (S-136808). [Scale bars: 100 μm(A); 50 μm(B); 25 μm(C); 1 μm(D); and 10 μm(E).] (A–D) Scanning electron micrographs. (E) Transmitted light micrograph.

Fig. 2.

Cladogram of recent Ephedra based on Bayesian analysis of five regions from the nuclear and chloroplast genomes (18S, 26S, ITS, rbcL, and rps4). Bayesian posterior probabilities are given above branches, parsimony bootstrap values below branches. Species marked * and ** were examined for papillae on the inner surface of apical part of the outer envelope. For species marked *, the DNA voucher material was investigated. For species marked **, other material has been used.