Divergence time estimates and the evolution of major lineages in the florideophyte red algae.

Eun Chan Yang,Gary W Saunders,Louis Graf,Andrew H Knoll,Debashish Bhattacharya,Sung Min Boo,Suzanne Fredericq,Hwan Su Yoon

doi:10.1038/srep21361

Abstract

The Florideophyceae is the most abundant and taxonomically diverse class of red algae (Rhodophyta). However, many aspects of the systematics and divergence times of the group remain unresolved. Using a seven-gene concatenated dataset (nuclear EF2, LSU and SSU rRNAs, mitochondrial cox1, and plastid rbcL, psaA and psbA genes), we generated a robust phylogeny of red algae to provide an evolutionary timeline for florideophyte diversification. Our relaxed molecular clock analysis suggests that the Florideophyceae diverged approximately 943 (817–1,049) million years ago (Ma). The major divergences in this class involved the emergence of Hildenbrandiophycidae [ca. 781 (681–879) Ma], Nemaliophycidae [ca. 661 (597–736) Ma], Corallinophycidae [ca. 579 (543–617) Ma], and the split of Ahnfeltiophycidae and Rhodymeniophycidae [ca. 508 (442–580) Ma]. Within these clades, extant diversity reflects largely Phanerozoic diversification. Divergences within Florideophyceae were accompanied by evolutionary changes in the carposporophyte stage, leading to a successful strategy for maximizing spore production from each fertilization event. Our research provides robust estimates for the divergence times of major lineages within the Florideophyceae. This timeline was used to interpret the emergence of key morphological innovations that characterize these multicellular red algae.

Highlights

Phylogenetic studies based on molecular data have resulted in a revised classification system that recognizes 29 orders in five subclasses: Ahnfeltiophycidae, Corallinophycidae, Hildenbrandiophycidae, Nemaliophycidae, and Rhodymeniophycidae[1,6,7,8]
The maximum likelihood (ML) phylogeny inferred from the seven-gene concatenated dataset is shown in Fig. 1
The analyses with red algal fossils only (C2ab nor) and the Bangiomorpha fossil (C1a nor) showed little difference for the age of node ‘r’, i.e., Δnode ‘r’ mean to C7 nor were 4 million years old (Ma) (0.2%) for C2ab nor and 20 Ma (1.2%) for C1a nor. These results indicate that constraint ‘a’ is more critical for divergence time estimates regarding red algae than the coralline fossils deposited in the Doushantuo formation

Summary

Introduction

Phylogenetic studies based on molecular data have resulted in a revised classification system that recognizes 29 orders in five subclasses: Ahnfeltiophycidae, Corallinophycidae, Hildenbrandiophycidae, Nemaliophycidae, and Rhodymeniophycidae[1,6,7,8]. Aguirre et al.[16] studied coralline red algal phylogeny and divergence times based on fossil records; well-preserved coralline skeletons in Mesozoic and Cenozoic sedimentary rocks include species placed within the Sporolithaceae (136–130 Ma), Hapalidiaceae (115–112 Ma), and Lithophylloideae (65.5–61.7 Ma), providing additional calibration points for molecular clock analysis[16,17]. Estimating divergence times using molecular data and fossil constraints can considerably advance the evolutionary study of florideophytes and algae in general[10]. The results are discussed in light of key morphological transitions, such as the origin of the triphasic life cycle and of a diversity of fertilization and diploidization modes in the Florideophyceae

Methods

Results

Conclusion