PHYLOGENETIC STUDIES OF THE CHARALES: THE CLOSEST LIVING RELATIVES OF LAND PLANTS

Abstract

The embryophytes (land plants) consist of organisms such as mosses, ferns, conifers and flowering plants. Although land plants have long been thought to be related to the green algal group Charophyceae, the nature of this relationship has been unresolved for over a century. A four-gene phylogenetic analysis supports the hypothesis that land plants arose from within the Charophyta and unambiguously identifies the Charales as the closest living relatives of plants. With a robust phylogeny, it is now possibly to explore phylogeny-dependant questions that were previously difficult to assess. Estimating the divergence time of the land plant lineage is one such question. A recent time estimate for the colonization of land by plants is 1,061 ± 109 mya and 703 ± 45 mya for the divergence of vascular plants and bryophytes, a result much older than the fossil record suggests (roughly 470 mya). Unlike most algae, a rich fossil record exists for the Charales in the form of calcified oospores. Representative fossils that can be attributed to five extant lineages in the Charales have been identified with reasonable accuracy. These multiple calibration points were used in conjunction with the four-gene DNA data set to estimate the divergence time of the land plant and Charales lineages. The Bayesian relaxed-clock approach estimated divergence of the Charales/land plant common ancestor in the Late Proterozoic (674.10 ± 99.96 MYA), modern land plants in the Cambrian (497.78 ± 75.66 MYA), and modern Characeae at the Paleozoic/Mesozoic boundary (247.75 ± 25.98 MYA). The genus Nitella is one of the most diverse genera in the Charales. Wood and Imahori's worldwide monograph divides Nitella into three subgenera with seventeen sections and radically modified the taxonomy of this group my submerging over 200 Nitella species into 53 loosely defined species. Phylogenetic analyses of rbcL sequence data from 79 Nitella species (plus outgroups) support the monophyly of Nitella and two subgenera (Hyella and Tieffallenia). Subgenus Nitella formed two paraphyletic lineages at the base of the genus. Few sections were monophyletic and species diversity is interpreted as being much higher than proposed by Wood.