The origin and early evolution of tracheids in vascular plants: integration of palaeobotanical and neobotanical data

Abstract

A life history involving alternation of two developmentally associated, multicellular generations (sporophyte and gametophyte) is an autapomorphy of embryophytes (bryophytes + vascular plants). Microfossil data indicate that Mid–Late Ordovician land plants possessed such a life cycle, and that the origin of alternation of generations preceded this date. Molecular phylogenetic data unambiguously relate charophycean green algae to the ancestry of monophyletic embryophytes, and identify bryophytes as early–divergent land plants. Comparison of reproduction in charophyceans and bryophytes suggests that the following stages occurred during evolutionary origin of embryophytic alternation of generations: (i) origin of oogamy; (ii) retention of eggs and zygotes on the parental thallus; (iii) origin of matrotrophy (regulated transfer of nutritional and morphogenetic solutes from parental cells to the next generation); (iv) origin of a multicellular sporophyte generation; and (v) origin of non–flagellate, walled spores. Oogamy, egg/zygote retention and matrotrophy characterize at least some modern charophyceans, and are postulated to represent pre–adaptative features inherited by embryophytes from ancestral charophyceans. Matrotrophy is hypothesized to have preceded origin of the multicellular sporophytes of plants, and to represent a critical innovation. Molecular approaches to the study of the origins of matrotrophy include assessment of hexose transporter genes and protein family members and their expression patterns. The occurrence in modern charophyceans and bryophytes of chemically resistant tissues that exhibit distinctive morphology correlated with matrotrophy suggests that Early–Mid Ordovician or older microfossils relevant to the origin of land plant alternation of generations may be found.