Lecture © Genaro R. Hernandez
BIOL 606 Session, University of Alberta, February 7, 2001
The first attempt to infer phylogeny of seed plants by means of cladistic analyses was proposed by Crane in 1985. Crane's phylogenetic analysis of seed plants was the first cladistic analysis that included both extinct and extant taxa. Two main groups of seed plants ("radiosperms" and "platysperms") were circumscribed, where the anthophyte clade was recognized within the "platysperms." The anthophyte clade includes Bennettitales (Cycadeoideales), Pentoxylon, Gnetales (Ephedra, Gnetum, and Welwitschia), and angiosperms. Anthophytes are plants with flower-like reproductive structures. The only two living taxa within the anthophytes are Gnetales and angiosperms. These two taxa are also considered as monophyletic. After Crane's analysis, several authors (Doyle and Donoghue 1992, Nixon et al. 1994, Rothwell and Serbet 1994, Doyle 1996) reassessed seed plant phylogeny. The new proposals generated distinct cladograms where coniferophytes (and/or conifers) and anthophytes are the only distinctive and well-supported clades. These further analyses support the existence of an anthophyte clade, and furthermore, they reinforced Gnetales as the sister group of angiosperms. However, relationships within the rest of the seed plants remain unresolved.
Initial molecular cladistic analyses were performed by several authors using plastid, mitochondrial, and nuclear gene sequences (Goremykin et al. 1996, Chaw et al. 1997). These molecular analyses suggest a relationship between Gnetales and conifers instead of angiosperms. However, most of these analyses lack strong statistical support. In more recent years, new molecular analyses based on mitochondrial and nuclear small subunit rRNA, rbcL, slowly evolving mitochondrial genes, and nuclear 18S rDNA sequences, among others (Chaw et al. 2000, Bowe et al. 2000), support initial results of molecular analyses. Therefore, these results contrast with those of morphological characters (Donoghue and Doyle 2000). This molecular evidence implies a close relationship between conifers and Gnetales, where Gnetales could be nested within or even derived from conifers (called the "Gnepine" hypothesis).
If these new molecular analyses are right, then the reproductive structures of Gnetales and conifers should be homologous. Reproductive structures of conifers are organized in cones, while gnetalean reproductive structures have been referred to as mega and microsporangiate cones, or even "flowers." Conifers have compound ovulate cones and simple pollen cones, while both gnetalean reproductive structures are compound. In this context, the pollen-bearing structures of conifers (simple) and Gnetales (compound) are not homologous. If the "Gnepine" hypothesis is reflecting real relationships among these taxa, then gnetaleans may have an ancestor within or closely related to conifers that shows compound pollen-bearing structures. However, pollen cones in all extant and almost all fossil conifers have been described as simple cones, and therefore many researchers have not accepted the "Gnepine" hypothesis. Nevertheless, a new primitive conifer with an undoubtedly compound pollen cone from the Upper Carboniferous of North America has been recently described. This new evidence supports ideas on the relationship between the coniferophytes and Gnetales.
Molecular and morphological characters, again, generate contrasting results. These contrasting results on the affinities of angiosperms, Gnetales and now conifers lead to a series of questions on how seed plant phylogeny has been assessed. Questions such as the role of certain genes in molecular analyses, the validity of both molecular and morphological analyses, and the current status of morphological characters were addressed in the discussion. In conclusion, more work has to be done to have a better understanding of seed plant phylogeny and the role of both molecular and morphological data.
Chaw, S. M., A. Zharkikh, H. M Sung, T. C. Lau, and W. H. Li. 1997. Molecular phylogeny of extant gymnosperms and seed plant evolution: analysis of nuclear 18S rRNA sequences. Mol Biol Evol 14:56-68.
Chaw, S. M., C. L. Parkinson, Y. Cheng, T. M. Vincent, and J. D. Palmer. 2000. Seed plant phylogeny inferred frrom all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers. Proc Natl Acad Sci USA 97:4086-4091.
Crane, P. R. 1985. Phylogenetic analysis of seed plants and the origin of angiosperms. Ann Missouri Bot Gard 72:716-793.
Donoghue, M. J., and J. A. Doyle. 2000. Seed plant phylogeny: demise of the anthophyte hypothesis? Current Biology 10(3):R106-R109.
Doyle J. A., and M. J. Donoghue. 1992. Fossils and seed plant phylogeny reanalyzed. Brittonia 44:89-106.
Doyle J. A. 1996. Seed plant phylogeny and the relationships of Gnetales. Int J Plant Sci 157:S3-S39.
Goremykin, V., V. Bobrova, J. Pahnke, J. Troitsky, A. Antonov, and W. Martin. 1996. Noncoding sequences from the slowly evolving chloroplast inverted repeat in addition to rbcL data no not support gnetalean affinities of angiosperms. Mol Biol Evol 13:383-396.
Nixon K. C., W. L. Crepet, D. Stevenson, E. M. Friis. 1994. A reevaluation of seed plant phylogeny. Ann Missouri Bot Gard 81:484-533.
Rothwell, G. W., and R. Serbet. 1994. Lignophyte phylogeny and the evolution of spermatophytes: A numerical cladistic analysis. Syst Bot 19:443-482.