Cone and ovule development in Cunninghamia and Taiwania (Cupressaceae sensu lato) and its significance for conifer evolution

We examined the early developmental stages of the seed cones and seeds of two conifer genera, Cunninghamiaand Taiwania, using scanning electron microscope (SEM) images of freshly collected material. In recent similar studies, these two taxa were not described. The present paper aims to fill that gap. Both genera appear to have features crucial to the understanding of the evolution of the cupressaceous cone, characteristic of the families Cupressaceae and Taxodiaceae, and provide further evidence for the need to merge these families. These features are: the ovuliferous scale in Cunninghamia develops as a small lobe with each of three ovules; in Taiwania these lobes are absent, but a small ridge could be a vestige of them. In neither of these two genera does an ovuliferous scale develop to maturity and only limited intercalary growth transforms the bracts, of which only their width and final shape distinguishes them from sterile leaves. Thus, the bracts, not the ovuliferous scales, form the mature cone in these two genera. This trend is continued in more derived genera of Cupressaceae. Another key extant taxon that has helped to elucidate the evolution of this type of conifer cone is Sciadopitys; similar studies have already been done on this genus, and we compared our findings to them. We also considered certain fossil Mesozoic conifer cones, which shed further light on the evolution of the cupressaceous cone. The evidence from these various genera strongly indicates that recently reconstructed phylogenies of gymnosperms based on molecular evidence from extant taxa do not reflect the evolution that actually happened. Such studies need to take into account nonmolecular evidence, as detailed here. Takaso and Tomlinson’s (1989, 1990, 1992) studies of early stages in the development of the ovuliferous cone of several taxa in Taxodiaceae have given us a much better understanding of their comparative structures. They have also raised questions about the interpretation of the cones’ parts in relation to theories about the evolution of the female conifer cone. Surveying the morphology of the early stages of the ovuliferous cones of all genera in Cupressaceae sensu lato (s.l.) (including Taxodiaceae), we found that Cunninghamiashares many characters with Taiwania and to a lesser extent with Athrotaxis (A. Farjon and S. Ortiz Garcia, unpublished data). The early development of the ovuliferous cone in the genus Sciadopitys, formerly in Taxodiaceae but now classified in its own family Sciadopityaceae (Hayata, 1931), has been described in the same manner (Takaso and Tomlinson, 1991). Although the genera Cunninghamiaand Taiwania were not included in Takaso and Tomlinson’s studies, they are interesting taxa because they appear to have characters in the ovuliferous cone that are intermediate between those of Sciadopitys and other members of Cupressaceae s.l. The cone and ovule development in the genus Athrotaxis was recently described by Jagel (2002). In all these studies detailed serial observations of the earliest stages using scanning electron microscopy (SEM) have been vital to understanding homology, and we adopted this method