Phylogenetic studies of other gymnosperms
Besides the great effort in resolving the phylogenetic position of Gnetales, many phylogenetic studies have been conducted on other gymnosperms. For instance, the monotypic genus Ginkgo is the sole survivor of ginkgos that originated at least 270 Mya, and its systematic position has been controversial for a long time . It was placed in the coniferophyte clade sense, comprising conifers, cordaites, ginkgophytes, and gnetophytes. Some studies suggested that Ginkgo is closer to conifers than cycads based on comparative development of the spermatozoids or intermediate between these two lineages based on embryogenesis. However, most molecular phylogenetic studies based on single or a few genes support Ginkgo as sister to a clade comprising conifers and gnetophytes. It is particularly interesting that most recent phylogenemic analyses support a sister relationships between Ginkgo and cycads which is consistent with the morphological characters shared between the two groups, such as an haustorial pollen tube and multiflagellated sperms