Characterization of MADS genes in the gymnosperm Gnetum parvifolium and its implication on the evolution of reproductive organs in seed plants

Gnetales, one of the extant gymnosperm orders, has traditionally been recog nized to be most closely related to flowering plants, because the reproduct ive organ of Gnetales has some morphological characteristics similar to flo wering plants. Most recent molecular phylogenetic studies do not support th e sister relationship of the Gnetales and flowering plants, but instead sup port a close relationship between Gnetales and other extant gymnosperms. Th e MADS genes are transcription factors, some of which are involved in repro ductive organ development in flowering plants. To resolve the discrepancy i n phylogenetic inferences, and to provide insights into the evolution of re productive organs in seed plants, four MADS genes (GpMADS1-4) were cloned f rom Gnetum parvifolium. GpMADS2 is likely to be a pseudogene and the other three genes were characterized. A MADS gene tree based on partial amino aci d sequences showed that GpMADS3 is included in the AGL6 group, but the othe r two genes do not cluster with any previously reported MADS gene. The thre e GpMADS genes were expressed during the early stage of ovule development i n the differentiating nucellus and three envelopes. A comparison of MADS ge ne expression among conifers, Gnetum, and flowering plants suggests that th e comparable reproductive organs in Gnetum and flowering plants evolved in parallel, and is likely to support the homology between the ovule-ovulifero us scale complex of conifers and the Gnetum ovules, including the three env elopes.