Does investigating the seed coat in Gnetum gnemon reveal how fruiting plants evolved?

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The evolution of angiosperms, flowering and fruiting plants is a problem in botany. Once they evolved, angiosperms diversified and spread across the world. But how did they develop from gymnosperms? One suggestion is that the Gnetales are the closest gymnosperms to the angiosperms, which may have developed from these. The reason for this argument is found around their ovules, the female organs that develop into seeds.

The seeds of gymnosperms have one integument, a layer that forms the outer layer of the seed. Angiosperms, in contrast, have two integuments, an inner and outer layer. The Gnetales are something in between. They have one integument but also have an envelope that wraps around the ovule. Could this envelope be something that developed into another integument in angiosperms? Cecilia Zumajo-Cardona and Barbara Ambrose decided to see if the genes known to work in building the integument in angiosperms were also found at work in the seed development of Gnetum gnemon. 

“Determining if their role in the development of the integument is conserved outside angiosperms allows us to better understand their impact on the morphological evolution of the ovules and to elucidate whether the ovule genetic network is conserved in seed plants,” write Zumajo-Cardona and Ambrose. “Moreover, an insight into the genes acting in integument development makes it possible to clarify the nature of the envelopes in G. gnemon from a molecular perspective.”

The botanists examined how genes from G. gnemon were expressed in developing seeds. Tracking how the gene expression varied over time let the scientists know what the genes were doing.

“These expression analyses made it possible to establish that the genes initially described as being involved in the development of the integument in angiosperms are not conserved in all seed plants. In turn, these genes appear to be involved in the development of the megasporangium in G. gnemon, suggesting that the ancestral function of the genes may be the development of sporangia. Furthermore, according to the expression patterns reported here in early-divergent seed plants, our results provide evidence that supports the interpretation of integuments as sterilized sporangia,” write Zumajo-Cardona and Ambrose.

The authors found that the envelopes in G. gnemon didn’t share genetic similarities with angiosperms. They conclude that G. gnemon quite definitely has just the one integument, like other gymnosperms. While answering some questions, Zumajo-Cardona and Ambrose point out that they have also found plenty of new puzzles.

“Differential expression analyses in dissected tissues from ovules are still required, which could reveal new candidate genes involved in the development of the different structures of seeds in Gnetum. Crucially, given the unique morphology of Gnetum ovules and the complex evolutionary history of these gene lineages, added to the uncertainty over monophyletic or polyphyletic origin of the ovule, it is difficult to extrapolate the results obtained here to other gymnosperms.”

If you’re interested in more detail, you can read the paper or watch a presentation they gave at Botany 2021 on YouTube.