Unearthing The Origins Of Flowering Plants

AsianScientist (May 27, 2021) – Just as a seed marks the start of many a plants’ lives, the seeds of an extinct plant group may be the key to mapping the family tree of flowering plants or angiosperms. This discovery fills a long-standing void in plant evolutionary history, as reported by a joint China-US team in Nature.

From dandelions to grape vines, many of the world’s most beautiful plants belong to the angiosperm group, their abundance born out of certain beneficial traits. For example, their seeds can survive for extended periods of time thanks to a special tissue called the endosperm, which nourishes the plant embryo, and a protective covering known as the integument.

This integument is also found in the gymnosperms, flowerless but seed-producing pioneers that include pines and fir trees. As gymnosperms are the older of the two groups, their integuments consist of only one layer. Meanwhile, angiosperm seeds have evolved to have an inner and outer integument, with their top portion uniquely bending backward.

But the origins of this second layer have for a long time remained a mystery, representing a gap in the plant evolutionary timeline. To find the missing members in this family tree, Professor Shi Gongle from the Chinese Academy of Sciences and colleagues from US institutions examined several hundred angiosperm-like plant fossils in peat excavated from Inner Mongolia, China.

Upon close examination, they found that the seeds bore a single integument, just like the gymnosperms, but were additionally covered by a cup-like structure called the cupule. Moreover, the cupules had a curled flap that resembled the angiosperm seeds, hinting that they may be the precursor to the outer integument of flowering plants.

The specimens came from the Early Cretaceous period, about 125 million years ago, right as angiosperms began to bloom. Given the relative recency of this timescale, the researchers sought out older fossils that pre-dated angiosperms. As it turned out, some seed plants dating to 250 million years ago also had cupules, tying together the gymnosperm and angiosperm relatives.

Taken together, these extinct angiosperm relatives—termed angiophytes—form part of the missing link between gymnosperm and angiosperm lineages, showing how the second integument arose from the cupules of extinct seed plants. While these fossil records had been stored in museum collections for several years, it was only recently that their significance was brought to light by Shi’s team.

Still, several branches of the plant family tree remain missing. More angiophyte subgroups are waiting to be discovered, as scientists look to identify the most immediate ancestors of the flowering plants. Completing the angiosperm evolutionary story also requires digging deep into the origins of other distinguishing features, including the endosperm and flower organs like the pollen-producing stamen.

“The concept of angiophytes presented here partially resolves the long-standing question of angiosperm origins and suggests that the close fossil relatives of angiosperms exhibit considerable reproductive diversity and have been hiding in plain sight for almost a century,” the authors wrote.

The article can be found at: Shi et al. (2021) Mesozoic cupules and the origin of the angiosperm second integument.

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Source: Chinese Academy of Sciences; Photo: Shutterstock.
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