Flowering plants have a unique characteristic that has contributed to their evolutionary success: their seeds are surrounded by two sheaths, so-called integuments, and can therefore mature well protected before they spread. More naked, like conifers, on the other hand, have only one integument. But how did the second shell develop and what were its predecessors? This question has puzzled evolutionary biologists and botanists for a long time. Researchers have now analyzed newly discovered and long-known fossils of putative precursors of flowering plants and established that even 250 million years ago there were plants with a structure from which the second integument probably emerged.
Around 90 percent of all land plants are now flowering plants, also called angiosperms or flowering plants. In contrast to naked samers, which include, for example, conifers, their ovules are enclosed in an ovary. This has two shells that protect the growing seed: the inner and the outer integument. Nudists, on the other hand, lack the external integument. How this crucial characteristic of flowering plants developed has long been a mystery.
Old and new fossils analyzed
A team led by Gongle Shi from the Chinese Academy of Sciences in Nanjing has now uncovered this mystery. The researchers were helped by new fossil finds from petrified peat in Inner Mongolia in China. The plants discovered in it are exceptionally well preserved and grew approximately 126 million years ago. At that time there were already the first flowering plants, but Shi and his colleagues concentrated on specimens that had an evolutionarily older characteristic instead of a second integument: a so-called cupula. This is a curved, cup-like structure that was discussed earlier as a precursor to the second integument. “These interpretations were made more difficult by insufficient information about the relevant fossils,” write Shi and colleagues.
Using the newly discovered fossil plants, they have now re-described the structure of the cupula in detail and drawn parallels to older fossil finds. To do this, they re-analyzed several museum specimens, including some 250 million year old representatives of the extinct Caytoniales order. The cupula was first described in these plants, which belong to the group of seed ferns. “A crucial question is what is the relationship between the cupula of Caytonia and the cupula of other plants from the Mesozoic Era,” the authors say. “The new material, combined with a re-examination of possibly related fossils, shows that the curved cups of several Mesozoic plant groups are all fundamentally comparable and that their structure corresponds to the curved shape and the development of the second integument in the ovules of the angiosperms.”
Original relatives of flowering plants
From this observation, Shi and his colleagues conclude that the cupula was indeed the forerunner of the external integument. Their analyzes reveal that there was considerable variation in the shape of the cupula among the different groups of plants, which likely indicate different reproductive strategies. What they all have in common, however, is the curved shape that can still be found today in the ovules of flowering plants. Due to the fundamental similarity, the authors combine the plants described in a group that they call angiophytes. The specimens from the Mesozoic era characterize them as stem angiophytes: “These are extinct seed plants that are more closely related to angiosperms than to any other existing group and which have one, but not all, of the key features of angiosperms,” the researchers say . Through further evolutionary developments, the angiosperms emerged from a representative of the angiophytes.
“The fossils described by Shi and colleagues do not correspond to the direct ancestor of the flowering plants,” emphasizes the botanist and evolutionary biologist Douglas Soltis from the University of Florida in an accompanying comment that was also published in the journal Nature. “Nonetheless, they provide missing, decisive information about the origin of the covering species. But more fossils are urgently needed in order to further clarify the origin of the flowering plants. ”Further open questions are, for example, how, when and from what other unique characteristics of the flowering plants that are still missing in the stem angiophytes, including the carpel and the stamens.
Source: Gongle Shi (Chinese Academy of Sciences, Nanjing, China) et al., Nature, doi: 10.1038 / s41586-021-03598-w