Ferns Reveal Important Information About Plant Evolution

Ferns show how early in plant history genes for producing seeds appeared. The genes responsible for spore production in ferns also function in plants that produce seeds and pollen.

Evolutionary Turning Point

An evolutionary turning point occurred when seed-producing plants appeared more than 300 million years ago. This event allowed plants to adapt to new settings and eventually gave rise to the blooming plants that now adorn our planet and provide most of our sustenance. But recently released DNA sequences imply that it wasn't as big a jump as it first appeared.

According to Shu-Nong Bai, a plant developmental biologist emeritus at Peking University who assisted in the sequencing of a species of the maidenhair fern genus, the fern and cycad genomes that have been published in a series of papers over the past few months "fill the gap of the gene flow during plant evolution." The genomes also reveal a second startling lesson: plants did not acquire part of their genes through mutation and selection but rather directly from fungus or other microorganisms through a contentious mechanism known as horizontal gene transfer.

Comparing the Observations

Most of the over 800 plant genomes sequenced so far have come from seed plants because of the intimidating size of most fern genomes and the concentration on crops like rice, wheat, and maize. Only two ferns-one with an abnormally tiny genome-were represented up to this point. As a result, according to Stanford University's Blaine Marchant, a plant evolutionary geneticist, "We have only gotten a brief picture of plant history."

His team and three other organizations have now taken on ferns with more conventional, huge genomes and a species of cycad, a non-flowering plant with naked seeds similar to those of pines and other conifers, thanks to improvements in sequencing lengthy lengths of DNA and decreases in costs. The sequencing of increasingly varied plant genomes is a good development, says evolutionary scientist Jennifer Wisecaver of Purdue University.

With almost 30,000 genes apiece, the fern genomes show a variety of genes that were once linked to flowering plants, which arose more than 200 million years later. For instance, a water fern called Ceratopteris richardii has 10 members of a gene family known to regulate flowering time, seed germination, and flower shape in the small flowering plant Arabidopsis, according to a report by Marchant and his colleagues published on September 1 in Nature Plants. Seven of these genes are active in the leaves where spores are formed, indicating they may play a part in fern reproduction in addition to that of seed plants; however, their functions in the fern remain unknown.

Adiantum capillus-veneris, a maidenhair fern, was studied by Jianbin Yan and colleagues at the Agricultural Genomics Institute of the Chinese Academy of Agricultural Sciences. Yan's team revealed in the same issue of Nature Plants that its DNA contained genes for transcription factors termed EMS1 and TPD1, proteins in maize and other seed plants control genes involved in pollen generation. These pollen gene controllers are active in the sporangia of maidenhair, the tissue where spores grow.

More Studies Needed

Botanist Verónica Di Stilio of the University of Washington in Seattle anticipates that the recently published genomes will reveal many surprises. She claims that the availability of reference genomes that cover the main plant lineages "opens up so many opportunities." "Genomes are resources; they are the tip of the iceberg."

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