Amborella trichopoda, a shrub from New Caledonia in the Pacific Ocean, is believed to be the last remnant of one of the most ancient lineages of angiosperms, or flowering plants. As William E. Friedman reports in this issue (Nature 441, 337–340; 2006), its red fruits, shown here, contain a relic of what seems to have been a period of intense evolutionary experimentation in early angiosperm history.
In nearly all angiosperms, the female gametophyte — egg-producing structure — is a sac containing seven cells and eight nuclei (see Fig. 1 of the paper on page 337). Apart from the egg cell itself, which is fertilized to become the embryo, a so-called central cell is fertilized to become the supporting tissue or endosperm. These are the elements required for the phenomenon of double fertilization unique to angiosperms. Making up the numbers are three antipodal cells and two synergids — sterile cells that accompany the egg.
Amborella dares to be different, and has one extra synergid. This may seem a trifling matter, but it is akin to finding a fossil amphibian with an extra leg: the developmental ramifications are equally important, for Friedman's detailed examination of Amborella development shows that the egg cell is — uniquely — a lineal sister of a synergid, rather than a mitotic sister of one of the nuclei of the central cell.
These findings add to the oddities of other relics of early angiosperm evolution, such as water lilies (Nymphaeales) whose mature female gametophytes contain only four cells. The major lineages of angiosperms are believed to have become established in an interval of around 15 million years, 130 million years ago. Amborella and other living fossils offer an immensely valuable window into how the reproductive structures of flowering plants evolved.
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Astrophysics and Space Science (2012)