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Secrets in a seed: Clues into the evolution of the first flowers
September 15, 2009Researchers work on part of Darwin's 'abominable mystery'
Approximately 120-130 million years ago, one of the most significant events in the history of the Earth occurred: the first flowering plants, or angiosperms, arose. In the late 1800s, Darwin referred to their development as an "abominable mystery." To this day, scientists are still challenged by this "mystery" of how angiosperms originated, rapidly diversified, and rose to dominance. (See the January 2009 issue of the American Journal of Botany at www.amjbot.org/content/vol96/issue1.)
Studies of key features of angiosperm evolution, such as the evolution of the flower and development of the endosperm, have contributed to our current understanding of relationships among the early families of flowering plants. Examining the development of seeds and embryos among early angiosperms may help to improve our understanding of how flowering plants evolved from the nonflowering gymnosperms.
A recent study by Dr. Paula Rudall and colleagues published in the September issue of the AJB (www.amjbot.org/cgi/content/full/96/9/1581) explores a piece of this mystery: the microscopic anatomy of seed development in Trithuria, a genus in the plant family Hydatellaceae, thought to be one of the earliest families of angiosperms-the so-called "basal angiosperms."
Rudall and colleagues' observations of the development of the embryo and endosperm (tissue that surrounds the embryo and provides nutrition) in Trithuria suggest that double fertilization occurs. Double fertilization is a unique feature of flowering plants where one sperm nucleus unites with the egg, producing the embryo, while another sperm nucleus unites with a separate nucleus from the female, producing the endosperm. The endosperm is divided into two regions-the micropylar and chalazal regions.
In Trithuria, the cells of the micropylar region divide many times to form the multi-celled endosperm. However, the chalazal region forms a single-celled haustorium, a structure that absorbs nutrients and ultimately degenerates to form an empty space in the seed. This situation is broadly similar to that of some waterlilies and some monocots but differs from that of many other early-diverging angiosperms such as Amborella, in which the endosperm is formed from the chalazal region.
One of the current hypotheses is that the endosperm originated as a monstrous proembryo that fails to develop into a plant. Rudall and colleagues' observations support this theory.
"Comparative studies of early endosperm development in extant 'basal' angiosperms (including Trithuria) tend to support this theory," Rudall said, "because there are similarities in early development of embryo and endosperm. In both cases, the first cell division produces two distinct domains that differ in their subsequent development." In the embryo, divisions of the chalazal cell produce most of the embryo. The micropylar cell develops into a stalk that attaches the embryo to the seed coat. In the endosperm of Trithuria, the chalazal haustorium may regulate early endosperm development of the micropylar region, in addition to facilitating transfer of nutrients from the perisperm, maternally derived nutritive tissue, to the embryo.
Rudall and colleagues' findings shed some light on the possible role of the endosperm in early angiosperms. "The endosperm of Trithuria, though limited in size and storage capacity, is relatively persistent," Rudall stated. "Coupled with the well-developed perisperm that occurs in Trithuria, this could indicate that the ancestral role of endosperm was to transfer nutrients from the perisperm to the embryo, rather than as a storage tissue."
American Journal of Botany
A recent study by Dr. Paula Rudall and colleagues published in the September issue of the AJB (www.amjbot.org/cgi/content/full/96/9/1581) explores a piece of this mystery: the microscopic anatomy of seed development in Trithuria, a genus in the plant family Hydatellaceae, thought to be one of the earliest families of angiosperms-the so-called "basal angiosperms."
Rudall and colleagues' observations of the development of the embryo and endosperm (tissue that surrounds the embryo and provides nutrition) in Trithuria suggest that double fertilization occurs. Double fertilization is a unique feature of flowering plants where one sperm nucleus unites with the egg, producing the embryo, while another sperm nucleus unites with a separate nucleus from the female, producing the endosperm. The endosperm is divided into two regions-the micropylar and chalazal regions.
In Trithuria, the cells of the micropylar region divide many times to form the multi-celled endosperm. However, the chalazal region forms a single-celled haustorium, a structure that absorbs nutrients and ultimately degenerates to form an empty space in the seed. This situation is broadly similar to that of some waterlilies and some monocots but differs from that of many other early-diverging angiosperms such as Amborella, in which the endosperm is formed from the chalazal region.
One of the current hypotheses is that the endosperm originated as a monstrous proembryo that fails to develop into a plant. Rudall and colleagues' observations support this theory.
"Comparative studies of early endosperm development in extant 'basal' angiosperms (including Trithuria) tend to support this theory," Rudall said, "because there are similarities in early development of embryo and endosperm. In both cases, the first cell division produces two distinct domains that differ in their subsequent development." In the embryo, divisions of the chalazal cell produce most of the embryo. The micropylar cell develops into a stalk that attaches the embryo to the seed coat. In the endosperm of Trithuria, the chalazal haustorium may regulate early endosperm development of the micropylar region, in addition to facilitating transfer of nutrients from the perisperm, maternally derived nutritive tissue, to the embryo.
Rudall and colleagues' findings shed some light on the possible role of the endosperm in early angiosperms. "The endosperm of Trithuria, though limited in size and storage capacity, is relatively persistent," Rudall stated. "Coupled with the well-developed perisperm that occurs in Trithuria, this could indicate that the ancestral role of endosperm was to transfer nutrients from the perisperm to the embryo, rather than as a storage tissue."
American Journal of Botany
Angiosperms Current Events and Angiosperms News RSSUT Knoxville professor finds unexpected key to flowering plants' diversity
What began with an off-the-cuff curiosity eventually led Joe Williams to hang from the limbs of a tree 80 feet above the soil of northeastern Australia.
A common genetic mechanism discovered in nitrogen-fixing plants
Some soil microorganisms are capable of forging associations with plant roots in the form of symbioses. Certain of these relationships play a highly important ecological and agronomic role.
Do we need a paradigm change? Disputing coevolution in herbivorous insects
Coleoptera (beetles) are one of the most successful groups of organisms on Earth. Their success in evolutionary terms is recognised by their extreme adaptive diversity (occupying almost every possible ecological niche) and their longevity (fossils from the Palaeozoic, 280 million years ago).
Research discovers oldest bee, evolutionary link
Researchers at Oregon State University have discovered the oldest bee ever known, a 100 million year old specimen preserved in almost lifelike form in amber, and an important link to help explain the rapid expansion of flowering plants during that distant period.
South Pacific plant may be missing link in evolution of flowering plants
A new University of Colorado at Boulder study involving a "living fossil plant" that has survived on Earth for 130 million years suggests its novel reproductive structure may be a "missing link" between flowering plants and their ancestors.
Massive Duplication of Genes May Solve Darwin's "Abominable Mystery" about Flowering Plants
Researchers from the Floral Genome Project at Penn State University, with an international team of collaborators, have proposed an answer to Charles Darwin's "abominable mystery:" the inexplicably rapid evolution of flowering plants immediately after their first appearance some 140 million years ago.
Ants are surprisingly ancient, arising 140-168 million years ago
Ants are considerably older than previously believed, having originated 140 to 168 million years ago, according to new Harvard University research published in the journal Science.
Nature press release for 31 January issue
[415512] CLIMATE: FLOOD WARNING (pp512-514; 514-517; N&V) Very wet winters will be up to five times more likely than today for much of central and northern Europe over the next century. The probability of very wet summers in the Asian monsoon region will rise by a similar amount, increasing the risk of flooding. So say Tim Palmer of the European Centre for Medium-Range Weather Forecast in Reading, UK, and Jouni R'¤is'¤nen of the Swedish Meteorological and Hydrological Institute in Norrköping, Sweden, in this week`s Nature. The duo have analysed the output of 19 climate models. In the same issue, Christopher Milly of the US Geological Survey, Princeton, New Jersey, and co-workers present evi
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