Where did duckweeds come from?
related to aroids.
|Duckweeds' Closest Living Relative||A Fossil Duckweed|
in the Duckweeds
Duckweeds are flowering plants, closely related to the aroid family.
Duckweeds are flowering plants (angiosperms). Duckweeds may appear to be very simple plants, but looks can be deceptive. The characteristics of their flowers, including the organization of their ovaries, the presence of double-fertilization, the structure of their seeds and other characteristics show that they are flowering plants. To see much of the evidence, you will need a magnifying glass, or, better, a microscope.
Seeds, contain both an embryo and an endosperm. The embryo, the portion from which a new plant will develop, has a single embryo leaf. The single embryo leaf makes duckweeds members of the monocots, along with the lilies, the grasses, and the aroids. Considering the anatomical organization of their flowers and seeds allows botanists to classify duckweeds as close relatives of the arum or aroid family (Araceae). This family also includes many decorative house plants and flowers, such as the Philodendron, the calla lily, and jack-in-the-pulpit. The aroids and the duckweeds (Lemnaceae) together form the Order Arales.
Pistia, closest living relative of the duckweeds?
Pistia, water lettuce, is considered by many
researchers to be the closest relative of the
duckweeds. For comparison, this illustration
Limnobiophyllum scutatum, a Fossil Duckweed
Above: Drawing of Limnobiophyllum scutatum, a fossil duckweed discovered in Canada, courtesy of R.A. Stockey. The scale bar is 2 cm.
Krassilov -- a fossil link between the Araceae and
Kvacek, Z. (1995) Aquatic Botany 50(1):49-62.
"Limnobiophyllum Krassilov includes fossil free-floating stoloniferous plants each with one or two sessile suborbicular to reniform leaves of different size as well as numerous simple and one or two longer branched roots on a reduced main stem. The venation consists of campylodromous curved primaries, up to 14 in number, among which irregular reticulate veins of higher orders can be preserved. There are no signs of lateral pouches characteristic of the Lemnaceae. However, aerenchyma and pigment cells are well developed in Limnobiophyllum. Two species are recognized - Limnobiophyllum scutatum (Dawson) Krassilov (latest Cretaceous to Oligocene of western North America and Palaeocene of East Asia) and Limnobiophyllum expansum (Heer) Kvacek, comb. n. (Miocene of Europe). Turion-like bodies are associated with L. scutatum. No fruits have been found in connection, but numerous isolated ribbed seeds are associated with L. expansum. They resemble some Araceae and also Lemna L. and Spirodela Schleid. Such seeds are known as Lemnospernum Nikitin from the Tertiary of Eurasia. Limnobiophyllum resembles in some respects Spirodela, but it is larger and the roots and the habit are more like young plants of Pistia L. (Araceae). It differs from Hydromystria Mey. (i.e. Limnobium Rich.) by venation, root system and habit; the Lemnaceae, although similar, differ by lateral pouches of leafy fronds and the lack of branched roots and higher-order veins. Limnobiophyllum is considered as an extinct link related to Pistia (Araceae), from which Spirodela (Lemnaceae) may have evolved by reduction."
Above: Photo of a fossil of Limnobiophyllum, courtesy of R.A. Stockey. This is a transverse section of a rosette plant with at least two leaves. The largest leaf cannot be seen in this section. There are many roots extending from the base. The scale bar is 5 mm. This fossil was collected by Stockey et al. at the Joffre Bridge area in the south-central Alberta, Canada.
The fossil monocot Limnobiophyllum scutatum: resolving the phylogeny of LemnaceaeRA Stockey, GL Hoffman and GW Rothwell, (1997) American Journal of Botany, Vol 84: 355-368.
"More than 200 specimens of Limnobiophyllum scutatum (Dawson) Krassilov have been recovered from lacustrine claystones in the Paleocene Paskapoo Formation near Red Deer, Alberta. The plant was a floating, aquatic angiosperm with helically arranged, ovate leaves attached in small rosettes. Rosettes are interconnected by stolons and bear simple adventitious roots as well as larger branching roots that appear to have vascular tissue. Leaves are pubescent, aerenchymatous, with 12-14 campylodromous primary veins that curve toward the apex, joining a fimbrial vein, often an apical notch. Staminate flowers with two, four-loculate stamens, are borne in the axils of second leaves. Anthers contain monoporate, globose, echinate pollen, 20-25 µm in diameter. The pollen wall is 0.8 µm thick, with a homogeneous foot layer, granular to slightly columellate infratectal layer and an echinate tectum. Pollen most closely resembles the sporae dispersae genus Pandaniidites Elsik. The completeness of L. scutatum has allowed for its inclusion in a numerical cladistic analysis to resolve relationships among taxa of the Lemnaceae, Pistia, and selected genera of Araceae. Results of the analysis indicate that the Lemnaceae plus Pistia form a monophyletic group within the Araceae."
relationships in the duckweeds and their relatives.
DNA sequencing has been used to clarify the evolutionary relationships of duckweeds and other Araceae. The chart below, adapted from the paper of Cabrera et al., shows that the duckweeds evolved independently from water lettuce (Pistia). Similar results were obtained by Rothwell et al.
Landoltia: A new genus of duckweeds?
A 1996 article (Les and Crawford, 1999) proposes that Spirodela punctata represents an evolutionary link between Spirodela and Lemna. The authors would rename this genus Landoltia, after Elias Landolt. Professor Landolt (personal communication, 2001) writes,
"Concerning the new genus Landoltia I only can comment that from a purely morphological point of view it was not necessary to create a new genus. The only species ("punctata") is morphologically between Lemna and Spirodela. It has some more characters together with Spirodela. However, if you look at the results of DNA and enzymatic investigations, it derived from Spirodela together with the other members of Lemnaceae and developed then parallel to the phylum Lemna - Wolffiella - Wolffia. Therefore, if you intend to form phylogenetically consistent taxa you have to separate S. punctata from the other two Spirodela species in a genus of its own."Some other experts remain skeptical and would keep it in the genus Spirodela.
More information and photos of Landoltia (Spirodela) punctata.
Eldred Corner, Prof. of Botany, Cambridge University, UK: Evolution in Contemporary Botanical Thought (Chicago: Quadrangle Books, 1961, p.97. In context, this quotation refers to the limited plant species found in the temperate zone. Prof. Corner was a tropical plant expert, and he intended to mean that to find the "missing links" between the major evolutionary groups of higher plants, one must study the transitional plants which are to be found mainly (in most cases, exclusively) in the tropics.
Pope John Paul II, Speech to the Pontifical Academy of Sciences, October 23, 1996. See:
Pope Pius XII, "Humani Generis" Encyclical Letter, August 12, 1950. http://www.newadvent.org/library/docs_pi12hg.htm
"...For these reasons the Teaching Authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter...."Landolt, E. Personal communication to J.W. Cross, 4 May 2001.
Les, D. H. & D. J. Crawford. 1999. "Landoltia (Lemnaceae), a new genus of duckweeds." Novon 9: 530-533.
Les, Donald H., Crawford, Daniel J., Landolt, Elias, Gabel, John D., Kimball, Rebecca T. (2002) "Phylogeny and Systematics of Lemnaceae, the Duckweed Family" Systematic Botany 27: 221-240.
Philbrick, C.T. and Les, D.H. (1996) "Evolution of aquatic angiosperm reproductive systems." BioScience 46 (11): 813-826.
Rothwell, G.W., Van Atta, M.R., Ballard Jr., H.E. and R.A. Stockey. 2004. "Molecular Phylogenetic Relationships among Lemnaceae and Araceae Using the Chloroplast trnL-trnF Intergenic Spacer." Molecular Phylogenetics and Evolution 30: 378-385.
Cabrera, L.I., Salazar, G.A., Chase, M.W., Mayo, S.J., Bogner, J., and P. Dávila. 2008. "Phylogenetic Relationships of Aroids and Duckweeds (Araceae) Inferred From Coding and Noncoding Plastid DNA." American Journal of Botany 95 (9): 1153-1165.
The following statement is commonly quoted by opponents of biological evolution:
"Can you imagine how an orchid, a duckweed and a palm tree have come from the same ancestry, and have we any evidence for this assumption?" [ Corner, 1961 ]Dr. Corner's statement was taken out of context, and in any event was written before the great explosion of bioscience knowledge in the last four decades of the twentieth century: He was writing before the advent of DNA cloning and genomic sequencing, before the discovery of homeobox genes, and before the theory of punctuated evolution. In short, today we benefit from a wealth of factual knowledge unavailable in 1961. No responsible scientist would agree with the above statement today.
Araceae. The family
of aroids, plants closely related to Lemnaceae. Pistia
is an aroid. See the chart above. To learn more
Arales. The order of plants that include both aroids and duckweeds.
Duckweed pollen fertilizes the egg cell of the ovule (which
develops into the seed embryo) and the polar nuclei in the
center of the ovule (which develops into the seed storage
Homeobox gene families are now known in all multicellular organisms from fruit flies to humans and in plants. The first homeobox gene, bithorax, was discovered in the fruit fly, Drosophila melanogaster. Edward Lewis, of the California Institute of Technology shared a Nobel Prize for his pioneering research on homeotic genes in the fruit fly. The first homeobox gene identified in plants was knotted1 which plays a major role in leaf development. Other homeotic genes are known to regulate floral development. Mutations in floral homeotic genes can convert one floral type into another. For example, the agamous mutant of Arabidopsis converts stamens into petals and replaces the fourth whorl carpels with sepals. Sequencing the agamous gene showed that it encodes a DNA transcription factor. Studying floral homeotic genes has led to unexpected insights into how floral organ type is determined.
Books about homeobox genes:
Ovaries. The ovary is the part of the flower from which the embryo and the endosperm will develop after fertilization. It contains the egg cell, the polar nuclei and other supporting cells.Master Control Genes in Development and Evolution : The Homeobox Story
by Walter J. Gehring and Frank Ruddle. (1998)
[ available from Amazon.com ]Lords of the Genes: Homeoboxes in Development and Evolution
by Walter J. Gehring (2000) [ available from Amazon.com ]
Guidebook to the Homeobox Genes
by Denis Duboule (1994 )
[ available from Amazon.com ]
theory of evolution states that the history of evolution is
concentrated in relatively rapid events of speciation rather
than taking place gradually as slow, continuous transformations
of established lineages. The discovery of homeotic genes provided a molecular
mechanism for such rapid changes and illustrations of how this
might work. The idea of punctuated equilibrium was first
formulated by Niles Eldredge and Stephen
Jay Gould in 1972. For a detailed discussion of
Jeffrey H. Schwartz, Sudden Origins: Fossils, Genes, and the Emergence of Species (1999) [ available from Amazon.com ]
[ Top of Page ] [ Return to The Charms of Duckweed ] [ Contact me ]
Last revised: June 9, 2013