I. As compared to a gymnosperm, what makes a plant an angiosperm??
A. Carpels. The presence of ovules enclosed within a carpel (megasporangium within an inrolled megasporophyll) is the best evidence that a plant is an angiosperm. The conduplicatecarpel hypothesis involves the formation of the carpel from a fertile megasporophyll that has folded in half and sealed along a suture (diagram). Evidence supporting this idea comes from extant plants with only partially closed carpels such as Degeneria (Degeneriaceae, Magnoliidae) which are considered primitive. Note, however, that Endress (1994) maintains that none of these carpels are truely closed because secretions from the inner lining of the carpel fill the gap. This photo shows Drimys which also displays a primitive form of carpel.
B. Reduced female gametophytes. No archegonium as is found in gymnosperms. Note that some gymnosperms also lack archegonia, e.g. within Gnetales, only Ephedra has one.
C. Double fertilization. Diagram of the micro- and megagametophytes in angiosperms. The production of triploid endosperm from the fusion of a sperm nucleus and a polar nucleus is unique to angiosperms. Friedman (Science 247:951, 1990) showed that a form of double fertilization occurs in Ephedra (Gnetales), but no endosperm is formed, simply multiple embryos. He hypothesized that if this process was homologous with angiosperms, endosperm could have evolved from such "supernumerary fertilization events." The position of Gnetales with conifers (not sister to angiosperms) as seen with molecular phylogenetic trees tells us that the fertilization tytpe in angiosperms and Gnetales is not homologous.
D. Pollen wall structure. Outer wall of the pollen grain is the ektexine. In angiosperms, ektexine is either columellate or tectate. Gymnospermous pollen lacks tectae (atectate). The inner part of the pollen wall (endexine) is spongy in gymnosperms, not spongy in angiosperms.
E. Pollen apertures (openings). In gymnosperms and Magnoliid angiosperms, only one aperture occurs, hence the pollen is monosulcate (monocolpate). More advanced angiosperms have three apertures trisulcate (tricolpate). So, pollen aperature number cannot be used to infer whether a plant is a gymnosperm or an angiosperm.
F. Vascular Tissue. Vessels in secondary xylem. Most gymnosperms lack vessels, but exceptions occur, such as in Gnetales. Moreover, some Magnoliid angiosperms lack vessels (e.g. the Winteraceae). Hence, the presence of vessels cannot be used as evidence that a plant is an angiosperm.
G. Leaves. Gymnosperms often have parallel venation with few branching (intercostal) veins. Angiosperms have either parallel or netted (reticulate) venation with intercostal veins. Extant gymnosperms such as Gnetum have strikingly angiospermous leaves. So as with E & F above, leaf morphology can only provide supportive, not conclusive evidence.
II. Gymnosperms Proposed to be Ancestors to Angiosperms
A. Extinct groups.
1. Briefly review the geologic time scale. Can get a pdf of the Geological Time Scale HERE. For a history of life as revealed by the fossil record, and some information on plate tectonics, go HERE. Note position of continents before separation (Pangea) during the late Triassic, early Jurassic here. Northern part separated, called Laurasia, southern part called Gondwana.
2. Many lineages of extinct
gymnosperm lineages are known from the fossil record. These plants were
derived from the Mesozoic seed ferns and are shown in your text as
occurring on the branch leading to angiosperms (Fig. 7.12). These groups included:
B. Crane (Ann. Mo. Bot. Gard. 72:716, 1985), Doyle and Donoghue (Bot. Rev. 52:321, 1986), and Nixon et al. (Ann. Mo. Bot. Gard. 81:484, 1994) conducted cladistic analyses of morphological characters of fossil and extant gymnosperms and angiosperms. All show Bennettitales as sister to a clade containing Gnetales and angiosperms (Crane cladogram). Crane (1985) went on to draw a possible way in which the megasporophylls of Caytonia (via a corystosperm) could have evolved into the "open carpel" of an angiosperm (Fig. 21).
III. Angiosperm Origins: The Fossil Record
A. The "Abominable Mystery". This phrase was used by Darwin with reference to the origin of flowering plants. He was referring to the dreadful lack of fossil evidence that would help link angiosperms to gymnosperms, seed ferns, or other extant or extinct groups. The lack of fossils was attributed to the thought that angiosperms arose in dry, upland areas that were not optimal for fossilization. Since Darwin's time, many advances have been made, including new fossils and lots of molecular data aimed at this mystery.
B. As shown in this figure (Crane 1995), the oldest indisputable angiosperms fossil is from the Valanginian of the Cretaceous period, ca. 130 mybp. Although this pollen is monosulcate (magnoliid), it is followed by the appearance of tricolpate pollen from the Barremian-Aptian boundary. By the upper Cretaceous, many of the extant angiosperm families and subclasses had already differentiated. This is considered the rapid radiation of the angiosperms which proceeded to become the dominant component of the world's flora.
C. Earlier Angiosperms? Does the presence of fossils in the lower Cretaceous and their absence in earlier strata indicate that angiosperms "began" at this time? in other words, did they branch off from gymnosperms then? NO! This is an important biological and evolutionary principle. Fossil evidence does not give the earliest divergence point, only that that group occurred at that time. The absence of data is not data! The assemblage of many angiosperms seen in Cretaceous represents the Crown Group for that clade. The ancestors of this crown group that evolved during the Triassic and Jurassic represent the Stem Group for which there are very few fossils.
D. New evidence. A paper was published in the November 27, 1998 issue of Science by Sun and colleagues who reported a macrofossil of a plant named Archaefructus from the Late (= Upper) Jurassic of China. The plant has a series of conduplicate carpels borne spirally on an elongate axis, but strangely, the axis is subtended only by a leaf-like structure (not floral whorls). In summary, this plant combined features of two magnoliid groups: the woody magnoliales and the herbaceous group called paleoherbs. More recent data indicates that the Archaefructus was actually from Cretaceous, not Jurassic strata.
E. An example of a fossil woody magnoliid: Archaeanthus (means ancient flower) have been found that are 98 million year old. Has many features found in the Magnolidae:
These features together support the "Magnolia is primitive" concepts originated by Bessey (1897) that have been supported by many others since (e.g. Bailey and Swamy, Cronquist, Takhtajan, Raven, Stebbins, Hickey and Wolfe, etc.). Compare above to the flower of Magnolia.
IV. Angiosperm Origins: Molecular Evidence
A. Up until fairly recently, the "anthophyte hypothesis", that shows Gnetales as sister to angiosperms, was the most popular concept. Figure 7.15 compares trees that show the anthophyte, gnepine and gnetipher hypotheses as well as a variant of the gnetifer tree with an alternate postion for the cycads (sister to angiosperms).
B. With the advent of molecular data, it became apparent that there was conflict between the anthophyte hypothesis and others that showed gymnosperms as monophyletic and sister to angiosperms. Some examples of molecular studies addressing this issue are:
C. Given that many molecular data sets do not support the anthophyte hypothesis,the apparent "synapomorphies" between angiosperms and Gnetales must be re-evaluated, for they likely represent parallelisms (e.g. presence of vessels, "double fertilization," leaf morphology in Gnetum, etc.).
