Key concepts -- Lecture 10 (Origin of angiosperms, part 1) IB 168 Spring 2009
What derived characteristics unite the angiosperms? Flowers?? (= stobili or cones of congested sporophylls, usually bisexual); flower-like strobili occur in other (extinct) groups of uncertain relationship to angiosperms (e.g., Bennettitales).
Clearly derived features of angiosperms: - Carpels - megasporophylls that are infolded and enclose the ovules (immature seeds) Provide protection to ovules (from physical & biotic environment). - Stigmatic pollen germination - germination on stigma rather than directly on ovule Allows for greater selectivity of reproductive partners by maternal parent. - Reduced gametophytes (even more so than in gymnosperms) - Microgametophytes (= pollen tubes) -- Only 3-celled at maturity; a tube cell + 2 sperm cells Megagametophytes (in most angiosperms, a 7-celled, 8-nucleate "embryo sac") - Endosperm A nutritive tissue that develops from a second fertilization event -- one of the two sperms from the pollen tube unites with two (polar) nuclei of megagametophyte (the other sperm unites with the egg). Involvement of both sperms in fusions with different cells of the megagametophyte is called double fertilization. Unlike other seed plants, angiosperms delay investment in nutritive tissue for the embryo until fertilization is assured (and make a relatively minor investment in each gametophyte compared to gymnosperms).
Darwin's abominable mystery The mystery was how to account for what appeared to be a sudden appearance of modern families of angiosperms in the Cretaceous. Transitional fossils appeared to be absent.
- Today, still no evidence of verifiable angiosperm fossils before early Cretaceous (Archaefructus described a few years ago as a Jurassic angiosperm but proved to be misdated -- actually from early Cretaceous). Fossil evidence suggests that angiosperms rapidly displaced other groups of seed plants during the Cretaceous.
- BUT fossils from early Cretaceous show evolutionary transitions in leaf and pollen characteristics; modern angiosperms do not suddenly appear. - Specifically, the fossil record shows an increase in species numbers and in morphological types that conform to our understanding of angiosperm relationships. - Leaves of Potomac formation (Doyle and Hickey) are simple, with disorganized venation patterns in earliest Cretaceous; lobed and compound leaves with organized venation in later Cretaceous. - Pollen record shows sequential emergence of different types, with derived types (triaperturate) emerging later than primitive types (uniaperturate).
Phylogenetic insights from comparisons across seed plant groups
Anthophyte Hypothesis (=flowering-plant hypothesis): Until recently, angiosperms were widely believed to be the sister group of gnetophytes based on results of preliminary molecular and morphological phylogenetic analyses. Some characteristics of putative relatives of gnetophytes (cycadioides or Bennettitales) believed to have flower-like strobili (although lacking carpels), thus the name "Anthophyte Hypothesis."
More recently, phylogenetic results from multiple-gene analyses provide more support for a sister-group relationship between gnetophytes and conifers (which both share various morphological features, such as compound seed cones).
Today, relationships among the five modern groups of seed plants (cycads, ginkgos, conifers, gnetophytes, and angiosperms) is otherwise unclear, although angiosperms may be sister to gymnosperms.
BUT seed plants taken together do appear to be a monophyletic group; one origin of the seed is the simplest hypothesis.
Many botanists believe that living groups of gymnosperms will not provide much clarification of angiosperm origins because they diverged so long ago from a common ancestor with the angiosperms; that is, the common ancestor between gymnosperms and angiosperms must date back to the Paleozoic or early Mesozoic (when the different gymnosperm groups appear in the fossil record), at least 100 million years before the first angiosperm fossils appear. Much change could have taken place in the angiosperm lineage during that 100+ million years between the time when it diverged from a common ancestor with other modern lineages of seed plants and the early Cretaceous, when angiosperms first appear in the fossil record.
The challenge is to identify whether any known or still-to-be-discovered fossil plants from that 100+ million year period might belong to the angiosperm lineage, possibly prior to origin of all of the features we now associate with flowering plants.