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Angiosperms are the regarded as one of the greatest terrestrial radiations of recent geological times. This occurred in the Cretaceous era (see MYBP time scale figure): The major lineages originated 130-90 MYBP, following by a dramatic rise to ecological dominance 100-70 MYBP, and >250000 angiosperm species are known today. Charles Darwin described the rapid rise and early diversification within the angiosperms as ‘an abdominable mystery’. This diversification is also manifested on the level of the seed, it’s longevity and germination control is part of the species adaptation to environmental factors, e.g. to the season: temperature, light and water.
Among other reasons, the ‘invention’ the double fertilization that gives rise to seeds with triploid endosperm is proposed to be a major reason for the evolutionary success of the angiosperms. Two hypotheses for the evolutionary origin of the endosperm are favored today (Friedman and Williams, 2004):
(1) The endosperm is a sterilized homolog of an embryo (Sargant 1900, Friedman 1995)
(2) The endosperm is the sexualized homolog of a portion of the megagametophyte (Strasburger 1900, Coulter 1911)
Primitive angiosperms, e.g. Nymphaceae, have a 4-celled, 4-nucleate embryo sac and double fertilization gives rise to a diploid endosperm. Based on this and on additional findings, Friedman and Wiliams (2004) are in support for the hypothesis that this diploid endosperm originated from a supernumerary embryo (altruistic sibling embryo, hypothesis 1). Modular duplication resulted in the 7-celled, 8-nucleate embryo sac of most of today’s angiosperms. In this case double fertilization gives rise to a triploid endosperm found in most angiosperms. Other researchers (e.g. Nowack et al., 2006) support hypothesis 2.
The endosperm is an embryo-nourishing tissue and is, depending on the species, parially or fully obliterated during seed development. However, most angiosperm species have retained an endosperm layer in the mature seed. In many of these cases, the endosperm not only functions as nourishing tissue, but is also involved in the control of seed germination in response to the environment and to developmental factors. Endosperm weakening is a prerequisite for the germination of many endospermic seeds including Solanaceae and Brassicaceae species.
God to Noah (after the great flood):
“As long as the earth endures,
seedtime and harvest,
cold and heat,
summer and winter,
day and night,
will never cease.”
Seed phylogeny – Morphological and physiological trends in seed evolution
Seed biodiversity has attracted the attention of many researchers and is a hallmark of seed biology. This great diversity of morphological and physiological features have evolved to control germination and dormancy in response to different environments. The evolution of seed structure, germination and dormancy is summarized by the Tansley review by Bill Finch-Savage and Gerd Leubner (2006) and the references cited therein. The cited references include key literature to seed evolution like the book “Seeds” from Baskin and Baskin (1998), and the publications by Baskin and Baskin (2004), by Forbis et al. (2002), and by Nikolaeva (2004). The work on seed morphology is based on a publication by Martin (The comparative internal morphology of seeds. The American Midland Naturalist 36: 513-660, 1946).
The most obvious morphological difference in mature angiosperm seeds is their “embryo to seed” size ratios resulting from the extent to which the endosperm is obliterated during seed development by incorporating the nutrients into the storage cotyledons. Based on the internal morphology of 1287 mature seeds Martin (1946) defined the following seed types with distinct embryo to endosperm ratios:
Structural seed types based on comparative internal morphology
Evolutionary trends of angiosperm seeds
Martin (The comparative internal morphology of seeds. The American Midland Naturalist 36: 513-660, 1946) arranged the seed types in a seed phylogenetic tree (see below) and proposed evolutionary seed trends. This has been revised and extended by Forbis et al. (2002) and is presented in the Tansley review by Finch-Savage and Leubner-Metzger (2006). In summary, the following general evolutionary seed trends are obvious:
(1) In mature seeds of primitive angiosperms a small embryo is embedded in abundant endosperm tissue. Such seed types (e.g. B1) are prevailing among basal angiosperms.
(2) The general evolutionary trend within the higher angiosperms is via the LA seed type (embryo linear axile and developed, endosperm abundance medium to high) towards FA seed types (embryo foliate axile and developed, often storage cotyledons, endosperm abundance low or endosperm obliterated) with storage cotyledons. The LA seed-type is typical for many Asterids, e.g. the endospermic Solanceae seeds. Further embryo dominance and endosperm reduction leads via the FA1 seed type to the diverted seed types FA2, FA3 and FA4. The FA seed types are typical for many Rosids, e.g. Brassicaceae seeds with more or less no endosperm at maturity.
(3) In addition to these general seed trends there are clade-specific seed type differences (“exceptions”), e.g. within the basal angiosperms (Laurales) and the Asterids (Aquifoliales).
(4) A small embryo is also found in primitive gymnosperms and an increase in the E:S values is also evident within the gymnosperms. An increase in relative embryo size appears therefore to be a general evolutionary trend within the angiosperms and the gymnosperms.
These morphological trends in seed evolution are of utmost importance for seed physiology, especially for the evolution of seed dormancy.
The evolution of whole seed size (e.g. dwarf seeds, MA) is discussed elsewhere, e.g. in Baskin and Baskin (2005), Baskin and Baskin (2007), and in Moles et al. (2005).
Martin (1946) investigated the embryo (form, size, position) and endosperm (plus additional storage tissue) in 1287 species and proposed seed types (B1 to B4, LA, P, MA, FA1 to FA4). Seed types with abundant endosperm (orange) and a tiny embryo (black) are basal (B1, B2, B3, B4). In the more advanced endospermic LA-type seeds, the embryo is linear axile. From this developed the FA-type seeds (FA1, FA2, FA3, FA4) where the embryo is foliate axile and, depending on the subtype, differs in shape and occupies more or less the entire seed. Mature FA-type seeds have little or no endosperm, and the nutrients are stored in the cotyledons.
Seed dormancy classes are indicated next to each family name: non-dormancy (ND), physiological dormancy (PD), morphological dormancy (MD), morphophysiological dormancy (MPD), physical dormancy (PY), combinatorial are explained on the web page “seed dormancy”.
Updated and modified from Martin (1946) based on work from Baskin and Baskin: 1998, 2004, 2005, and personal communication.
Seed types Angiosperms are the regarded as one of the greatest terrestrial radiations of recent geological times. This occurred in the Cretaceous era (see MYBP time scale figure): The major