The origin of the ovule

Abstract

Of several theories for the origin of the ovule advanced in this century and based largely on fossil evidence, the telomic concept is widely considered the most plausible. Its principal tenet is the evolution of the integument through fusion of sterile branches or telomes around a terminal megasporangium. The only point of agreement in these theories is that the entire nucellus is a megasporangium that retains a single megaspore and the endosporic female gametophyte. Their differences center on the origin of the integument. A new concept offered here on evidence from ovules of both Paleozoic and extant seed plants significantly alters the telomic theory. It proposes that the nucellus is a sporangiophore of stem origin that bears a terminal megasporangium; that at least some of the fused integumentary telomes were fertile; and that among all features cited to characterize ovules, the unique nature of the retained megaspore alone defines the seed habit. Changes in the seed plant megaspore that extended the period of nutrient absorption over the whole course of female gametophyte development, along with complex physiological changes in the nucellus, were probably achieved along a single phylogenetic line beginning in a Late Devonian population of progymnosperms. For such a combination of events to have occurred more than once is highly unlikely, and, therefore, a monophyletic origin for seed plants is proposed. Several primitive features in ovule structure, some not evidenced since the Lower Carboniferous Period, occur in a mutant form of Arabidopsis thaliana isolated from genetically transformed plants. Their recurrence provides additional support for the proposed concept of ovule origin and also suggests that the genetic mechanisms for expression of primitive features in advanced taxa could be initiated in each case by mutation of a single homeotic gene.

Gynoecium diversity and systematics of the Magnoliales and winteroids

“…The thicker the young ovulc is and the longer it takes until the upper rim surpasses the ovular apex to form the micropyle the greater the constraint becomes to form lobes or irregular thickening. Herr's (1995) contention that lobation especially of the inner integument ofMupolza like that of Gnetum is reminiscent of the tclonie-like integument of Palaeozoic seeds is difficult to substantiate, since in the taxa studied here lobation is also common in the outer integument and occurs in various forms and combinations between the two integuments (see also van Heel, 1970van Heel, , 1976, for some eudicot taxa).…”

“…(4) A fourth feature that has been discussed as phylogenetically interesting is lobation of the integuments (Umeda et al, 1994;Herr, 1995). However, a critical evaluation of this feature has to consider that it may be an architectural necessity especially in ovules with thick nucelli (as occurs commonly in magnoliids and many other angiosperms), and thus not a fundamental organizational feature but only a by-product of large size.…”

Gynoecium diversity and systematics of the Laurales

“…Again, from the variation of lobation or lack of lobation in both integuments there is little support for the contention that a lobed inner integument is the basal condition in angiosperms (Herr, 1995). Lobation also seems to be a by-product of developmental constraints on the formation of a closed micropyle from an open annular structure, as discussed in Igersheim & Endress (1997).…”

Functional Analysis of Sporophytic Transcripts Repressed by the Female Gametophyte in the Ovule of Arabidopsis thaliana

“…The ovules of flowering plants are most often formed as elongated primordia emerging from the inner surface of the young carpel, with the integument mounds initiating from periclinal divisions of the epidermal layer [1], [2], [3], [4]. A differentiated ovule is composed of a nucellus and two integuments, and is usually attached by a funiculus to the placental tissue of the gynoecia.…”