Embryology of Amborella (Amborellaceae): Descriptions and Polarity of Character States

Abstract

We present the first study on the embryology of Amborela frichopoda Baill. which, based on recent analyses of multiple genes, was identified as the first branch in the angiosperm evolution. Comparisons with two other basal angiosperms (Nymphaeaceae and Illiciales), as well as with the other angiosperms, showed that most of the embryological features of Ambom//a are plesiomorphies. They include: the tetrasporangiate anther; the fibrous endothecium; the glandular tapetum composed of two-nucleate cells; two-celled mature pollen; formation of the hypostase; bitegmic and crassinucellate ovules; the Polygonum type embryo sac; ephemeral antipodal cells, a lack of vascular bundles in the integuments; ab infio Cellular type endosperm; non-pachychalzal ovule or seed; albuminous mature seed. Contrary to all previous descriptions, Amborela has hemianatropous, rather than anatropous or orthotropous, ovules. For the determination of the polarity of ovule curvature as well as of some other characters such as the presence or absence of the nucellar cap and the obturator, the formation of the micropyle, and seed coat structure, additional studies on Nymphaeaceae and llliciales are needed.

Modularity of the Angiosperm Female Gametophyte and Its Bearing on the Early Evolution of Endosperm in Flowering Plants

“…Because these regions of the female gametophyte differ in shape and possess distinct and presumably autonomous patterns of free-nuclear development at all later stages of ontogeny, we hereafter refer to them as domains (BoisnardLorig et al 2001). The club shape of the female gametophyte is retained up to and beyond fertilization, and is also characteristic of other members of the Nymphaeaceae, Cabomba, Trimeniaceae, and some eumagnoliids (Earle 1938;Batygina et al 1982;Endress and Sampson 1983;Heo et al 1998), but not of other basal angiosperms such as Amborella, Acorus, Chloranthaceae, or Ceratophyllaceae (Buell 1938;Vijayaraghavan 1964;Batygina et al 1982;Tobe et al 2000).…”

“…Thus, the common, and presumably Polygonum-type female gametophyte has served as the standard against which to compare variant female gametophyte types. Because the Polygonum type is itself variable, we here reconstruct the ontogeny typical of early lineages of angiosperms with monosporic seven-celled/eight-nucleate female gametophytes based on recent studies of Amborella and basal lineages of eumagnoliids, eudicots, and monocots (Batygina et al 1982;Palser et al 1989;Grayum 1991;Johri et al 1992;Tobe et al 1993;Floyd et al 1999;Tobe et al 2000).…”

“…However, early angiosperm lineages with Polygonum-type development appear to be variable in their pattern of vacuolation and nuclear positioning. The first nucleus may be positioned in the micropylar, chalazal, or central region, and this trait may even vary within species (Palser et al 1989;Floyd et al 1999;Tobe et al 2000). Irrespective of initial nuclear positioning, in all Polygonum-type female gametophytes, the first freenuclear mitosis produces two daughter nuclei that migrate to opposite poles.…”

“…Thus, at the time of cellularization, the Polygonum-type female gametophyte is seven-celled and eight-nucleate (Johri et al 1992). Early angiosperm lineages with Polygonum-type female gametophytes are characterized by rapid degeneration of antipodals (the three cells at the chalazal end of the female gametophyte) and fusion of the two polar nuclei (within the central cell) before fertilization (Johri et al 1992;Tobe et al 2000). In more derived angiosperm taxa, there is a high degree of variation in both the timing of nuclear fusion within the central cell, and the fate of the three antipodals, which may degenerate, fuse with each other, undergo endoreduplication, enlarge, and/or proliferate (McLean and Ivimey-Cook 1956;Dahlgren 1991;Johri et al 1992).…”

“…All indicate the presence of either a monosporic seven-celled/eight-nucleate Polygonum-type female gametophyte or a monosporic four-celled/four-nucleate Nuphar-like female gametophyte. Only one publication to date provides definitive micrographic and developmental data, that of Tobe et al (2000) for Amborella. Of the other published studies, many conflict with each other, and drawings or micrographs of critical developmental stages are often lacking, or are inconsistent with the accompanying text.…”