Comparative ovule and megagametophyte development in Hydatellaceae and water lilies reveal a mosaic of features among the earliest angiosperms.
OBJECTIVE
The embryo sac, nucellus and integuments of the early-divergent angiosperms Hydatellaceae and other Nymphaeales are compared with those of other seed plants, in order to evaluate the evolutionary origin of these characters in the angiosperms.


METHODS
Using light microscopy, ovule and embryo sac development are described in five (of 12) species of Trithuria, the sole genus of Hydatellaceae, and compared with those of Cabombaceae and Nymphaeaceae.


RESULTS
The ovule of Trithuria is bitegmic and tenuinucellate, rather than bitegmic and crassinucellate as in most other Nymphaeales. The seed is operculate and possesses a perisperm that develops precociously, which are both key features of Nymphaeales. However, in the Indian species T. konkanensis, perisperm is relatively poorly developed by the time of fertilization. Perisperm cells in Trithuria become multinucleate during development, a feature observed also in other Nymphaeales. The outer integument is semi-annular ('hood-shaped'), as in Cabombaceae and some Nymphaeaceae, in contrast to the annular ('cap-shaped') outer integument of some other Nymphaeaceae (e.g. Barclaya) and Amborella. The megagametophyte in Trithuria is monosporic and four-nucleate; at the two-nucleate stage both nuclei occur in the micropylar domain. Double megagametophytes were frequently observed, probably developed from different megaspores of the same tetrad. Indirect, but strong evidence is presented for apomictic embryo development in T. filamentosa.


CONCLUSIONS
Most features of the ovule and embryo sac of Trithuria are consistent with a close relationship with other Nymphaeales, especially Cabombaceae. The frequent occurrence of double megagametophytes in the same ovule indicates a high degree of developmental flexibility, and could provide a clue to the evolutionary origin of the Polygonum-type of angiosperm embryo sac.
Rudall
Paula J
PJ
Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, UK. p.rudall@kew.org
Remizowa
Margarita V
MV
Beer
Anton S
AS
Bradshaw
Elizabeth
E
Stevenson
Dennis W
DW
Macfarlane
Terry D
TD
Tuckett
Renee E
RE
Yadav
Shrirang R
SR
Sokoloff
Dmitry D
DD
D005075:Biological Evolution; D035264:Flowers / Q000033:anatomy & histology / Q000196:embryology / Q000648:ultrastructure; D019684:Magnoliopsida / Q000033:anatomy & histology / Q000196:embryology* / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D030721:Nymphaea / Q000033:anatomy & histology / Q000196:embryology* / Q000648:ultrastructure; D012639:Seeds / Q000033:anatomy & histology / Q000196:embryology* / Q000648:ultrastructure; D013997:Time Factors