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1.  Unique stigmatic hairs and pollen-tube growth within the stigmatic cell wall in the early-divergent angiosperm family Hydatellaceae 
Annals of Botany  2011;108(4):599-608.
Background and Aims
The ultrastructure of the pollen tubes and the unusual multicellular stigmatic hairs of Trithuria, the sole genus of Hydatellaceae, are described in the context of comparative studies of stigmatic and transmitting tissue in other early-divergent angiosperms.
Methods
Scanning and transmission electron microscopy and immunocytochemistry are used to study the structure and composition of both mature and immature stigmatic hair cells and pollen-tube growth in Trithuria.
Key Results
Trithuria possesses a dry-type stigma. Pollen tubes grow within the cell walls of the long multicellular stigmatic hairs. Immunocytochemistry results suggest that arabinogalactan proteins are involved in attracting the pollen tubes through the stigmatic cuticle. Most tubes grow along the hair axis towards its base, but some grow towards the hair apex, suggesting that pollen tubes are guided by both physical constraints such as microfibril orientation and the presence of binding factors such as unesterified pectins and adhesive proteins.
Conclusions
The presence of a dry-type stigma in Trithuria supports the hypothesis that this condition is ancestral in angiosperms. Each multicellular stigmatic hair of Hydatellaceae is morphologically homologous with a stigmatic papilla of other angiosperms, but functions as an independent stigma and style. This unusual combination of factors makes Hydatellaceae a useful model for comparative studies of pollen-tube growth in early angiosperms.
doi:10.1093/aob/mcr021
PMCID: PMC3170147  PMID: 21320877
Angiosperm evolution; Hydatellaceae; immunocytochemistry; pollen-tube growth; stigma; Trithuria; ultrastructure
2.  A new type of specialized morphophysiological dormancy and seed storage behaviour in Hydatellaceae, an early-divergent angiosperm family 
Annals of Botany  2010;105(6):1053-1061.
Background and Aims
Recent phylogenetic analysis has placed the aquatic family Hydatellaceae as an early-divergent angiosperm. Understanding seed dormancy, germination and desiccation tolerance of Hydatellaceae will facilitate ex situ conservation and advance hypotheses regarding angiosperm evolution.
Methods
Seed germination experiments were completed on three species of south-west Australian Hydatellaceae, Trithuria austinensis, T. bibracteata and T. submersa, to test the effects of temperature, light, germination stimulant and storage. Seeds were sectioned to examine embryo growth during germination in T. austinensis and T. submersa.
Key Results
Some embryo growth and cell division in T. austinensis and T. submersa occurred prior to the emergence of an undifferentiated embryo from the seed coat (‘germination’). Embryo differentiation occurred later, following further growth and a 3- to 4-fold increase in the number of cells. The time taken to achieve 50 % of maximum germination for seeds on water agar was 50, 35 and 37 d for T. austinensis, T bibracteata and T. submersa, respectively.
Conclusions
Seeds of Hydatellaceae have a new kind of specialized morphophysiological dormancy in which neither root nor shoot differentiates until after the embryo emerges from the seed coat. Seed biology is discussed in relation to early angiosperm evolution, together with ex situ conservation of this phylogenetically significant group.
doi:10.1093/aob/mcq062
PMCID: PMC2876005  PMID: 20338953
Hydatellaceae; morphophysiological dormancy; embryo; desiccation; seed; evolution; Trithuria submersa; Trithuria austinensis; Trithuria bibracteata
3.  Comparative Ovule and Megagametophyte Development in Hydatellaceae and Water Lilies Reveal a Mosaic of Features Among the Earliest Angiosperms 
Annals of Botany  2008;101(7):941-956.
Background and Aims
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.
Key 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.
doi:10.1093/aob/mcn032
PMCID: PMC2710223  PMID: 18378513
Embryo sac; megagametophyte; ovule; Hydatellaceae; Trithuria
4.  Seedling Diversity in Hydatellaceae: Implications for the Evolution of Angiosperm Cotyledons 
Annals of Botany  2007;101(1):153-164.
Background and Aims
Cotyledon number has long been a primary morphological feature distinguishing monocots from other angiosperms. Recent placement of Hydatellaceae near the early-divergent angiosperm order Nymphaeales, rather than in the monocot order Poales, has prompted reassessment of seedling morphology in this poorly known family.
Methods
Seedlings of six species representing all eco-geographical groups of Hydatellaceae are described using light and scanning electron microscopy.
Key Results
Two seedling types were discovered. Material examined of Trithuria submersa, T. bibracteata, T. austinensis and T. filamentosa possess a transparent bilobed sheathing structure that surrounds the main axis below the first foliage leaf. The seed coat is attached to the sheathing structure. Seedlings of Trithuria lanterna and T. konkanensis lack a sheathing structure, and the seed coat is attached to a short, narrow lateral outgrowth on the main axis of the seedling.
Conclusions
The sheathing structure that is present in seedlings of some Hydatellaceae could be homologized with the two united cotyledons of water lilies. It also resembles the single cotyledon of some monocots, and hence demonstrates a possible pathway of the origin of a monocot-like embryo, though no homology is implied. The sheathing structure is reduced in Trithuria lanterna and T. konkanensis, and the short, narrow outgrowth of its seedling could represent a single cotyledon. This synapomorphy suggests that the only Indian species of Hydatellaceae, T. konkanensis, is closer to the northern Australian T. lanterna than to the south-western Australian T. bibracteata.
doi:10.1093/aob/mcm274
PMCID: PMC2701838  PMID: 18032428
Seedling; cotyledon; monocot; dicot; Hydatellaceae; Trithuria

Results 1-4 (4)