Upper Cretaceous sulcate pollen from the Timerdyakh Formation,Vilui Basin (Siberia)

One inaperturate and 16 monosulcate pollen types are described from the latest Campanian to earliest Maastrichtian sediments of the Vilui basin, Siberia, using both light and scanning electron microscopy, and assigned systematically when possible to modern families or subfamilies. Despite their scant occurrence, the pollen grains show considerable diversity. Two new genera have been erected. Lasioideaecidites with two species, Lasioideaecidites hessei and Lasioideaecidites bogneri, represents the earliest record of the subfamily Lasioideae (Araceae). Aristolochiacidites with one species, Aristolochiacidites viluiensis, is assigned to the subfamily Aristolochioideae (Aristolochiaceae) and represents the first fossil pollen record of the family. A new species of Liliacidites, Liliacidites goldblattii, is closely similar to pollen of Isophysioideae (Iridaceae) and/or Doryanthaceae and is the first fossil evidence of the clade that includes these closely related families. A further nine new fossil species are described (Clavatipollenites timerdyakhensis, Liliacidites palaeofritillaria, Retimonocolpites longosucatus, R. microreticulatus, R. microrugulatus, R. lysichitonoides, R. chapmaniae, Monosulcites parvus and Arecipites tyungensis) and assigned to Chloranthaceae, Araceae (Orontioideae, ?Pothoideae), Liliaceae, ?Hypoxidaceae and Arecaceae. The Hydatellaceae (Nymphaeales) may also be present in the palynoflora of the Vilui Basin, represented by pollen similar to M. rivularis Braman from the Santonian to Campanian (Upper Cretaceous) Milk River Formation, Canada.     

A new type of specialized morphophysiological dormancy and seed storage behaviour in Hydatellaceae, an early-divergent angiosperm family

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.     

Reproductive ecology of the basal angiosperm Trithuria submersa (Hydatellaceae)

Background and Aims

Trithuria, the sole genus in the family Hydatellaceae, is an important group for understanding early angiosperm evolution because of its sister relationship to the ancient lineage, Nymphaeales (water lilies). Although also aquatic, Trithuria differs from water lilies in that all species are extremely small, and most have an annual life form and grow in seasonal wetlands. Very little is known about their reproductive ecology. This paper reports on reproductive timing, mode of pollination and characteristics of the breeding system of Trithuria submersa in Western Australia.

Methods

Mass collections of open-pollinated plants from different ecological settings were used to characterize the reproductive developmental sequence and natural pollen reception. Hand-pollination, caging and emasculation experiments were used to measure outcross + geitonogamous pollen reception versus autonomous self-pollination in two populations over two field seasons.

Key Results

Natural outcross or geitonogamous pollination was by wind, not by water or insects, but pollen reception was extremely low. Pollen production was very low and pollen release was non-synchronous within populations. The pollen to ovule (P/O) ratio was 23·9, compared with 1569·1 in dioecious Trithuria austinensis. Stigmas became receptive before male phase and remained so until anthers dehisced and autonomous self-pollination occurred. Natural pollen loads are composed primarily of self pollen. Self- and open-pollinated plants had equivalent seed set (both >70 %). Self-pollinated plants produced seed within 17 d.

Conclusions

Autonomous self-pollination and self-fertilization are predominant in T. submersa. The low P/O ratio is not an artefact of small plant size and is inconsistent with long-term pollination by wind. It indicates that T. submersa has evolved a primarily autogamous breeding system. Selfing, along with the effect of small plant size on the speed of reproduction, has enabled T. submersa to colonize marginal ephemeral wetlands in the face of unpredictable pollination.     

Comparative ovule and megagametophyte development in Hydatellaceae and water lilies reveal a mosaic of features among the earliest angiosperms

Background and Aims: The embryo sac, nucellus and integuments of the early-divergentangiosperms Hydatellaceae and other Nymphaeales are comparedwith those of other seed plants, in order to evaluate the evolutionaryorigin of these characters in the angiosperms. Methods: Using light microscopy, ovule and embryo sac development aredescribed in five (of 12) species of Trithuria, the sole genusof Hydatellaceae, and compared with those of Cabombaceae andNymphaeaceae. Key Results: The ovule of Trithuria is bitegmic and tenuinucellate, ratherthan bitegmic and crassinucellate as in most other Nymphaeales.The seed is operculate and possesses a perisperm that developsprecociously, which are both key features of Nymphaeales. However,in the Indian species T. konkanensis, perisperm is relativelypoorly developed by the time of fertilization. Perisperm cellsin Trithuria become multinucleate during development, a featureobserved also in other Nymphaeales. The outer integument issemi-annular (‘hood-shaped’), as in Cabombaceaeand 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 monosporicand four-nucleate; at the two-nucleate stage both nuclei occurin the micropylar domain. Double megagametophytes were frequentlyobserved, probably developed from different megaspores of thesame tetrad. Indirect, but strong evidence is presented forapomictic embryo development in T. filamentosa. Conclusions: Most features of the ovule and embryo sac of Trithuria are consistentwith a close relationship with other Nymphaeales, especiallyCabombaceae. The frequent occurrence of double megagametophytesin the same ovule indicates a high degree of developmental flexibility,and could provide a clue to the evolutionary origin of the Polygonum-typeof angiosperm embryo sac.     

Cabomba as a model for studies of early angiosperm evolution

BACKGROUND: The angiosperms, or flowering plants, diversified in the Cretaceous to dominate almost all terrestrial environments. Molecular phylogenetic studies indicate that the orders Amborellales, Nymphaeales and Austrobaileyales, collectively termed the ANA grade, diverged as separate lineages from a remaining angiosperm clade at a very early stage in flowering plant evolution. By comparing these early diverging lineages, it is possible to infer the possible morphology and ecology of the last common ancestor of the extant angiosperms, and this analysis can now be extended to try to deduce the developmental mechanisms that were present in early flowering plants. However, not all species in the ANA grade form convenient molecular-genetic models. SCOPE: The present study reviews the genus Cabomba (Nymphaeales), which shows a range of features that make it potentially useful as a genetic model. We focus on characters that have probably been conserved since the last common ancestor of the extant flowering plants. To facilitate the use of Cabomba as a molecular model, we describe methods for its cultivation to flowering in the laboratory, a novel Cabomba flower expressed sequence tag database, a well-adapted in situ hybridization protocol and a measurement of the nuclear genome size of C. caroliniana. We discuss the features required for species to become tractable models, and discuss the relative merits of Cabomba and other ANA-grade angiosperms in molecular-genetic studies aimed at understanding the origin of the flowering plants.     

Seedling diversity in Hydatellaceae: implications for the evolution of angiosperm cotyledons

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.     

Immunolocalization of arabinogalactan proteins (AGPs) in reproductive structures of an early-divergent angiosperm,Trithuria (Hydatellaceae)

Background and Aims Trithuria

is the sole genus of Hydatellaceae, a family of the early-divergent angiosperm lineage Nymphaeales (water-lilies). In this study different arabinogalactan protein (AGP) epitopes in T. submersa were evaluated in order to understand the diversity of these proteins and their functions in flowering plants.

Methods

Immunolabelling of different AGPs and pectin epitopes in reproductive structures of T. submersa at the stage of early seed development was achieved by immunofluorescence of specific antibodies.

Key Results

AGPs in Trithuria pistil tissues could be important as structural proteins and also as possible signalling molecules. Intense labelling was obtained with anti-AGP antibodies both in the anthers and in the intine wall, the latter associated with pollen tube emergence.

Conclusions

AGPs could play a significant role in Trithuria reproduction, due to their specific presence in the pollen tube pathway. The results agree with labellings obtained for Arabidopsis and confirms the importance of AGPs in angiosperm reproductive structures as essential structural components and probably important signalling molecules.