The developmental basis of an evolutionary diversification of female gametophyte structure in Piper and Piperaceae

Female gametophyte and embryo development in Helleborus bocconei Ten. (Ranunculaceae)

In this study, we investigated cytohistochemistry, cycle progression, and relative DNA content of the female gametophyte cells of Helleborus bocconei Ten. before and after fertilization process. The early stages of embryo development were also investigated. H. bocconei possesses a monosporic seven-celled/eight-nucleate Polygonum type female gametophyte, characterized by a morpho-functional polarity. The cells of the embryo sac showed abundant reserves of polysaccharides, strongly increasing in the egg cell just before fertilization. With different timing in DNA replication during cell cycle progression, synergids, egg cells, and polar nuclei showed a haploid DNA content at the end of their differentiation, while antipodes underwent three DNA endoreduplication cycles. Programmed cell death symptoms were detectable in synergid and antipodal cells. After double fertilization, the central cell quickly underwent many mitotic cycles forming the endosperm, which exhibited a progressive increase in protein bodies and starch grains. Close to the developing embryo, the endosperm differentiated a well-defined region rich in a fibrillar carbohydrate matrix. The zygote, that does not start immediately to divide after double fertilization, developed in to an embryo that reached the heart stage at fruit maturation time. A weakly differentiated embryo at this time indicates a morpho-physiological dormancy of seeds, as a survival strategy imposed by the life cycle of this plant with seed dispersal in spring and their germination in the following winter.

Natural variation in epigenetic pathways affects the specification of female gamete precursors in Arabidopsis

In angiosperms, the transition to the female gametophytic phase relies on the specification of premeiotic gamete precursors from sporophytic cells in the ovule. In Arabidopsis thaliana, a single diploid cell is specified as the premeiotic female gamete precursor. Here, we show that ecotypes of Arabidopsis exhibit differences in megasporogenesis leading to phenotypes reminiscent of defects in dominant mutations that epigenetically affect the specification of female gamete precursors. Intraspecific hybridization and polyploidy exacerbate these defects, which segregate quantitatively in F2 populations derived from ecotypic hybrids, suggesting that multiple loci control cell specification at the onset of female meiosis. This variation in cell differentiation is influenced by the activity of ARGONAUTE9 (AGO9) and RNA-DEPENDENT RNA POLYMERASE6 (RDR6), two genes involved in epigenetic silencing that control the specification of female gamete precursors. The pattern of transcriptional regulation and localization of AGO9 varies among ecotypes, and abnormal gamete precursors in ovules defective for RDR6 share identity with ectopic gamete precursors found in selected ecotypes. Our results indicate that differences in the epigenetic control of cell specification lead to natural phenotypic variation during megasporogenesis. We propose that this mechanism could be implicated in the emergence and evolution of the reproductive alternatives that prevail in flowering plants.

Developmental origins of the conjoined twin mature embryo sacs in Smilax davidiana, with special notes on the formation of their embryos and endosperms

PREMISE OF THE STUDY

Smilax davidiana, a perennial deciduous vine in Smilacaceae, is widely distributed in East and South China. A recent investigation revealed the existence of twin embryo sacs in this species. This study describes their mature structure, analyzes their developmental origins, and discusses the formation of their embryos and endosperms.

METHODS

Conventional paraffin section, light microscopy, and various staining techniques were used to document the course of megasporogenesis and mature embryo sac structure in pistillate flowers.

KEY RESULTS

Two chalazal megaspores of the tetrad became functional, giving rise to twin embryo sacs. Two mature embryo sacs of the same size (equal-conjoined twins); a dominant mature embryo sac and a subordinate one (unequal-conjoined twins); or very rarely, fully fused twin mature embryo sacs sharing a common chalazal domain are formed.

CONCLUSION

A single embryo occurring in conjoined twins is the outcome of two eggs competing for sperm rather than a result of contention of two pre-embryos (or young embryos) for nutrition. Conjoined twin embryo sacs could provide a new pathway to the formation of pentaploid endosperms.

Characterization of the basal angiosperm Aristolochia fimbriata: a potential experimental system for genetic studies

BACKGROUND

Previous studies in basal angiosperms have provided insight into the diversity within the angiosperm lineage and helped to polarize analyses of flowering plant evolution. However, there is still not an experimental system for genetic studies among basal angiosperms to facilitate comparative studies and functional investigation. It would be desirable to identify a basal angiosperm experimental system that possesses many of the features found in existing plant model systems (e.g., Arabidopsis and Oryza).

RESULTS

We have considered all basal angiosperm families for general characteristics important for experimental systems, including availability to the scientific community, growth habit, and membership in a large basal angiosperm group that displays a wide spectrum of phenotypic diversity. Most basal angiosperms are woody or aquatic, thus are not well-suited for large scale cultivation, and were excluded. We further investigated members of Aristolochiaceae for ease of culture, life cycle, genome size, and chromosome number. We demonstrated self-compatibility for Aristolochia elegans and A. fimbriata, and transformation with a GFP reporter construct for Saruma henryi and A. fimbriata. Furthermore, A. fimbriata was easily cultivated with a life cycle of just three months, could be regenerated in a tissue culture system, and had one of the smallest genomes among basal angiosperms. An extensive multi-tissue EST dataset was produced for A. fimbriata that includes over 3.8 million 454 sequence reads.

CONCLUSIONS

Aristolochia fimbriata has numerous features that facilitate genetic studies and is suggested as a potential model system for use with a wide variety of technologies. Emerging genetic and genomic tools for A. fimbriata and closely related species can aid the investigation of floral biology, developmental genetics, biochemical pathways important in plant-insect interactions as well as human health, and various other features present in early angiosperms.

Polar bodies--more a lack of understanding than a lack of respect

Polar bodies are as diverse as the organisms that produce them. Although in many animals these cells often die following meiotic maturation of the oocyte, in other organisms they are an essential and diverse part of embryonic development. Here we highlight some of this diversity and summarize the evolutionary basis for their utility.

Verhuellia is a segregate lineage in Piperaceae: more evidence from flower, fruit and pollen morphology, anatomy and development

BACKGROUND AND AIMS

The perianthless Piperales, i.e. Saururaceae and Piperaceae, have simple reduced flowers strikingly different from the other families of the order (e.g. Aristolochiaceae). Recent molecular phylogenies proved Verhuellia to be the first branch in Piperaceae, making it a promising subject to study the detailed structure and development of the flowers. Based on recently collected material, the first detailed study since 1872 was conducted with respect to morphology, anatomy and development of the inflorescence, pollen ultrastructure and fruit anatomy.

METHODS

Original scanning electron microscopy (SEM), transmission electron microscopy (TEM) and light microscopy (LM) observations on Verhuellia lunaria were compared with those of Piperaceae, Saururaceae and fossils.

KEY RESULTS

The inflorescence is an indeterminate spike with sessile flowers, each in the axil of a bract, developing in acropetal, helical succession. Flowers consist of two (occasionally three) stamens with basifixed tetrasporangiate anthers and latrorse dehiscence by a longitudinal slit. The gynoecium lacks a style but has 3-4 stigma branches and a single, basal orthotropous and unitegmic ovule. The fruit is a drupe with large multicellular epidermal protuberances. The pollen is very small, inaperturate and areolate, with hemispherical microechinate exine elements.

CONCLUSIONS

Despite the superficial similarities with different genera of Piperaceae and Saururaceae, the segregate position of Verhuellia revealed by molecular phylogenetics is supported by morphological, developmental and anatomical data presented here. Unitegmic ovules and inaperturate pollen, which are synapomorphies for the genus Peperomia, are also present in Verhuellia.

Female gametophyte and early seed development in Peperomia (Piperaceae)

The evolution of female gametophyte development provides an example of how minor ontogenetic modifications can impact the functional biology of seeds. Mature Peperomia-type female gametophytes are normally depicted as 16-nucleate, nine-celled structures. However, recent ultrastructural data have demonstrated that many previous reports were incorrect, suggesting that our understanding of the Peperomia-type ontogeny is incomplete. In this investigation, female gametophyte and early seed development is described in Peperomia dolabriformis, P. jamesoniana, and P. hispidula. Nuclear positioning, nuclear division, and vacuole morphology are documented during the syncytial stages of development, and two mature female gametophyte cellular configurations are described. Endosperm ploidy is measured in each species using microspectrofluorometry. We conclude that a 10-celled construction is likely the most common cellular configuration in Peperomia and that a three-celled female gametophyte exists in P. hispidula. We also describe how developmental modifications of wall formation could produce the diverse cellular configurations observed throughout Peperomia. Interestingly, the onset of female gametophyte diversification within Piperales correlates with the origin of the perisperm in the common ancestor of Piperaceae + Saururaceae. We posit that the origin of the perisperm may have relaxed selection on endosperm genetic constructs, thereby promoting diversification of female gametophyte ontogeny.