Why Study Plant Development and Reproductive Biology?
Plants are continuously building their bodies - adding new cells, tissues, and organs to respond to the challenges they face in their environments while striving to acquire nutrients and reproduce. It's important to integrate different views of plant biology - including anatomy, morphology, development, genetics, genomics, and evolution - to understand the factors that determine how plants grow.
From flowers (and cones or sporangia) to the offspring they produce, reproductive biology in plants is important! As for all sexual reproduction, chromosome rearrangements and segregation during gamete formation introduce genetic, and thus the potential for phenotypic, variation - which is after all the raw material for natural (or artificial) selection.
For plants in particular, sexual reproduction includes many of the few opportunities that these stationary organisms have to move their genes around (such as pollen and seed dispersal). In addition, seeds and flowers are indispensable parts of terrestrial ecosystems and human cultures. Perhaps most importantly - seeds are the cornerstones of our agriculture and our diets.
From flowers (and cones or sporangia) to the offspring they produce, reproductive biology in plants is important! As for all sexual reproduction, chromosome rearrangements and segregation during gamete formation introduce genetic, and thus the potential for phenotypic, variation - which is after all the raw material for natural (or artificial) selection.
For plants in particular, sexual reproduction includes many of the few opportunities that these stationary organisms have to move their genes around (such as pollen and seed dispersal). In addition, seeds and flowers are indispensable parts of terrestrial ecosystems and human cultures. Perhaps most importantly - seeds are the cornerstones of our agriculture and our diets.
Plant development and reproduction is essential for producing the food that we eat and for forming the ecosystems that we live in - and is puzzling, exciting, and beautiful.
Evolution of Interparental Conflict and Imprinting Mechanisms in Seed Development
Endosperm is a novel structure for flowering plants (angiosperms) - it is the product of a second fertilization event that occurs at the inception of every angiosperm seed. It is genetically biparental (has maternal and paternal genetic contributions) and is essential for ensuring offspring growth and survival. While endosperm has long been appreciated as a mediator of mother-offspring nutritional relationships, it has more recently been placed in the context of how interparental conflict has shaped angiosperm evolution. Evidence for this conflict can be seen the phylogenetically wide-spread phenotypic trends that results from interploidy crosses, in which the endosperm and embryo possess altered maternal-to-paternal genomic ratios.
However, the molecular mechanisms behind the different developmental outcomes have only been characterized in three highly derived eudicot (Arabidopsis) and monocot systems (maize and rice). In these systems, it has been discovered that epigenetic modifications like DNA and histone methylation underlie cases of parent-of-origin effects on gene expression and seed development. However, between these species there is variation in both which genes are imprinted, as well as in global patterns of epigenetic modification -- begging the question of when imprinting mechanisms and patterns arose during angiosperm evolution.
To address this discrepancy, I have:
1) Performed reciprocal interploidy crossing Nymphaea thermarum (Nymphaeales, one of the earliest-diverging lineages of flowering plants) to test for the presence of reciprocal phenotypes that suggest imprinting at work
2) Used RNA seq to explore gene expression patterns at several key points in seed development.
Associated Publications:
Povilus, R. A., P. K. Diggle, and W. E. Friedman. 'Evidence for parent-of-origin effects and interparental conflict in seeds of an ancient flowering plant lineage
'. Proc. R. Soc. B. 285: 20172491. https://doi.org/10.1098/rspb.2017.2491
Povilus R.A. & Friedman W.E. (2022). Transcriptomes across fertilization and seed development in the water lily Nymphaea thermarum (Nymphaeales): evidence for epigenetic patterning during reproduction. Plant Reprod. doi:10.1007/s00497-022-00438-3
However, the molecular mechanisms behind the different developmental outcomes have only been characterized in three highly derived eudicot (Arabidopsis) and monocot systems (maize and rice). In these systems, it has been discovered that epigenetic modifications like DNA and histone methylation underlie cases of parent-of-origin effects on gene expression and seed development. However, between these species there is variation in both which genes are imprinted, as well as in global patterns of epigenetic modification -- begging the question of when imprinting mechanisms and patterns arose during angiosperm evolution.
To address this discrepancy, I have:
1) Performed reciprocal interploidy crossing Nymphaea thermarum (Nymphaeales, one of the earliest-diverging lineages of flowering plants) to test for the presence of reciprocal phenotypes that suggest imprinting at work
2) Used RNA seq to explore gene expression patterns at several key points in seed development.
Associated Publications:
Povilus, R. A., P. K. Diggle, and W. E. Friedman. 'Evidence for parent-of-origin effects and interparental conflict in seeds of an ancient flowering plant lineage
'. Proc. R. Soc. B. 285: 20172491. https://doi.org/10.1098/rspb.2017.2491
Povilus R.A. & Friedman W.E. (2022). Transcriptomes across fertilization and seed development in the water lily Nymphaea thermarum (Nymphaeales): evidence for epigenetic patterning during reproduction. Plant Reprod. doi:10.1007/s00497-022-00438-3
Genomic signatures of vascular cambium loss - perspectives from the genomes of Nymphaea thermarum and other cambium-less flowering plants
For ∼225 million years, all seed plants were woody trees, shrubs, or vines. Shortly after the origin of flowering plants ∼140 million years ago, Nymphaeales (water lilies) became one of the first seed plant lineages to become herbaceous through loss of the meristematic cell population known as the vascular cambium. We sequenced and assembled the draft genome of the water lily Nymphaea thermarum and compared it to genomes of other plants that have retained or lost the vascular cambium. By using both genome-wide and candidate-gene analysis, we found lineage-specific patterns of gene loss and divergence associated with cambium loss. Our results reveal divergent genomic signatures of convergent trait loss in a system characterized by complex gene-trait relationships.
Associated Publication:
Povilus, R. A., J. M. DaCosta, C. Grassa, P. R. V. Satyaki, M. Moeglein, J. Jaenisch, Z. Xi, S. Mathews, M. Gehring, C. C. Davis, W. E. Friedman (2020) Water lily (Nymphaea thermarum) genome reveals variable genomic signatures of ancient vascular cambium losses. Proceedings of the National Academy of Sciences (PNAS), DOI: 10.1073/pnas.1922873117
^ This paper was highlighted in a Commentary article:
Water lilies, loss of woodiness, and model systems. Peter R. Crane, Else Marie Friis. Proceedings of the National Academy of Sciences (2020), https://doi.org/10.1073/pnas.2005075117
Associated Publication:
Povilus, R. A., J. M. DaCosta, C. Grassa, P. R. V. Satyaki, M. Moeglein, J. Jaenisch, Z. Xi, S. Mathews, M. Gehring, C. C. Davis, W. E. Friedman (2020) Water lily (Nymphaea thermarum) genome reveals variable genomic signatures of ancient vascular cambium losses. Proceedings of the National Academy of Sciences (PNAS), DOI: 10.1073/pnas.1922873117
^ This paper was highlighted in a Commentary article:
Water lilies, loss of woodiness, and model systems. Peter R. Crane, Else Marie Friis. Proceedings of the National Academy of Sciences (2020), https://doi.org/10.1073/pnas.2005075117
Floral Biology and Seed Development in the Waterlily, Nymphaea thermarum
Nymphaea thermarum is a member of the Nymphaeales, of one of the most ancient lineages of flowering plants. This species was only recently described and then declared extinct in the wild, so little is known about its reproductive biology. In general, the complete ontogeny of ovules and seeds is not well documented among species of Nymphaea and has never been studied in the subgenus Brachyceras, the clade to which N. thermarum belongs.
In this study we document the reproductive development of N. thermarum from floral bud emergence through fertilization and seed development to germination. The goals are to correlate the timing of key events during floral and ovule development with pollination and seed development in order to provide an integrated view of the reproductive biology of N. thermarum.
You can find out more about Nymphaea thermarum by clicking here!
Associated Publication:
Povilus, R. A., J. M. Losada, and W. E. Friedman. 'Floral Biology And Ovule And Seed Ontogeny Of Nymphaea thermarum, A Water Lily At The Brink Of Extinction With Potential As A Model System For Basal Angiosperms'. Annals of Botany 115.2 (2014): 211-226. doi: 10.1093/aob/mcu235
In this study we document the reproductive development of N. thermarum from floral bud emergence through fertilization and seed development to germination. The goals are to correlate the timing of key events during floral and ovule development with pollination and seed development in order to provide an integrated view of the reproductive biology of N. thermarum.
You can find out more about Nymphaea thermarum by clicking here!
Associated Publication:
Povilus, R. A., J. M. Losada, and W. E. Friedman. 'Floral Biology And Ovule And Seed Ontogeny Of Nymphaea thermarum, A Water Lily At The Brink Of Extinction With Potential As A Model System For Basal Angiosperms'. Annals of Botany 115.2 (2014): 211-226. doi: 10.1093/aob/mcu235
Ovule and Megagametophyte (female gamete-producing structure) Development in Aquilegia caerulea 'origami'
Aquilegia is emerging as a model system for studying evolution of reproductive traits, from development of floral form to pollinator-flower interactions across time and space. However, the process of gamete production, fertilization, and embryology throughout seed development are not well known. We are addressing by correlating key stages of floral development with events in ovule and pollen development, and comprehensively documenting the ontogeny of major seed tissues throughout seed development.
In addition, I have done preliminary work to look at the activity of auxin-related genes (associated with the biosynthesis, transport, and antagonism of auxin) during female gametophyte development in Aquilegia. Many auxin-related genes are expressed early in ovules development in Aquilegia, while only an auxin antagonists showed higher expression in mature ovules - this is similar to patterns described in Arabidopsis. By looking at the evolutionary relationships of the Aquilegia and Arabidopsis members of these families via phylogenetic methods, and then mapping which copies are expressed during ovules development, it is evident that coding sequence similarity is not a good predictor of function during ovule development. Associated Publication: Povilus, R. and W.E. Friedman. (2012) Female gametophyte development and auxin regulation in Aquilegia. Conference of the Botanical Society of America; poster presentation. (Click to see poster) |
Developing Microscopy Methods - opening up difficult tissues for high-throughput imaging
Ovules and seeds present special challenges for microscopists:
- Ovules are tiny and buried within several layers of tissue, which make it difficult to orient them during sectioning. The female gametophytes in particular are delicate structures, which are difficult to capture in good condition.
- Seeds are surrounded with a hard seed coat. This both prevents proper infiltration of fluids during sample processing, and makes sectioning difficult. Many samples are often lost or destroyed during the sectioning process.
- Seed coats also present a challenge for confocal microscopy - they are often filled with compounds that strongly auto-fluoresce, which prevents imaging the enclosed tissue
- Many techniques for imaging ovules and seeds in whole-mount (without sectioning) do not allow for the use of histochemical staining. This means that important information about the type of compounds present in the sample cannot be collected.
These challenges means that ovules and seed development is often understudied. During my research, I am always working on ways to allow high-throughput, high-quality imaging of seeds, often with the aim of producing 3d models to help better understand the structure of the tissues inside of seeds.
Associated Publication:
Povilus, R. and W. E. Friedman (2014) Optimizing whole mount confocal microscopy for ovules and seeds. Plant Biology Initiative annual Symposium; poster presentation. (Click to see poster)
- Ovules are tiny and buried within several layers of tissue, which make it difficult to orient them during sectioning. The female gametophytes in particular are delicate structures, which are difficult to capture in good condition.
- Seeds are surrounded with a hard seed coat. This both prevents proper infiltration of fluids during sample processing, and makes sectioning difficult. Many samples are often lost or destroyed during the sectioning process.
- Seed coats also present a challenge for confocal microscopy - they are often filled with compounds that strongly auto-fluoresce, which prevents imaging the enclosed tissue
- Many techniques for imaging ovules and seeds in whole-mount (without sectioning) do not allow for the use of histochemical staining. This means that important information about the type of compounds present in the sample cannot be collected.
These challenges means that ovules and seed development is often understudied. During my research, I am always working on ways to allow high-throughput, high-quality imaging of seeds, often with the aim of producing 3d models to help better understand the structure of the tissues inside of seeds.
Associated Publication:
Povilus, R. and W. E. Friedman (2014) Optimizing whole mount confocal microscopy for ovules and seeds. Plant Biology Initiative annual Symposium; poster presentation. (Click to see poster)