Reproduction and Seed Development in the Water Lily Nymphaea Thermarum – a New Perspective on the Evolution of Flowering Plant Seeds

Abstract

Almost a century of research connects the origin of double fertilization, a major evolutionary innovation of flowering plants (angiosperms), to conflicting parental interests over offspring provisioning during seed development. Furthermore, work in a small handful of model systems has revealed that imprinting, an epigenetic phenomena based on chromatin methylation patterns, underlies key components of parent-of-origin effects on seed development. However, neither parent-of-origin effects on development nor genetic imprinting have been studied in any early-diverging angiosperm lineage – hindering our understanding of the evolutionary relationships between interparental conflict, parent-of-origin-effects, and chromatin methylation. For my dissertation I identified and leveraged the experimental tractability of the water lily Nymphaea thermarum (Nymphaeales) to perform reciprocal interploidy crosses and to document patterns of gene expression related to chromatin methylation during reproductive development. In Chapter 1, we describe the floral biology, female reproductive development, fertilization, and seed ontogeny of N. thermarum, and thereby provide knowledge necessary for further experiments. In Chapter 2, we create autotetraploid lines of N. thermarum, and perform reciprocal interploidy crosses to test for parent-of-origin effects on seed development. By measuring the size of seed components and rates of embryo development in different types of crosses, we provide the first evidence that parent-of-origin effects on offspring development may date to the evolutionary origin of flowering plants. We also demonstrate that that the evolutionary transfer of embryo-nourishing function from a genetically biparental endosperm to a genetically maternal storage tissue (perisperm) can function as a maternal strategy to recapture control of resource distribution among progeny. In Chapter 3, we sequence transcriptomes of whole ovules and seeds from three key stages of reproductive development in N. thermarum. In particular, we examine the expression of genes associated with chromatin methylation – processes known to be essential for parent-of-origin effects on seed development in other angiosperms. We find evidence for a dynamic chromatin methylation landscape during reproductive development. However, genes involved in establishing, maintaining, and removing methylation marks associated with genetic imprinting show a mix of conserved and unique expression patterns between N. thermarum and other angiosperms, providing perspective on how the regulation of imprinting has changed throughout angiosperm evolution.