Speaker: Kaitlin Higgins, graduate student 

Title: "Genomic Imprinting in Maize: A comprehensive exploration of regulation, expression across time, and manipulation of genomic imprinting"

Major: Genetics and Genomics

Major professors: Professor Erik Vollbrecht and former Assistant Professor Sarah Anderson

Abstract: Genomic imprinting is a phenomenon in which alleles are expressed in a parent-of-origin dependent manner. Due to the parent-of-origin nature of imprinted expression, imprinted genes fall into one of two categories, maternally expressed genes (MEGs), and paternally expressed genes (PEGs). Imprinting in flowering plants occurs primarily in the endosperm and has been shown to affect seed viability. Although the first evidence of imprinted expression was discovered over five decades ago, recent technological advances have enabled us to investigate the regulation of imprinted expression as well as identify imprinted genes that don’t lead to a visible phenotype. These advances have led to hundreds of imprinted genes being identified in maize, A. Thaliana, and other species enabling us to more thoroughly investigate and understand how imprinting is regulated and the extent of imprinting activity within the endosperm. Maternal de-repression of R1 (Mdr1) was first identified as a regulator of imprinted expression through disruption of a phenotypic imprinted gene. A recent study evaluated Mdr1’s role as a DNA glycosylase and identified differentially methylated regions between mutant mdr1 and wild-type endosperm. We examine transcriptional changes occurring in the mutant and associate differentially expressed genes with differential methylation, transposable elements, and imprinted expression. We find that Mdr1 regulates over half of maternally expressed genes in the W22 genome, is strongly associated with helitron transposable elements, and may share redundancy of regulation with another endosperm active DNA glycosylase. Prior studies of imprinting have focused on evaluating imprinted expression at one time point during endosperm development, but this focus may restrict our understanding of imprinted expression due to the dynamic nature of endosperm development. We evaluate imprinted expression across four time points during endosperm development and find patterns of imprinting consistency and correlate these patterns to conservation of parentally biased expression across eight diverse maize lines reciprocally crossed with B73. Through these analyses we also identify three imprinted zeins. Finally, in an effort to create a visible marker of imprinted expression that may not impact seed viability, we design eight transgenic promoter fusions with green fluorescent protein (GFP). Of these eight constructs we find two with easily identifiable GFP expression in T2 seed, both of which are designed based on zein promoters associated with imprinting. We find one of these constructs displays higher GFP when inherited maternally versus paternally, in line with what we would expect for an imprinted gene. Future testing will enable us to delineate imprinted expression from allelic dosage effects. Together this work provides a substantial contribution to our understanding of imprinted expression in maize.