Body of Abstract: The RKD transcription factors are an ancient group of genes required for gametogenesis and the transition to zygotic development. These transcription factors are recorded in the literature to have a vital role in egg and sperm cell differentiation in Marchantia polymorpha (Koi, 2016). Previous research has identified possible effects of the mutated transcription factors, mainly affecting pattern formation and embryogenesis. For example, RKD4 has been identified as a key regulator in the earliest stages of plant development as it is required for polar growth of the zygote (Waki, 2011 and Jeong et al., 2011).
Single mutations of the four other RKD genes show no discernible effect on development. I argue that this lack of obvious defects in rkd1, rkd2, rkd3, and rkd5 single mutant plants indicates that these genes have functional overlap. Analyzing higher order mutants provides an opportunity to evaluate the distinct function of each gene. By creating a transmission table of a population created from the reciprocal cross of wild type pollen and plants heterozygous for rkd1, rkd2, rkd3, rkd4 and rkd5 mutations, information about the viability of RKD mutation combinations was examined. Specific combinations were underrepresented, thus alluding to these genes being essential for the formation of the gametes. In support of this view, the phenotypes of a rkd1-2, rkd2-19 double-mutant population were analyzed to investigate the possible role of RKD genes in embryogenesis. Research has shown that the double-mutant combination of rkd1-2, rkd2-19d may lead to arrested development of the polar embryo. In these phenotypes, fertilization occurs, but the zygote does not establish a polar axis and no further cell division occurs. The transmission test led to the prediction of multiple-mutant informative plants that reveal more about the specific function of the RKD genes in gametogenesis and the transition from gametophytic to zygotic development.