Seminar "Genetics and Epigenetics of Hybrid Vigour"

Speaker: Dr. Elizabeth Dennis, Chief scientist at the plant division, CSIRO Canberra.

Title: "Genetics and Epigenetics of Hybrid Vigour"

Abstract:

Hybrids have been widely used in agriculture because of the increased yields of hybrids relative to their parents. The molecular basis of hybrid vigour is largely unknown. Hybrids lose the yield advantage in the F2 and successive generations forcing farmers to buy seed each planting season. We have developed pure breeding lines (hybrid mimics) which retain the hybrid advantage in vigour using a process of recurrent selection of the F1- like plants in the F2 generation. The availability of the hybrid mimic lines has enabled us to approach the question of which genes are important for hybrid vigour? We have noted that many of the gens which are differentially expressed in hybrids relative to parents have the same alteration in expression in the hybrid mimics suggesting the vigour phenotype is achieved in the same way in hybrids and mimics. Altered metabolic pathways that are present in both include the auxin biosynthetic and downstream signalling pathways that are regulated by the PIF4 transcription factor.

Hybrids are heterozygous throughout the genome but hybrid mimics are homozygous at each locus but have chromosome segments from both parents. This suggests trans interactions between genes and gene products are important for achieving the correct level of gene expression. These may be interactions between transcription factors and their targets, epistatic interactions or result from epigenetic alterations which occur when hybrids are formed. We have described the phenomenon of Trans Chromosomal Methylation in hybrids where the methylation pattern of one allele in a hybrid is altered to resemble the other allele. This appears to be associated with the presence of 24nt RNAs and  is mediated by the RdDM pathway and can affect the level of gene expression. We and others have now shown that the RdDM pathway is not necessary for the generation of heterosis but another epigenetic pathway, the DDM pathway, is necessary for wild type levels of heterosis.