Development 1990 Supplement. 21-28 Printed in Great Bntmn © The Company of Biologists Limited 1990 21 Gene expression and parental dominance in hybrid plants J. S. HESLOP-HARRISON Karyobtology Croup, Jl Centre for Plant Science Research, Colney Lane, Norwich, NR4 7UJ, UK Summary Genomic imprinting, where the genes from one parent have different expression properties to those of the other parent, occurs in plants. It has potentially significant consequences because of the importance of hybrids in plant evolution and plant breeding, and provides a mechanism that can hide genetic variation for many generations. The study of nuclear organization shows that chromosome and genome position relates to imprinting in F| hybrids, with peripheral genomes tending to be expressed preferentially. In some inbred, polyploid hybrids, such as Triticale (a wheat x rye hybrid), treatment with the demethylation agent azacyti- dine releases hidden variation, which was perhaps lost because of imprinting phenomena. Key words: gene expression, parental dominance, hybrid plants, Triticale, demethylation, azacytidine. Occurrence and importance Genetic phenomena that do not follow Mendelian segregation ratios have been noted - and occasionally published - in hybrid plants over the last 50 years. They have been explained by phrases including 'block transferance of characters,' 'genetic affinity,' 'sup- pression,' 'selectivity' of expression and 'cryptic struc- tural differentiation.' Other unusual segregation ratios have been explained using 'skewed backcross ratios,' 'polygenes,' 'expression modifiers/modification,' 'link- age' and 'homoeostasis.' In some cases, elimination of chromosome sets, leaving the chromosomes of only one parent, may be involved (e.g. Davies, 1958, 1974; Lange, 1971). However, some of the non-Mendelian events can be explained by genomic imprinting - where genes from one parent have different expression properties to those of genes from the other parent, solely as a consequence of their parental origin. In the present work, 'parents' will be used to describe both the direct parents of an individual plant, and, for inbred and hybrid plants, the original lines used to make the cross that gave rise to the stock. Even within inbred lines, parental differences may give imprinting, but, in plants, the evidence for this remains limited and largely unpublished. In crop plants When breeding crop plants, phenomena leading to genomic imprinting can prevent the production of new plant hybrids that combine desirable characteristics of the parents. Another consequence of imprinting can be the unexpected and undesirable release of variation after several generations of inbreeding, which would lead to non-uniformity in crop stands. Although the mechanism of imprinting is not certain, it is possible that imprinting might be manipulated. For example, particular sets of genes could be incorporated in a crop, which could them be activated in a subsequent generation. Such a property would be useful both for the induction of new characteristics at particular times (e.g. the period when cereal grains are filling) or years (e.g. drought). Alternatively, genomic imprinting could be used to ensure the expression of certain desirable groups of genes in all the progeny of a cross, without the possibility of non-expression because of dominance relationships or supression. In wide hybrids In the plant kingdom, sexual interspecific and even intergeneric hybrids can be made relatively easily (see, e.g. Stephens, 1949; Finch and Bennett, 1980). In evolutionary terms, such wide hybridization may lead to genetic introgression in plants, where genes are transferred between different evolutionary lines. The recombination of characters between alien species is also important for enabling the introduction of new genetic variation into the gene pools of crops which may have been restricted by many centuries of inbreeding or intensive selection for homozygosity. Many such hybrids do not resemble intermediates between the parental species, but exhibit a form of genomic imprinting or parental dominance, which is not necess- arily gamete specific. The present paper aims to show that such forms of species-specific imprinting occur in plants in both F, hybrids and inbred lines.