Development Suppl. 108:21-28 (1990)
Gene expression and parental dominance in hybrid plants.
J. S. Heslop-Harrison
Karyobiology Group, JI Centre for Plant Science Research, Norwich, 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 F1 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 azacytidine releases hidden variation, which was perhaps lost because of imprinting phenomena.

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,' 'suppression,' 'selectivity' of expression and 'cryptic structural differentiation.' Other unusual segregation ratios have been explained using 'skewed backcross ratios,' 'polygenes,' 'expression modifiers/modification,' 'linkage' 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lines, parental differences may give imprinting, but, in plants, the evidence for this remains limited and largely unpublished.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 necessarily gamete specific. The present paper aims to show that such forms of species-specific imprinting occur in plants in both F1 hybrids and inbred lines.


Correlations of phenotype with chromosome position

The physical positioning of the chromosomes in the hybrid H. vulgare x S. africanum has been investigated by reconstructing metaphases from sets of electron micrographs of serial sections. These results have shown that the chromosomes originating from the two parental genomes are not randomly positioned within the hybrid, but tend to be spatially separated with those from one species lying in a peripheral domain, while those from the other are more central. In the H. vulgare x S. africanum hybrid, the chromosomes of H. vulgare origin tend to be central in the nucleus, while those of S. africanum origin are peripheral (Bennett, 1982, 1984; Finch and Bennett, 1981; Bennett, 1988; Heslop-Harrison and Bennett, 1984). However, at metaphase, the chromosomes are generally inactive in gene expression and DNA replication, so it is necessary to know if the genomes are also spatially separated at interphase, when the chromosomes are active in gene expression and DNA replication.

In hybrids where the DNA from the two parents is sufficiently different, the chromosomes of the two genomes can be distinguished at all stages of the cell cycle by a method using in situ hybridization of total genomic DNA. The basis of the technique is illustrated in Fig. 3, which shows Southern transfers of restriction enzyme digests of DNA from two parents and an intergeneric hybrid. These have been hybridized with labelled genomic DNA from one of the parents, H. chilense, alone (Fig. 3A), or in the presence of a large excess of unlabelled genomic DNA from the other parent, S. africanum, to block sequences that are common between the two genomes so the labelled DNA cannot hybridize (Fig. 3B; see Anamthawat-Jónsson et al. 1990, for further details). The tracks with DNA from the H. chilense parent show stronger hybridization than the S. africanum tracks, and the differentiation is emphasized in the blocked blot where the two parents and hybrid can be distinguised clearly.

Fig. 4 shows the results following in situ hybridization using genomic probe and blocking (Heslop-Harrison et al. 1990; Schwarzacher et al. 1989) in the F1 hybrid H. vulgare x S. africanum. The micrographs show that the two genomes, originating from different parents, tend to be spatially separated both at prophase and during interphase, as well as at metaphase. However, these data are from spread material, where it is possible that differential penetration of probe or detection reagents could lead to the observed results. Fig. 5 shows a single section through nuclei of the hybrid, which demonstrates that the genome separation is also seen in sectioned material where the three-dimensional structure of the cell has been preserved (Leitch et al. 1990). Reconstructions now being made from such interphase nuclei tend to confirm the impression of the spatial separation of parental chromosome sets at interphase in the wide hybrids (Leitch, Schwarzacher and Heslop-Harrison, unpublished data).

The two hybrids H. vulgare x S. africanum and H. chilense x S. africanum have been studied in greater detail than others. However, further examples of hybrids where the phenotype resembled the parent that contributed the peripheral chromosomes in the metaphase have also been reported (Bennett, 1984).

Parental dominance in segregating populations after hybridization

Another class of parental dominance occurs in the hybrid between two species of cotton, Gossypium hirsutum and G. barbadense. The characteristics and behaviour of this hybrid were first described many years ago (see Stephens, 1950 and Wallace, 1960, for references). In summary, after interspecific crossing and self-pollination of subsequent generations, the phenotype of the progeny often reverted to resembling one or other of the parental species showing that groups of parental characters were preserved (Schwendiman, 1974). In more formal terms, the progenies of interspecific hybrids do not give clear Mendelian ratios for the segregation of genes that are known to be allelomorphic on the basis of intraspecific tests. In some cases, which are well documented and comprehensively discussed by Stephens (1950), interspecific hybrids followed by backcrossing to the female parent can give a marked deficiency in the expression of the gene from the male parent. He goes on to report that the deficiency cannot be due to the effect of the introduced gene itself because with successive backcrosses the normal 1:1 segregation ratio tends to be more closely approached. Cytoplasmic effects cannot play a major role since there will be little introgression of the male cytoplasm into the hybrids and backcrosses. Although some results can be explained by the segregation of expression modifiers, and by cryptic structural differentiation of chromosomes which leads to selection against chromosome segments introduced from one parent, it seems probable that some of the effects are the result of the imprinting of genes, and certainly the area would be worthy of further investigation.

Parental dominance within inbred cereal hybrids

The phenotype of first generation hybrids between different cereals is discussed above. Intergeneric hybrids between cereals can be made and the chromosome number doubled to give polyploid plants, which are fertile and can be grown as a crop. One such crop is Triticale (2n=6x=42 x Triticosecale Witt.), a hybrid between a tetraploid wheat (2n=4x=28; Triticum durum) and rye, S. cereale (2n=2x=14). This crop is widely grown, particularly in eastern Europe and Canada, for use in animal feed and occasionally for milling for flour. However, like many of the other hybrids discussed above, it does not resemble a true intermediate between the parental species, but has many features of rye rather than wheat (see, e.g. Percival. 1923).

Hetero-Fertilization / Endosperm Failure