Who's dominating?

It is a question that has always confounded scientists. What is the role of the father and the mother in embryonic development? Earlier studies say that in the case of animals, the existence of one male parent and one female parent does not automatically mean that the contributions of both play equal roles at all times. With mammals — which includes humans — the situation is different: both maternal and paternal genes are important for embryonic development. But what about plants? A new study, conducted by Vielle-Calzada, Baskar and Grossniklaus of the Cold Spring Harbor Laboratory, usa, now suggests that the genes of the ‘father’ and the ‘mother’ are not equally active after fertilisation, at least in the case of the weed Arabidopsis thaliana ( Nature , Vol 404, p91-94).

Just like humans and animals, plants too have fathers and mothers. Every plant that is derived from a seed is the result of the fertilisation of a female egg cell by a male pollen cell. Given this, we can legitimately think of the egg donor as the mother, and of the pollen donor as the father, of the plant that we are looking at. All the genes carried by the embryo are present in two copies, one of which comes from the male parent and the other from the female parent.

Earlier work had uncovered genes in Arabidopsis that, when mutated, affected the early embryonic development and caused the embryo to die. But for the effect to be seen, both copies of the gene — the maternal as well as the paternal copy — had to be aberrant. This had been generalised to draw the inference that maternal and paternal genomes played comparable roles in early development. Vielle-Calzada and colleagues used a clever technique known as ‘enhancer trapping’ to study the relative roles of the male and female parents in regulating the expression of about 20 genes both in the embryo proper and in the endosperm, which is a supportive tissue formed in plants as the result of a second act of fertilisation. This second fertilisation is by different a pollen grain, and what is fertilised is not the egg cell of the mother (which carries just one copy of her genes), but a complete ‘diploid’ cell of hers (which carries both copies of her own genes).

In enhancer trapping, a foreign gene is inserted into the plant genetic material — the site of insertion is random. The foreign gene codes for an easily detectable product, commonly a product that is coloured (or can be treated to give a particular colour) or is capable of producing fluorescent light. But the gene lacks something important, namely the regulatory signals that are needed to get the product actually made at a particular place and time.

But what happens when the foreign gene gets inserted into the genome in a location where it does not cause any damage, but comes under the influence of some regulatory signal? Normally, the signal would cause some other gene, the gene whose function it is ‘supposed’ to regulate, to be expressed. This means that its message would be decoded — in a particular place and at a particular time. But now, because of the presence of the foreign gene, the regulator also causes, for example, a patch of blue colour to appear in some tissue. The blue colour can be taken as a sign that some gene would normally be active in the region of tissue so marked. Then one can legitimately ask the question: is the blueness the result of the activity of both maternal and paternal copies of the gene? Or is either copy sufficient? Or is just the maternal copy (or just the paternal copy) necessary, with the other one being more or less redundant?

So, first the foreign gene was inserted into the embryo or endosperm at an early stage, and its presence verified by the colour that it produced. Because the researchers were interested in genes that were expressed during early development, they restricted themselves to those cases in which the colour was seen in the developing embryo, or endosperm, soon after fertilisation. Once that was assured, breeding experiments were carried out to check whether the gene had got into the germ line — to check, in other words, whether the gene could be passed on to the next generation via pollen or eggs. After that was done, it was a simple matter to carry out crosses in which the foreign gene was present only in the male parent but not in the female parent, or vice-versa.

Nineteen cases were obtained in which the foreign gene had been successfully integrated into the germ line of the host. In all 19 cases, the pattern of expression of the gene was the same when the gene came from both parents as when it came only from the mother. However, if the gene was provided by the father alone, absolutely no colour was detectable: the gene was not expressed.

The nature of the experiment was such that the 19 instances must have mirrored the activities of a random set of early-acting genes. If as many as 19 random cases displayed a pattern such as the one seen, an obvious inference could be drawn. The activity of a large number of early-acting genes — perhaps the overwhelming majority — appears to depend solely on the contribution from the mother. If this is true, the implication points to something unprecedented, perhaps a wholesale silencing of paternal genes during early plant development.

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