Bulblist 3/3/2004


The question of superfoetation, or fertilization by two pollens, has been debated since the late 18th century (at least). Evidence for has battled with theory against; the theory being that a single ovum is fertilized by a single pollen nucleus, resulting in an embryo combining inheritance from both parents.

Much has been learned in the past 200+ years. We now know that fertilization is not as simple as it originally seemed. For one thing, double fertilization has been found to be the rule among flowering plants. A pollen tube delivers two nuclei to the female gametophyte (egg sac). One nucleus fertilizes the ovum to produce the embryo, the other fertilizes the central cell (often a diploid cell formed by the fusion of two monoploid cells). The fertilized central cell develops into the endosperm.

In addition, there is evidence that cells other than the ovum and central cell are occasionally fertilized by a wandering pollen nucleus. And two nuclei entering the same ovum is not out of the question.

What is important to know, however, is whether the nuclei may be delivered by different pollen grains. They can! At least in maize.

As most gardeners know, F1 Hybrid corn (maize) is produced by inbreeding several strains of maize, then crossing the different inbred lines to produce the uniform and very productive F1 Hybrids. This is a lengthy process since it can take several years to produce the inbred lines. Decades ago some experimenters found a shortcut — some strains of maize, when used as pollen parents, induced a proportion of haploid embryos. When the chromosomes of these haploids were doubled — spontaneously or by colchicine — the resulting diploids (doubled haploids) were about as close to being perfectly homozygous as one could hope. The "inbred" lines were produced in a single generation. Apparently some of the pollen from the haploid-inducing lines only managed to deliver a single nucleus, which could fertilize either the central cell (leaving the monoploid ovum to develop as a haploid embryo) or the ovum (leaving the central cell to develop into a diploid — rather than triploid — endosperm).

Then someone noticed that they got more haploids by hand pollination than by open pollination — even though the same parents were used. A single hand pollination results in nearly 100% seed set, but repeated pollination by the same pollen (every time the wind blows) reduced the frequency of haploids. The only plausible explanation is that the first pollination fertilized some central cells but left the ova unfertilized. Subsequent pollen tubes delivered another nucleus to fertilize the ova. Some of them, anyway. And so, the missing haploids must have been fertilized by a pollen other than the one that fertilized the central cell. This is called heterofertilization.

In another experiment, pollen from a haploid-inducing line was mixed with pollen from the seed parent. The haploid-inducing line carried a dominant gene that puts some anthocyanin pigment on the top of the endosperm and embryo. The seed parent lacked this gene. When the seeds matured it was easy to sort the seeds according to their probable parentage, but took more effort to measure the chromosome numbers of embryos and endosperms. At least 5.3% of the seeds had been heterofertilized. In a parallel experiment, using a strain that does not induce haploids, only 1.5% of the seeds were heterofertilized.

The relevance of all this to plant breeding in general may not be immediately obvious. What does it matter that the embryo and endosperm have been fertilized by different pollen grains?

It often happens, particularly in wide hybrids, that the hybrid endosperm fails to develop normally even though the embryo does fine. In such cases the rate of success might be improved if a mixture of pollen is used — some from the desired pollen parent, some from the maternal species. If heterofertilization occurs, the hybrid embryo may be accompanied by a perfectly developed and entirely maternal endosperm. A good seed from an incompatible cross.

A second possibility is more speculative. Experiments pollinating a tomato by a wild relative were more successful  when the "wild" pollen was applied repeatedly. Certainly the wild pollen was normal — it worked perfectly well on flowers of its own species. Perhaps something in the tomato flower interfered with the normal behavior of its nuclei. A difference in pH, for example, might make the two nuclei stick together. One could fertilize the ovum while the other got stuck outside. A second fertilization, from a different pollen grain, could be just as abnormal, but fertilize the central cell instead.

Or, in plants (such as Lilium species) where a single pollen nucleus undergoes mitosis shortly before fertilization, something in the foreign environment may inhibit this mitosis. In which case, full fertilization (of ovum and central cell) would require two pollen tubes, potentially from different parents if a mixture of pollens is used.

Experiments have shown that pollen tubes have some means for avoiding each other. That is, if one tube is making its way towards the female gametophyte, other tubes are warned off — presumably by some repellant substance. But once a pollen tube has done its job, as well as it can in the circumstances, it may stop releasing the repellant. Thus, if the second pollen comes along a few hours later, it may not be able to detect the first one. And, because only the ovum or central cell has been fertilized, the unfertilized one may still be releasing its pollen tube attractant.

I have no direct evidence of pollen nuclei sticking together in a foreign ovule, nor of a failure of pollen nucleus mitosis. Maybe a different principle is involved. The fact remains that a tomato pollinated once by L. peruvianum gave only a few, poorly developed seeds. Only one germinated. But repeated pollinations by the same parents gave more seeds with better viability. Heterofertilization may not be the only possibility, but I think it's worth considering. And it may explain how Rosa x hardii came into existence by accident, while "scientific" breeders have been unable to duplicate it.