Quoted by Sherwood, N.N., Garden Peas. J. RHS v 22, 299-260 (1898)
|*Knight, T. ( 1799) Experiments on the fecundation of vegetables. Philosophical Transactions of the Royal Society of London 89, 195–204.|
In the "Philosophical Transactions" for 1799* appears an account of some experiments on the fecundation of vegetables, made by Mr. T. A. Knight, then President of the Royal Horticultural Society. Mr. Knight says :—
"I had a Pea in my garden, which having been long cultivated in the same soil had ceased to be productive, and did not appear to recover the whole of its former vitality when removed to a soil of a somewhat different quality: on this my first experiment in 1787 was made. When the blossoms were matured I introduced the farina of a large and luxuriant grey Pea into the one half, leaving the others as they were. The pods of each grew equally well, but I soon saw that in those whose blossoms I had not fertilised the seeds remained undeveloped and finally withered. Those in the other pods attained maturity, but were not sensibly different to those of other plants of the same variety.
"In the succeeding spring, however, the difference became very obvious, for the plants rose from them with increased luxuriance, and the colour of their leaves and stems clearly indicated that they had changed their whiteness for the colour of the male parent, the seeds produced in autumn being dark grey. By introducing the farina of another white variety (or, in some instances, by simple culture) this colour was easily discharged, and a numerous variety of new kinds produced, many of which were in size and every other respect much superior to the original white kind, and grew with excessive luxuriance, some to the height of more than 12 feet. I observed a stronger tendency to produce purple blossoms and coloured seeds than white ones, for when I introduced the farina of a purple blossom into a white one the whole of the seeds the next year became coloured; but when I tried to discharge this colour by reversing the process a part only afforded plants with white blossoms, this part sometimes occupying one end of the pod, and being at other times irregularly interspersed with those which when sown retained their colour.
"As the offspring of a White Pea is always white unless the farina of a coloured kind is used on it, and as the colour of the grey one is always transferred to its offspring, it occurred to me that if the farina of both were mingled or applied at the same moment the offspring of each could be readily distinguished.
"My first experiment was not altogether successful, for the offspring of five pods (the only ones which escaped the birds) received their colour from the coloured male. There was, however, a strong resemblance to the other male in the growth and character on more than one of the plants, and the seeds of several closely resembled it in everything but colour. In this experiment I used the farina of a White Pea, which possessed the remarkable property of shrivelling excessively when ripe; and in the second year I obtained white seeds from grey ones, above mentioned, perfectly similar to it. I am strongly disposed to believe that the seeds were here of common parentage.
"Again I prepared blossoms of the little Early Frame Pea. I introduced its own farina, and immediately afterwards that of a very large and late grey kind, and I sowed the seeds thus obtained. Many of them retained the colour and character of the small Early Pea, not in the slightest degree altered, and blossomed before they were 18 in. high, whilst others (taken from the same pods), whose colour was changed, grew to the height of more than 4 ft., and were killed by the frost before any flowers appeared."
CybeRose note: The bold paragraphs are not entirely clear, unless we assume that the grey seeded parent was the same one mentioned previously. If so, the experiment involved three strains:
The first was pollinated by the other two at the same time, but all the offspring bore only grey seeds, some apparently being wrinkled. In the second generation, however, some of the plants had wrinkled white seeds. It is possible, guessing from a safe distance, that the two white seeded strains differed in the nature of their whiteness. That is, if they were crossed with no possible interference from the grey seeded pollinator the seeds would have been grey. But this would have to be determined by experiment. Do any two strains of white peas produce grey-seeded progeny?
It is unlikely that either #1 or #2 carried the recessive wrinkled trait, because this fact would have been observed earlier ... and Knight would not have regarded the shrivelling of #3 as "remarkable".
This case is reminiscent of Tracy's cross of Cuzco Corn x Black Mexican sweet corn. In this case a variety with smooth, white seeds was pollinated by one with wrinkled black. Some of the kernels were smooth and white, suggesting that stray pollen from the same variety were involved. Others were black and wrinkled, as expected, but a third group of kernels were wrinkled and white.
Plants grown from the wrinkled white kernels were pollinated by a yellow sweet corn. Despite selection for white kernels, red and black kept turning up in later generations . Tracy did not mention variegation specifically, it is possible that some of the kernels were. Black Mexican is known to carry a silenced transposon (jumping gene) that can become active in hybrids, resulting in variegated kernels. Thus, the deep coloration may become nearly recessive (reduced to dots or streaks), then reappear in later generations when the transposon again becomes silenced.
Zakharova also found evidence of dual paternity in maize.
It is also worth noting that Tschermak raised pied (variegated) peas when he crossed the green 'Telephone' with yellow varieties. Considering that Knight's wrinkled peas were the most popular (and perhaps the first variety of the type widely grown), it is very possible that 'Telephone' (bred by Culverwell from Laxton's Supreme and Veitch's Perfection) was descended from Knight's strain through one or both of its parents.
The suggestion is that silencing sometimes may be responsible for the dominance (apparent and sometimes temporary) of one trait over another. Research has shown (Palauqui, et al.) that silencing can pass from stock to scion in grafted plants: "The transmission of co-suppression occurs when silenced stocks and non-silenced target scions are physically separated by up to 30 cm of stem of a non-target wild-type plant." It should pass just as easily between embryos growing in close proximity. Thus, a given seedling may receive "genes" from only one pollen parent, but still be influenced by another—indirectly, through a neighboring embryo derived from that other pollen parent.