The Evolution of Genetic Systems (1958)
C. D. Darlington
What then is the relationship of nucleus and plastid? It seems in some cases that the nucleus controls the colour of the plastid. Yet in other cases it seems that the plastid is as autonomous and permanent as any gene. This apparent conflict is resolved by certain critical experiments with Oenothera.
As we saw earlier, the hybrid species Oenothera muricata produces two types of gametes, curvans and rigens. The homozygous species Oe. Hookeri on the other hand produces only one kind of gamete whose haploid nuclear complex is known as hHookeri. These two species can be crossed reciprocally to give two kinds of progeny in respect of the nucleus: and there are two kinds of each of these in respect of the cytoplasm. If we take only the curvans progeny we find that the progeny of the reciprocal crosses is different:
Hookeri x muricata hHookeri.curvans: yellow
muricata x Hookeri hHookeri.curvans: green
This means that Hookeri plastids are yellow with the H.c nucleus. Yet they are green with a pure Hookeri nucleus, and the muricata plastids are green with the H.c nucleus.
The table shows us what is happening:
Now the yellow seedlings die, but a few Hookeri plastids have been brought into the green seedlings with the pollen where Hookeri was the male parent. Some of these develop yellow flakes and yellow layers by sorting out during development. Yellow-over-green shoots should then breed from their yellow subepidermal layers as though they had pure Hookeri plastids. When selfed these shoots gave the two kinds of progeny that we should expect on this view (curvans.curvans being, as we know, lethal):
(i) hHookeri . curvans : all yellow
(ii) hHookeri.hHookeri : all green
As we should expect also, when these shoots are back-crossed with muricata pollen all the progeny is of the first type; with Hookeri pollen, of the second type.
Thus Hookeri plastids which become yellow with the H.c nucleus become green again with the pure Hookeri nucleus, and indeed with certain other nuclei in other crosses.