Multiranking in the floral and vegetative phase

--Walton C. Galinat

In teosinte, the ancestor maize, as in most other grasses, the phyllotaxy of the floral and vegetative phase is the same, namely two-ranked. But when man domesticated teosinte, he selected for a multiranked phyllotaxy in the uppermost spike of both the tassel and ear clusters during the floral phase. At low levels of condensation, multiranking has a fragile penetrance that is normally confined to these points in space and time. Almost any form of stress during development such as disease, injury, heat or drought may cause multiranking to revert to two-ranking. The tassel branches, the secondary ears, the spikelets and florets all normally remain two-ranked in the floral phase. Therefore, if there is an epigenetic switch to multiranking during development of the uppermost ear and the central spike of the tassel, the reversion to the two-ranked state in preparation for the vegetative phase of the next generation usually is either unnecessary, as with pollen from the tassel branches, or is reverted within the two-ranked spikelets.

Most corn breeders have at some time observed individual corn plants that express multiranking in the vegetative phase (at a frequency of perhaps 1/100,000). On trying to propagate this condition, it has usually been found either not to be inherited or just a tendency within certain lines. We have discovered a more stable mutant form of vegetative multiranking in which the leaves occur in whorls of two, three or four.

The situation with the phytomers in the areas of multiranking is interesting. There has been a mirror-image repetition of phytomeric instructions such that rather than just a solo performance, it is played out as a duet with four ranking, a trio with six ranking or a quartet with eight ranking, etc., with the whorled phytomers sharing a common internode. The concept of expressing simultaneously different instructions is known to apply elsewhere in the case of overlapping phases in the corngrass and teopod mutants.

In the case of the mutant genes such as Bif and isp which I have described (MNL 62) as delaying the switching on of the floral phase after the vegetative phase is turned off, an alternative explanation is that the simultaneous mirror-image instructions for multiranking are temporarily cancelling each other out in the barren areas.

Our studies that are underway involve pollinations with pollen collected separately from the multiranked central spike of the tassel in comparison with pollinations by pollen from its two-ranked lateral branches. The pollen from the two sources has been placed on ears at different multiranking levels including two ranking in teosinte. Spikelet morphology of the vegetative multiranked lines is being examined in terms of possible multiranking within the spikelets.

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