Morphological study of male sterile-silky (ms-si or si1)

--D. W. Dales, R. I. Greyson and D. B.Walden

The gene "male sterile-silky" (ms-si or si1) is located twenty centimorgans from Adh1 on the long arm of chromosome six (MNL 62:143, 1988). The phenotype was described by A. C. Fraser (J. Hered. 24:41, 1933) as "ears bearing silks in great profusion and a small number of thin, wiry silks developing from the tassel. The tassel normally produces no, or an extremely small quantity of pollen." As part of a broader study of this trait we have discovered a number of items to add to this brief description. These include:

A) Tassel:

1) No filament elongation or anther extrusion.

2) The protrusion of wiry silks or "silk-like structures" (SLS) varied among ms-si plants and among spikelets of the same tassel. There was a range from zero to three SLS which emerged from a single spikelet. In those spikelets with no SLS emerging, SLS were frequently enclosed within the glumes.

3) The length of the SLS varied and ranged from 0-5cm.

4) The upper floret of each ms-si spikelet contained three abnormal stamens while the lower floret aborted. The extent of stamen development varied between spikelets on the same tassel and between plants, and ranged from no development to a structurally normal, but non-functional, stamen.

5) ms-si differed from other male sterile (genic or cytoplasmic) maize due to a failure in stamen development rather than a failure of the tapetum and/or microsporogenesis.

6) SLS developed from the tip of the connective cells of the abnormal stamen.

B) Ears: 1) The number of silks on the ms-si ear was approximately four times that found on a ms-si/+ ear.

2) An isolated wild type spikelet had only one silk, but a ms-si had SLS in addition to the one normal silk attached to the upper floret ovary. The number of SLS per ms-si spikelet varied and ranged from two to six, with three being most common.

3) SLS in ears emerged from "supernumerary ovaries" (SO) which developed, in addition to the central gynoecium, in each spikelet. One SLS was associated with each SO.

4) SEM analysis revealed that the SO developed from the organs that we normally identify as the aborted stamens of the ear spikelet. In addition the lower floret developed to some degree.

5) SO development varied. Generally the two SO located opposite each other in the upper floret and one from the lower floret were larger at maturity than the others.

6) The ms-si gynoecium, after fertilization, developed into a mature seed (kernel) while the SO became dried remnants and remained attached to the kernel.

7) After de-husking a mature ms-si ear, the SLS remained attached to the dried remnants of the SO while the silk of the gynoecium abscised.

In summary, ms-si is a recessive trait which is expressed in the early stages of flower development. It is developmentally distinct from other genic maize mutants with its restricted aberration of stamen development and development of SO in the ear spikelet and SLS that protrude from the abnormal stamens and SO. Because of the variation in expression, we hope that the ms-si is open to environmental manipulation and possible chemical reversion. Various plant growth regulators (PGR) are being used in these studies.

Seeds used in our study were from UWO stocks obtained originally from Maize Genetics Cooperation Stock Center.

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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