Cytology of pollen abortion in cytoplasmic male sterile corn anthers

Recently, Warmke and Lee (J. Hered. 68:213-222) have reported that early mitochondrial degeneration occurs in tapetal cells of cms-T corn anthers shortly after meiosis. The degeneration of mitochondria is similar to that observed ultrastructurally in cms-T corn lines treated with H. maydis, race T toxin (H. C. Aldrich et al., Tissue and Cell 9:167-177). A study of pollen abortion in cms-C and cms-S cytoplasms of maize was initiated to determine whether similar mitochondrial degeneration occurs in these sources of cytoplasmic male sterility. Examinations of anthers in various stages of development from cms-T, cms-C, and cms-S versions of inbred W182BN, W64A, and NY821LERf were made at light and electron microscopic levels.

Anther development of cms-C and non-male sterile versions of inbred W182BN was similar through meiosis. Differentiation of male sterile from normal anthers was first seen in tapetal cells at the tetrad stage of development. Two types of tapetal differentiation occurred in cms-C. One type of tapetal variant had a dense cytoplasm, was binucleate, and had an increased number of small vacuoles when compared to male fertile anthers. In this type, tapetal inner and radial walls broke down as in fertile types but the deposition of Ubisch bodies was irregular and quantitatively less than in fertile anthers. The tapetum and the microspores disintegrated at the intermediate microspore stage.

The other type of tapetal variant exhibits a less dense cytoplasm and the presence of large vacuoles at the early tetrad stage. The tapetal cells increased in radial thickness by 2-2.5 times as compared to the meiosis stage. The cells usually contained only one nucleus and they developed rather thick cell walls which stained red with safranin. In this type of anther, microspores began to disintegrate a little earlier than in the other type of cms-C anther and the tapetal walls remained intact and thick when the tetrads aborted. The tapetal cells extended radially to fill most of the locule and the aborted tetrads were compressed into the center.

The tetrads of both types of tapetal variants generally appeared normal in their early development. Microspores of both types initiated exine development. However, none of the cms-C microspores was seen to form a single large vacuole and undergo mitosis as the fertile microspores did. Microspore development appeared to stop at an intermediate stage when the cytoplasm began to disintegrate.

At an electron microscopic level, differences in cytoplasmic density and cell wall thickness were observed between the two tapetal types, but changes in mitochondrial structure were not observed. The cytoplasm of the dense type appeared relatively normal except for the degree of vacuolation. Vacuoles tended to develop at the tetrad stage and enlarge during the early microspore stage. The cytoplasm became less dense and contained vacuoles, free ribosomes, and dilated endoplasmic reticulum just before degeneration at an intermediate microspore stage. The other type of tapetum was even more vacuolate, containing large vacuoles at even the early tetrad stage. The cytoplasm appeared to be less dense and endoplasmic reticulum occurred in a single elongate form or in aggregates. Multivesicular bodies and aggregates of ER occurred frequently near the radial cell wall.

The importance of the two patterns of tapetal breakdown is not known. Usually individual anthers exhibited one or the other type of tapetal breakdown but rarely a mixture of the two types was seen in different locules of the same anther. It is not known whether the two types of tapetal response represent two different mechanisms of pollen abortion. Although the latter would seem to be more compatible with current thoughts on cms-C sterility and restoration, more data on the nature of the cms-C type sterility are needed.

Preliminary studies on the cms-S type of cytoplasm revealed a completely different pattern of pollen abortion as might be expected since the cms-S cytoplasms follow a gametophytic pattern of pollen fertility restoration. The microsporogenesis of the cms-S system was identical to normal in all aspects of tapetal and microspore development. Deviation occurred late at the stage when pollen was nearly mature after the tapetum had disappeared. Pollen organelles appeared to be normal in structure during microsporogenesis. At a relatively late stage in pollen development, abortion occurred rapidly; however, distinct changes in organelles including mitochondria were not observed. Thus, changes in mitochondrial structure observed in cms-T pollen abortion and after treatment with H. maydis, T toxin were not seen in either cms-C or cms-S pollen abortions.

Sheu-Ling Janet Lee, Vernon Gracen and E. D. Earle

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

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