Genetics and cytology of the ms4 male-sterile soybean

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Abstract
Genetic studies of a new male-sterile, female-fertile soybean mutant found in the cultivar ‘Rampage’ showed that the gene involved was not allelic to any of the previously identified male sterility genes (ms1, ms2, or ms3). Therefore, it was assigned gene symbol ms4 The heterozygous form (T274H) has been added to the Soybean Genetic Type Collection. Linkage tests showed that ms4 is not located on trisomics A, B, or C and is not linked to the breakpoint of a homozygous chromosome translocatlon from PI 101,404B. Cytological studies of anthers from male-sterile plants showed development of sporogenous tissue to be normal until the end of meiosis. Tetrad formation did not occur, however, and four-nucleate coenocytic microspores were formed. These microspores either degen erated soon or underwent some irregular divisions, forming various kinds of 1- to 4-celled aggregates that degenerated sooner or later. Some of the aggregates persisted until anthesis. In some instances, functional pollen grains were released from the aggregates. A strong vacuolization of tapetum often was observed at the time of early degeneration of the microspores. The formation of coenocytic microspores has been observed in another male-sterile mutant in soybean (ms1), but those microspores never exhibited the same type of division as observed in ms4 The presence of cellular aggregates Instead of individ ualized pollen grains seems to be a unique feature of ms4 male-sterile plants.