a)Genetic Male Sterility (msms) Msms X Msms 1Msms Imsms (不育株)（可育株） (不育株) (可育株) 甲型两用系 Msms X Msms 1msms:1Msms (不育株)（可育株） (不育株) (可育株) 乙型两用系 b)Cytoplasmic Male Sterility(s) c)Cytoplasmic-Genetic Male Sterility (smsms) Cytoplasmic-genetic male sterility is widely used in hybrid seed production of a large number of crops.This is indeed the most convenient and practical way of producing hybrid seeds on an extensive scale.The following factors are important however,and need to be considered before embarking upon an ambitious hybrid seed production programme based upon cytoplasmic-genetic male sterility system. Male sterility must be stable.Any breakdown leads to partial self-fertilization. >Extensive use of a single cytoplasmic source may increase vulnearability to disease epidemics. Environmental influences,such as day length,light intensity,temperature, relative humidity and plant density,etc.have been reported to affect the fertility behaviour of restorer genes in some crops. Seed production must be economical,that is,yield and quality advantages of the hybrids must be substantial over non-hybrids.11 a) Genetic Male Sterility (msms) 甲型两用系 乙型两用系 b) Cytoplasmic Male Sterility (s) c) Cytoplasmic-Genetic Male Sterility (smsms) Cytoplasmic-genetic male sterility is widely used in hybrid seed production of a large number of crops. This is indeed the most convenient and practical way of producing hybrid seeds on an extensive scale. The following factors are important however, and need to be considered before embarking upon an ambitious hybrid seed production programme based upon cytoplasmic-genetic male sterility system.  Male sterility must be stable. Any breakdown leads to partial self-fertilization.  Extensive use of a single cytoplasmic source may increase vulnearability to disease epidemics.  Environmental influences, such as day length, light intensity, temperature, relative humidity and plant density, etc. have been reported to affect the fertility behaviour of restorer genes in some crops.  Seed production must be economical, that is, yield and quality advantages of the hybrids must be substantial over non-hybrids