In sesame, both GMS and cytoplasmic male sterility (CMS) systems have been identified (Kumar and Abraham1941; Osman and Yermanos1982; Tu et al.1995; Anitha and Ganesan2000; Kavitha et al.2000; Wang and Wang et al.2002). Among them, some are unstable in male sterility expression. For example, the sib-mated progenies derived from a chemical induced male sterile mutant produce a lower frequency of male sterile plants than the expected proportion of 50%, indicating that the induced GMS has not attained stability (Anitha and Ganesan2000). In the present study, we have developed a new GMS line which is complete (Table 2; Figure 2) and stable across different genetic backgrounds even different environments (Wuhan and Sanya, China), which is similar to a previous report by Kavitha et al. (2000) who found that the expression of pollen sterility is stable in progenies of backcross generation (BC11F1) at two different locations and that it exhibits male sterility (>98%) even at high day temperature (36°C).
Although a number of male sterility lines have been reported, most of them cannot be used in breeding practice mainly due to incomplete male sterility, poor agronomic traits, or poor general combining ability (GCA). For instance, Anitha and Ganesan (2000) obtained three chemical-induced completely sterile plants, with no or sterile pollen. However, these plants show general reduction in the values of all biometrical characters compared to wild parent. The new sterile line developed in present study, although also being classified as recessive GMS, has a number of advantages when compared to previous ones. Being developed from a spontaneous mutant in Zhuzhi 4, a widely used cultivar in China, D248A has very good agronomic performance such as growth vigor, long BD (Table 2) and high GCA (unpublished data), which can be directly used in breeding practice. Currently, four F1 hybrids with 47-142% yield advantage over check variety have been developed from D248A (Table 3), which will be further tested in National Regional Trail.
GMS has great potential for hybrid seed production for several crops such as rice (Borkakati and Virmani1996), wheat (Liu et al.1997), cotton (Jyotiba et al.2010) and rapeseed (Chen et al.1993). The obvious merits of recessive GMS include the completeness of sterility, no potential adverse cytoplasmic effect and the availability of large number of restorer lines. These merits insure more chances of developing super F1 hybrid and less risk of producing impure hybrid seeds. However, like previous GMS lines, D248A also requires the removal of 50% fertile plants for hybrid production, which would be laborious and cost inefficient. To mitigate this situation, several approaches including pleiotropism and nonallelic interaction have been proposed (Rao et al.1990). The most promising approach, however, is from rapeseed (Brassica napus L.). Chen et al. (1993) reported a new recessive GMS line (9012A) in which the sterility is conditioned by two recessive genes and an epistatic suppression gene (esp). Later, they successfully developed it into a ‘three-line’ system (i.e. sterile line, temporary maintainer line and restorer line), which is as effective as traditional CMS system for massive hybrid seed production (Chen and Hu19982003). In our study, the possibility of a similar esp gene in sesame cannot be ruled out. In fact, Wang and Wang (2002) have already shown some evidences for this possibility. In their study, the male sterility of sesame is controlled by 1–2 recessive genes plus an epistatic gene. Screening sesame germplasm to identify such a gene is now underway in our lab. Much more efforts should be made on developing D248A into a ‘three-line system’ in the future.