The lower number of female than male lines is because it is easier to breed restorer than maintainer lines.
Many indica cultivars are known to carry restorer genes for WA-CMS lines and cannot be used as maintainer lines.
A major technical handicap in the development of hybrid rice using WA-CMS is a limited source of maintainer lines, as many indica elite cultivars are known to carry restorer genes for WA-CMS lines and cannot be used as maintainer lines (Virmani 1994).
Huge efforts had been made to establish maintainer lines for WA-CMS based on indica varieties through a maintainer-breeding program at the International Rice Research Institute (Virmani 1994).
Recently, pyramiding of blast R gene has been successfully implemented in rice breeding program, thus leading to the development of the maintainer lines with broad-spectrum resistances to blast (Jiang et al. 2012).
Fu et al. (2011) found that MH63 was the most tolerant variety to high temperate in a total of frequently-used 39 restorer and maintainer lines at flowering stage.
