**5. Identification of high-yielding hybrids**

After a large number of prospective parental lines (CMS, restorers or EGMS) have been identified, there is the need to select the most promising ones by their ability to give superior hybrids. This is normally achieved using the line by tester design [17]. Although it is still not well understood, the positive correlation between genetic distance of hybrid parents and the resulting F1 heterosis is accepted phenomenon. Heterosis levels in rice are reported to increase in the direction of *Japonica* × *japonica* < *indica* × *indica* < japonica × *javanica* < *indica × javanica* < *indica* × *japonica* [14]. Utilization of intersubspecific heterosis has been regarded as a promising strategy for increasing rice productivity. Large efforts have been invested in the last decades for breeding *indica*–*japonica* hybrids. However, such efforts have been hindered by hybrid sterility that frequently occurs in intersubspecific crosses. Discovery of Wide Compatibility Varieties (WCVs) brought hope for breaking the sterility barrier between *indica* and *japonica* subspecies and provided a possibility for exploiting the strong heterosis between them. The WCVs could produce fertile F1 hybrids when crossed with *Indica* or *Japonica* lines. The key approach was to introduce wide compatible genes into the restorer or CMS lines for developing widely compatible restorer or CMS lines which will permit the production of fertile F1 hybrids from either subspecies. Through marker-assisted selection, Guo *et al*. [21] successfully pyramided the *indica* allele (*S*-i) at four loci (*Sb*, *Sc*, *Sd* and *Se)* and the neutral allele (*S*-n) at *S5* locus in *japonica* genetic background to develop

*Indica*-Compatible *Japonica* Lines (ICJLs). These lines have a great promise of overcoming the intersubspecific hybrid sterility and exploiting the high heterosis between them. There is also a super hybrid rice breeding project in China trying to exploit intersubspecific heterosis in combination with ideal plant type [17].
