**4.2 The two-line method**

*Recent Advances in Rice Research*

**4. Hybrid rice breeding methods**

**4.1 The CMS (three-line) method**

A-line to produce hybrid seeds.

Yu *et al*. [18] provided evidence of the importance of epistasis as a possible genetic basis of heterosis in rice. Since none of these theories exclusively explains heterosis, suggestions are to ascribe the phenomenon to a combination of the three models.

Hybrids rice is currently produced either by the three-line (CMS) or the twoline (EGMS) method [19]. The three-line method employs three lines; the CMS (A line), maintainer (B line) and restorer (R line). An A-line is genetically identical to the B-line except that the B-line has a normal (N) cytoplasm while the A-line has a male sterile (S) cytoplasm. An R-line has dominant fertility restorer genes in its nucleus which restore male fertility to the F1 hybrid. The B-line is crossed to the A-line to maintain/produce seeds of the A-line whiles the R-line is crossed to the

Due to the relatively complex nature of the three-line (CMS) breeding system,

breeding materials are grouped into separate nurseries or stages for efficient handling and development of parental lines. Experimental hybrids normally pass through the required stages and are finally tested on farmers' fields before their release. The nurseries include the source nursery, the testcross nursery, backcross nursery, re-testcross nursery and combining ability nursery [19]. The source nursery contains elite breeding and CMS lines with the potential of becoming parents of commercial hybrids. In the testcross nursery, F1s from the CMS and tester lines in the source nursery are evaluated for their pollen and spikelet fertility to identify prospective maintainers (B-lines) and restorers (R-lines). Re-testcross nursery is for confirming and purifying prospective R-lines and backcross conversion of prospective B lines into CMS lines. Combining ability nursery evaluates general and specific combining abilities of selected CMS and R lines. This nursery helps identify higher yielding hybrids and is very crucial in the hybrid rice development process [17, 19]. Various modifications of the IRRI's system described above are made to improve breeding efficiency. Elite tester inbreds are selected from R-lines to cross onto new A-lines and select testers from A and B-lines to identify new R-lines. Once commercial R and A-lines are identified, they are used as testers instead of crossing a bunch of new R-lines by new A-lines. For instance, in China, IRRI's elites IR24, IR26 were directly used as restorer lines. These restorers were also used as restorer gene resources to develop new restorer lines [17]. Conversion of identified maintainer into a CMS line. Often lines introduced to new areas are not adapted to the target environment. They may be susceptible to particular local pests and diseases or may lack the desirable grain quality attributes. This necessitates the conversion of the available CMS lines into locally adapted and desirable ones. Introduced CMS lines maintainer identified from a cross of local lines onto a CMS source should be backcrossed to develop adapted CMS lines [19]. Backcrossing is continued to the 5th -6th generation (BC5-BC6) where the nuclear content of the original CMS source is replaced almost completely with that of its corresponding maintainer. Development of new CMS lines is normally difficult due to the limited chances of identifying stable sterile lines from local germplasm [19]. Developing new CMS lines using African cultivars has proven difficult due to sterility instability in the

*4.1.1 Organization of hybrid rice breeding program using the CMS system*

**50**

BC1 and BC2 generations [15].

In this system, male sterility is conditioned by the interaction of nuclear genes with environmental factors such as photoperiod, temperature or both. Such lines are referred to as environment-genetic male sterility (EGMS). The ones conditions by lines temperature are technically known as thermosensitive-genetic male sterility (TGMS), ones by photoperiod are photo-genetic male sterility (PGMS) and by both temperature and photoperiod are referred to as photo-thermo-genetic male sterility [17, 20]. Organization and seed production are simpler in this system than the CMS system. The EGMS lines are multiplied by sowing these lines in such a way that the sensitive period coincides with photoperiod/temperature that is conducive for inducing fertility. Hybrid seed production is taken up by sowing these lines in such a way that the sensitive stage coincides with the photoperiod or temperature conducive for inducing complete male sterility [20]. Magnitude of heterosis in two-line hybrids is 5 to 10% higher than in three-line hybrids. The major constraints to developing and using TMGS lines in the tropics are limited availability of stable TGMS germplasm. Since the PGMS, TGMS and PTGMS are controlled by recessive gene (s), when these lines are crossed with a fertile line, the hybrids are fully fertile, irrespective of the day length and temperature conditions prevailing during the growth season. Although attractive and potential as a tool for exploiting heterosis, the EGMS system has some advantages and disadvantages. Compared to the two-line system, the use of the three-line system is expensive and labor intensive but much more reliable. Since the two-line system does not require a maintainer line, any line can be used as pollen donor. This increases the chances of identifying higher yielding hybrids in the two-line system than the three-line system. However, any sudden changes in the environmental conditions during the hybrid production season affect the sterility of the temperature sensitive line and the requirement of additional land in different day length areas limits the ability to reliably produce hybrid rice using the two-line system [20].
