INHERITANCE OF FERTILITY RESTORATION INVOLVING WILD ABORTIVE CYTOPLASMIC MALE STERILITY SYSTEM IN RICE ( O ryza sativa L . )

Inheritance of fertility restoration was studied in crosses involving ten elite restorer lines of rice viz. BR6839-41-5-1R, BR7013-62-1-1R, BR7011-37-1-2R, BR10R, BR11R, BR12R, BR13R, BR14R, BR15R and BR16R and one male sterile line Jin23A with WA sources of cytoplasmic male sterility. The segregation pattern for pollen fertility of F2 and BC1 populations of crosses involving Jin23A indicated the presence of two independent dominant fertility restoring genes. The mode of action of the two genes varied in different crosses revealing three types of interaction, i.e. epistasis with dominant gene action, epistasis with recessive gene action, and epistasis with incomplete dominance.


INTRODUCTION
Precise understanding of genetics of fertility restoration is useful in planning a sound breeding strategy for development of superior restorers in a hybrid breeding program.It may also help in the efficient transfer of restorer genes into other agronomical desirable genotypes.In rice, several sources of cytoplasmic genetic male sterility (CMS) have been reported (Virmani and Edwards, 1983).However, extensive research work on identification of restorers and maintainers and on inheritance of fertility restoration has been done on the WA (Wild Abortive) type cytoplasmic source only.Cytoplasmic male sterility (CMS), which causes the production of non-functional pollen and inherited maternally, is important in commercial hybrid seed production (Kaul, 1988) and breeding programmes.A number of studies on the relationship between CMS and fertility-restorer genes (Rf) have been conducted in various plants and may enable a better understanding of genetic differentiation and the interaction between cytoplasmic and nuclear genomes in plants (Budar and Pelletier, 2001).
The wild-abortive (WA) type of CMS in rice has been used extensively in commercial production, and its fertility is sporophytically restored by the dominant restorer genes (Shen et al., 1998, Jing et al., 2001).Fertility-restorer genes are important in the production of hybrid rice.Although a variable number of restorer genes have been proposed in various restorer lines, one or two dominant restorer alleles (Rf3 and Rf4) are usually suggested to be responsible for the fertility (Yao et al., 1997, Tan et al., 1998).A major dominant gene with sporophytic action has been reported to be involved in fertility restoration of the WA source (Huang et al., 1986, Zhang andShen, 1987).However, most studies involving restorers of WA cytoplasm suggest the presence of two pairs of independent, dominant fertility restorer genes with one gene pair being stronger in action than the other (Yang and Lu, 1984k, Li and Yuan, 1986, Zhang and Lu, 1987, Raj and Virmani, 1988, Muker 1990).The maintenance and transference of CMS within natural population of wild rice cannot be separated from Rf and it is easy to assume that the restorer genes exist in wild rice.However, the information about the origin, evolutionary relationships and distribution of the fertility-restorer genes for WA-CMS system is fragmentary.To be able to recognize the Rf in wild rice would facilitate not only the exploitation of new Rf alleles but also give a better understanding of the origin and evolution of the fertility-restorer genes.With this view, the present study was undertaken to determine the genetic control of fertility restoration of WA-CMS system.

MATERIALS AND METHODS
The experiment was conducted at the experimental field of Bangladesh Rice Research Institute (BRRI), Gazipur.Three consecutive seasons such as Boro 2008-09, T. Aman 2009 and Boro 2009-10 has been deployed for this experiment.The material consisted of one male sterile line Jin23A belonging to WA type source of cytoplasmic male sterility and ten prospective restorer lines.The restorer lines were BR6839-41-5-1R, BR7013-62-1-1R, BR7011-37-1-2R, BR10R, BR11R, BR12R, BR13R, BR14R, BR15R and BR16R.First season F 1 was raised.The resulting F 1 's were selfed as well as backcrossed with their respective female parents to generate F 2 and BC 1 populations in the second season and final season evaluation was made with pollen and spikelet fertility of each plant in the F 2 and BC 1 populations.From each plant, five anthers from different spikelets were collected and their pollen grains were stained in 1% Iodine Potassium Iodide (IKI) solution.Plants were classified on the basis of pollen fertility analysis as fertile (61-100% pollen stained), partially fertile (31-60%), partially sterile (1-30%) and sterile (<1%) based on their shape and extend of staining under an optical microscope.One panicle from each plant was bagged before flowering for spikelet fertility analysis and spikelet fertility of bagged panicle was count at maturity stage.At maturity, the bagged panicle was examined for seed set and classified as fertile (81-100% seed set), partially fertile (31-80%), partially sterile (1-30%) and sterile (<1%).The parental lines, F 1 progenies, 250 F 2 plants for each segregating population were grown and evaluated in the same conditions for phenotypic and pollen fertility rate was used as the main criteria for the evaluation of fertile and sterile plants.F 1 's contained two rows, F 2 seven rows, BC 1 five rows with 37 plants/row and non replicated.For inheritance analysis plants with less than 1% stained pollen were categorized into the sterile class, and all others were regarded as fertile.Chi-square analysis was used to estimate the distribution pattern of Rf alleles with WA type source of CMS lines.Measurement of pollen fertility was made on all plants in each of the entries.Twenty to thirty spikelets were collected from each primary panicle and fixed in 70% alcohol.From these spikelets 10-12 anthers were collected at random and smear in IKI (1%) and examined under optical microscope.Plants were classified on the basis of pollen fertility analysis as fertile (61-100% pollen stained), partially fertile (31-60%), partially sterile (1-30%) and sterile (<1%) based on their shape and extend of staining under an optical microscope.The pollen fertility were computed and expressed in percentage for each F 1, F 2 and BC 1 population as follows:-

RESULTS AND DISCUSSION
Pollen fertility, spikelet fertility and standard error of mean of F 1 's along with their parents were shown in Table 1.Pollen fertility of F 1 ranged from 71-85 % and the highest was obtained from Jin23A/BR15R while the lowest from Jin23A/BR14R.On the other hand, spikelet fertility of the same cross combinations ranged from 71-82% and suggested that pollen fertility little bit differ with spikelet fertility at the time of maturity.It might be due to pollen abortion at different cell division stage.Pollen fertility and spikelet fertility of the parents ranged from 72-88 % and 71-82 % respectively.According to pollen and spikelet fertility classification category all entries fallen into fertile class.
Segregation patterns for pollen fertility in F 2 population of Jin23A with ten restorer lines are stated in Table 2.The pollen fertility results indicated that relatively few genes were involved in fertility restoration in the crosses studied.Li and Zhu (1988) have reported that the Rf exist mainly in the varieties from Southern Asia and South-eastern Asia and the varieties from North America, Latin America and Africa have no restoring ability.For further confirmation of the segregation patterns, pollen fertility of BC 1 generations were also studied and presented in Table 3.
On the other hand, the crosses of Jin23A with BR7013-62-1-1R, BR15R and BR16R displayed epistasis with incomplete dominance as their F 2 and BC 1 populations segregated in 9:6:1 and 1:2:1 ratios, respectively, for full fertile, semi fertile and complete sterile plants.The cross Jin23A with BR12R showed segregation ratio in F 2 and BC 1 populations as 9:3:4 and 1:1:2 indicated the pattern of recessive epistasis.This finding is in line with the findings of Sohu and Phul (1995) but cyto sources of the CMS line was Gambiaca.From this, it is clearly evident that WA sources of CMS line Jin23A followed similar pattern of fertility restoration system like Gambiaca sources of CMS lines.

CONCLUSION
Based on the results obtained, it was concluded that the inheritance of male sterility and fertility restoration was conditioned by a pair of alleles, dominant and recessive, at a single locus interacting with the male sterile cytoplasm, and that the mode of action of the two genes varied in different crosses revealing three types of interaction, i.e. epistasis with dominant gene action, epistasis with recessive gene action, and epistasis with incomplete dominance.

FF=
Full fertile, PF = Partial fertile, PS = Partial sterile, PF + PS = SF (Semi fertile), CS = Complete sterile Similar trend were observed in F 2 population with restorer line BR15R and BR16R.

Table 2 .
Segregation for pollen fertility restoration in F 2 populations involving WA type source of CMS line Jin23A FF = Full fertile, PF = Partial fertile, PS = Partial sterile, PF + PS = SF (Semi fertile), CS= Complete sterile

Table 3 .
Segregation for pollen fertility restoration in BC 1 populations involving WA sources of CMS line Jin23A