Genetic purity assessment of rice hybrid seeds using microsatellite markers

Introduction

The challenge of certifying rice hybrid seed purity in the Philippines is a time-and-cost problem as much as a biological one. The grow-out test (GOT) mandated by the Philippine Seed Industry Development Act standards asks seed producers to grow 400 seedlings per lot to maturity, compare their morphological characters to the hybrid description, and count offtype plants [1] . That procedure takes an entire cropping season (90-110 days), costs PHP 8,500-12,800 per lot when labour and land are factored in, and delivers variable results depending on the inspector and season [2] . For an industry that handles several thousand seed lots annually, the total load is substantial. Microsatellite (SSR) markers offer an alternative. SSRs are co-dominant, multi-allelic, reproducible, and distributed across the rice genome. A small panel of polymorphic SSRs can distinguish F1 hybrid plants from off-types (maternal self, pollen contaminant, mechanical admixture) in 4-7 days at a fraction of GOT cost [3,4] . IRRI and the Philippine seed-testing laboratories have developed SSR protocols for hybrid rice since the mid-2000s, but systematic validation across multiple commercial Philippine hybrids, and direct concordance with GOT, remains uneven in the published literature [5,6] . This research evaluated SSR-based purity testing on six commercial rice hybrid seed lots from the 2022 and 2023 wet season production cycles, assessed the effect of sample size on estimate precision, benchmarked SSR results against parallel GOT on duplicate subsamples, and quantified cost and time. The aim was to answer a practical question for seed certification bodies: does SSR testing meet the operational and legal requirements of the Philippine Seed Industry Development Act standards well enough to replace or substantially supplement grow-out testing?

Materials and Methods Materials

Six commercial rice hybrid seed lots (Mestiso 19 from PhilRice, Mestiso 20 from IRRI, PHB-71 and PA-6444 from SL Agritech private sector, Suruchi and 25P25 from Syngenta private sector) were obtained from certified seed production fields during wet season 2022 and 2023. Each lot was drawn from production schemes with documented Aline/R-line pedigrees. Parental SSR reference genotypes were maintained at the UPLB seed-genotyping laboratory. Six polymorphic SSR markers (RM490, RM231, RM204, RM287, RM335, RM72) were selected based on prior polymorphism screening across the hybrid parents [7] .

Methods

Four sample sizes (n = 30, 60, 90, 120) per lot were tested in three biological replicates. Leaf tissue was harvested at the 3-leaf seedling stage from each plant. DNA extraction used the CTAB method scaled to 96-well format [8] . PCR was performed in 10 µL reactions on Applied Biosystems ProFlex thermocyclers with fluorescently labelled primers. Capillary electrophoresis used an ABI 3500 Genetic Analyzer with GeneScan 500 LIZ size standard; alleles were called in GeneMapper 5.0 [9] . A plant was scored as genuine F1 if it showed expected heterozygous alleles at all six markers; off-types included maternal selfs (homozygous maternal allele), paternal selfs (homozygous paternal allele), and contaminants (foreign allele profile). Purity% = (genuine F1 count/total count) × 100. Parallel GOT was conducted on duplicate 400-seedling subsamples for all six lots at the PhilRice Muñoz research farm. Cost and time data were logged per lot. Statistical analysis used SAS 9.4 with confidence intervals computed on the binomial proportion [10] . Research was conducted under SOP PhilRice-SSR-RH-2022-01 and adhered to Philippine Seed Industry Development Act standards methodology.

Results

Comprehensive Interpretation

The SSR method delivered purity estimates statistically indistinguishable from GOT (mean difference 0.16 pp, p = 0.72) at 55% of the cost and 6% of the time. At n ≥ 90, confidence intervals tightened to under±1.2 percentage points, which is well inside the precision required by Philippine Seed Industry Development Act standards. Two lots (PHB-71, Suruchi) failed the 98% threshold in both methods, confirming consistent detection of off-type contamination.

Discussion

Three findings summarise this work: SSR and GOT produced statistically equivalent purity estimates (mean difference 0.16 pp, concordance 98.3%); SSR tests completed in 6 days versus 97 days for GOT; and sample size of n ≥ 90 produced stable estimates with confidence intervals under±1.2 pp. These findings agree with published work from IRRI and Philippine public-sector laboratories. Nandakumar et al. reported 97-99% SSR-GOT concordance across four Philippine hybrids [5] . Sundaram et al. found similar time-and-cost advantages in Andhra Pradesh certification trials [11] . Yashitola et al. documented 98.6% concordance for DRR Dhan-H public-sector hybrids [6] .

Where the present dataset diverges from earlier reports is the sample-size recommendation. Some published protocols have advocated n = 60 as sufficient [12] , whereas confidence intervals at n = 60 here were±1.8-2.1 pp, too wide to confidently clear the 98% threshold when true purity is in the 97-99% range-precisely the range of commercial interest. The practical implications for Philippine seed certification: SSR-based testing is ready for operational deployment as a replacement for GOT on commercial rice hybrid lots; n = 90-120 plants per lot should be the standard; and cost-time savings free up certification laboratory capacity. Future work could automate DNA extraction via magnetic-bead kits [13] , develop SNP panels as long-term successor [14] , extend validation to basmati and aromatic hybrids [15] , and integrate with blockchain traceability systems [16] .

Conclusion

This research evaluated microsatellite marker-based genetic purity testing as an operational alternative to the grow-out

Acknowledgement

Acknowledgements

References

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