Rice Genetics IV - IRRI books - International Rice Research Institute

Search for apomixis in riceThere are many reports of apomixis in rice (Guo 1991, Chen 1992, Tan et al 1994, Yeet al 1994, 1995, Mu et al 1996, Yao et al 1997) but they remain to be validated. Thecrucial criterion for declaring apomixis is the demonstration, preferably through theuse of molecular markers, that progeny are identical in genotype to the apomicticparent. Rigorous progeny analysis has not been performed for the presumed apomicticlines of rice; evidence for apomixis has been based on microscope observations ofvarious abnormalities in the sexual reproductive pathway. Most intensively studied iscultivar 84-15. Chen (1992) gathered together 39 papers describing aspects of thehistology, genetics, and agronomy of 84-15, including the report that nucellar embryoscompete with the zygotic embryo for dominance. Rigorous progeny analysis isneeded. Shi et al (1996) examined 84-15 for the abnormalities reported by Chen (1992).They found megasporogenesis and embryo sac development to be normal in 97.3%of 1,380 ovules. Fertilization and embryo and endosperm development were alsonormal. They concluded that 84-15 is not an apomict.All validated, naturally occurring apomictic species of the Poaceae are polyploid(Huang and Chen 1999). For this reason, when IRRI’s germplasm collection wasexamined for evidence of apomixis, polyploid accessions were included along withdiploid accessions. The cytological search failed to reveal any abnormal structuressuggestive of apomixis in diploid or polyploid wild species of rice (Brar et al 1995).Synthetic apomixis for hybrid riceStrategies for achieving apomixis in riceIn the past decade, several laboratories embarked on programs to introduce someform of apomixis into rice. Han et al (1999) attempted to transfer apomixis to ricefrom Panicum maximum by protoplast fusion. Protoplasts were derived from suspensioncells of japonica line 02428, inactivated with iodoacetamide, and fused withprotoplasts of P. maximum that had been treated with soft X-rays. Southern hybridizationconfirmed the somatic hybrid character of regenerated plants, but an analysisof apomictic character was not reported. Most other laboratories have focused on thetransfer of isolated genes through genetic engineering (Peacock 1992, Jefferson 1994).Peacock (1992) proposed a genetic engineering and mutagenesis approach based onthe assumption that there is one major gene (a putative “embryo sac induction” gene)with two allelic forms controlling apomixis. The sexual allele of this gene would beswitched on in the megaspore after meiosis, but the apomeiosis allele would be expressedearly, prior to meiosis, in nucellar cells. Peacock (1992) hypothesized that amutation equivalent to the apomeiosis allele could be identified if an appropriatehigh-throughput genetic screen could be devised. He proposed that Arabidopsisthaliana would offer major advantages in the search for the relevant gene. Subsequentresearch on Arabidopsis led to the isolation of mutants displaying fertilizationindependentseed (fis) production (Peacock et al 1995) and fertilization-independentendosperm (fie) production (Ohad et al 1996) and isolation of the corresponding genes(Luo et al 1999, 2000, Ohad et al 1999, Vinkenoog et al 2000). Howden et al (1998)382 Bennett et al