Positional cloning and characterization of the papaya diminutive mutant reveal a truncating mutation in the CpMMS19 gene.

The papaya diminutive mutant exhibits miniature stature, retarded growth and reduced fertility. This undesirable mutation appeared in the variety 'Sunset', the progenitor of the transgenic line 'SunUp', and was accidentally carried forward into breeding populations. The diminutive mutation was mapped to chromosome 2 and fine mapped to scaffold 25. Sequencing of a bacterial artificial chromosome in the fine mapped region led to the identification of the target gene responsible for the diminutive mutant, a gene orthologous to MMS19 with a 36.8 kb deletion co-segregating with the diminutive mutant. The genomic sequence of CpMMS19 is 62 kb, consisting of 20 exons and 19 introns. It encodes a protein of 1143 amino acids while the diminutive allele encodes a truncated protein of 287 amino acids. Expression of the full-length CpMMS19 was able to complement the thermosensitive growth of the yeast mms19 deletion mutant while expression of the diminutive allele resulted in increased thermosensitivity. Over-expression of the diminutive allele in Arabidopsis met18 mutant results in a high frequency of seed abortion. The papaya diminutive phenotype is caused by an alteration in gene function rather than a loss-of-function mutation. SCAR (sequence characterized amplified region) markers were developed for rapid detection of the diminutive allele in breeding populations.