Molecular Analysis of the Papaya Diminutive Mutant Revealed a Large Deletion in CpMMS19 Gene

Papaya diminutive mutant exhibits miniature stature, retarded growth, and reduced fertility. The diminutive mutation appeared in the variety Sunset, the progenitor of the transgenic line SunUp, which is a parent of the PRSV-resistant hybrid cultivar Rainbow, the leading commercial cultivar in Hawaii. An F₂ mapping population was developed from a cross between UH918 and SunUp diminutive mutant for characterization of the diminutive mutant using a map-based cloning approach. All F₁ plants were phenotypically wild-type, indicating the diminutive mutant is caused by a recessive mutation. However, 215 F₂ plants segregated into 191 wild-type and 24 diminutive plants. Using the mapping population and the physical map of the papaya genome, a 36.8 kb deletion was co-segregating with the diminutive mutant. This deletion interrupted the functions of a gene orthologous to MMS19, named CpMMS19. MMS19 has been characterized in yeast and mammals, playing essential roles in DNA replication and repair. The abnormal segregation ratio of the F₂ mapping population is likely caused by the low survival rate due to loss of function of CpMMS19 genes. The CpMMS19 gene consists of 20 exons and 19 introns, covering a genomic region of 62kb with 2 introns larger than 10kb. We mapped the CpMMS19 gene on chromosome 4 (LG2) and the deletion is between introns 8 and 18. RT-PCR result showed CpMMS19 was widely expressed in papaya tissues and 12 splicing variants of the CpMMS19 gene were observed in the papaya leaf tissue. Transformation of CpMMS19 alleles into yeast MMS19Δ cells is ongoing for functional analysis of CpMMS19 alleles.