Genetic diversity analysis in a set of Caricaceae accessions using resistance gene analogues

In order to assess genetic diversity of a set of 41 Caricaceae accessions, this study used 34 primer pairs designed from the conserved domains of bacterial leaf blight resistance genes from rice, in a PCR based approach, to identify and analyse resistance gene analogues from various accessions of Carica papaya, Vasconcellea goudotiana, V. microcarpa, V. parviflora, V. pubescens, V. stipulata and, V. quercifolia and Jacaratia spinosa.

Of the 34 primer pairs fourteen gave amplification products. A total of 115 alleles were identified from 41 accesions along with 12 rare and 11 null alleles. The number of alleles per primer pair ranged from 4 to 10 with an average of 8.21 alleles/ primer pair. The average polymorphism information content value was 0.75/primer. The primers for the gene Xa1 did not give any amplification product. As a group, the Indian Carica papaya accessions produced a total of 102 alleles from 27 accessions. The similarity among the 41 accessions ranged from 1% to 53%. The dendrogram made from Jaccard's genetic similarity coefficient generated two major clusters showing that the alleles of Jacaratia spinosa and Vasconcellea accessions were distinctly different from those of Carica papaya accessions. All the alleles were sequenced and eleven of them were allotted accession numbers by NCBI. Homology searches identified similarity to rice BLB resistance genes and pseudogenes. Conserved domain searches identified gamma subunit of transcription initiation factor IIA (TFIIA), cytochrome P450, signaling domain of methyl-accepting chemotaxis protein (MCP), Nickel hydrogenase and leucine rich repeats (LRR) within the sequenced RGAs.

The RGA profiles produced by the 14 primer pairs generated high genetic diversity. The RGA profiles identified each of the 41 accessions clearly unequivocally. Most of the DNA sequences of the amplified RGAs from this set of 41 accessions showed significant homology to the conserved regions of rice bacterial leaf blight resistance genes. These information can be used in future for large scale investigation of tentative disease resistance genes of Carica papaya and other Caricaceae genus specially Vasconcellea. Inoculation studies will be necessary to link the identified sequences to disease resistance or susceptibility.