A new wild type of Citron (Citrus medica L., Rutaceae) identified through morphology and psbM-trnD spacer region of chloroplast DNA

Abstract

This study showed that Themachhi is genetically very close to C. medica and consistently grouped with the typical citron in all the phylogenetic trees; hence revealed the identity of ‘Themachhi’ as a natural wild type of C. medica. Taxonomic identity and well-supported phylogenetic relationships of doubtful and taxonomically uncertain taxa are highly requisite for developing crop improvement programmes and suitable conservation strategies. In Citrus, several natural varieties and hybrids of ambiguous origin have been existing due to several factors such as frequent intergeneric and interspecific hybridization, apomixis, bud mutations, polyploidy and diverse eco-geographical distribution. In Northeast India, an important unidentified type of Citrus, i.e. ‘Themachhi’ is found naturally in Garo Hills, Meghalaya, which is morphologically very close to C. medica and C. limon. The sequences of chloroplast non-coding region psbM-trnD of 16 accessions of Citrus were utilized to confirm the taxonomic identity of ‘Themachhi’ and also to infer its phylogenetic affinities to related taxa. The nucleotide frequencies were 32.86 % for A and T and 17.4 % for C and G. Pairwise genetic divergence between the accessions of Citrus ranged from 0.002 to 0.036 with an average of 0.017. ‘Themachhi’ showed least genetic divergence with C. medica (0.013) and highest divergence with C. limon (average 0.024). All the accessions of Citrus were grouped into three main clusters (C. medica, C. reticulata and C. maxima clusters) in both the MP and ML trees with high bootstrap value (>60 %). In the MP and ML trees, both accessions of ‘Themachhi’ were closely attached with C. medica cluster with high bootstrap value (98 %). Close genetic affinity and consistently grouping of “Themachhi” with typical citron (C. medica) in both the phylogenetic trees revealed the identity of ‘Themachhi’ as a natural wild type of C. medica.