Phylogeny and Biogeography of Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] Based on Chloroplast, Nuclear Sequence and AFLP Molecular Marker Data

Abstract

Watermelons [Citrullus lanatus (Thunb.) Matsum. & Nakai], together with cucumbers, melons of various sorts, summer squashes, winter squashes and pumpkins are the principal food plants of the gourd family (Cucurbitaceae). The phylogeny of C. lanatus was estimated from separate and combined analysis of noncoding regions of chloroplast (trnS-trnG and trnR-atpA) and nuclear G3pdh sequence data and amplified fragment length polymorphisms (AFLP) marker data. Sequences from 18 taxa included in the study provided an aligned length of 869 bp for trnS-trnG and 610 bp for trnR-atpA (with total length of the chloroplast sequences of 1479 bp and the outgroup C. colocynthis (34256) sequence of 1442 bp). The sequenced G3pdh region covers one intron and a short section of the transit peptide region with the length of around 900 bp. Combined sequence analysis of cpDNA and G3pdh divides the C. lanatus accessions into 11different haplotypes. The cultivated watermelon, C. lanatus var. lanatus accessions, grouped into one major clade, the citron type, var. citroides, into another clade. Two distinct lineages within C. lanatus var. citroides were detected. Since accessions from southern Africa contain ancestral haplotypes and the highest frequency (44.4%) of different var. citroides haplotypes, it can be considered the area of origin for C. lanatus var. citroides, with colonization patterns to Zaire, USA, Europe, and India. Combined analysis of cpDNA and G3pdh sequences showed the accumulation of unique nucleotide substitutions in C. lanatus var. lanatus, suggesting that the two varieties (lanatus and citroides) diverged from a common ancestor. AFLP marker data also indicate low levels of genetic diversity, possibly as a result of the domestication of watermelon.