Morphological Markers as a Tool for Investigating Genetic Diversity of Some Lime Genotypes

Citrus fruits are the most common semi-tropical crops in the world. Acid lime (Citrus aurantifolia Swingle) is an important commercial fruit crop, cultivated in the south of Iran. High variation of acid lime fruits is observed in the south of Iran due to crossing within the other citrus species and frequent bud mutation. Recently, Witch’s Broom Disease of Lime (WBDL) become a major limiting factor for lime production in the South of Iran  as well as the main threating factor for lime industry in Iran. Having knowledge about the genetic of this plant is helpful for designing citrus breeding program. Therefore, in this research morphological traits were used to understand the genetic relationships and diversity of this gene pool.
Thirty citrus samples, including 23 undefined local and native genotypes and seven known cultivars were collected from four regions in Iran (Darab (54.53E, 28.75N), Manojan (57.49E, 27.40N), Minab (57.07E, 27.14N) and Ramsar (50.64E, 36.92N)). Forty-one (32 qualitative and nine quantitative) traits were investigated using leaves, fruits and seeds. The selection of morphological traits were made based on IPGRI descriptors. The similarity was calculated by simple matching coefficient and dendrogram was designed based on UPGMA algorithms. Principal components analysis was performed.
Genetic diversity of 23 lime genotypes and seven commercial cultivars were investigated by using 41 morphological characters. Based on gained data similarity matrix (Jaccard, Dice and Simple maching) were calculated and dendrogram based on UPGMA algorithms were designed. To finding better similarity coefficient, cophenetic test was done, it showed that if used SM coefficient 79 percent of data of similarity matrix was shown in designed cluster based on UPGMA algorithm. According to results, range of similarity was between 0.141 until 0.683. Maximum similarity was observed between two lime cultivars (Persian lime and Mexican lime). Average of similarity was calculated 0.39. In cluster analyses of studied samples at 32 level of similarity samples were divided into four main groups. First group has only one member and it was pomelo, as it is one of the ancestors of citrus fruits, this is quite acceptable. The notable point for second main group that it included all examined samples of Minab. As in this region most of cultivated citrus are lime, there is little gene mixing with other citrus. The third group consist of two cultivars sweet orange and Citon that intended in present investigation. Fourth group which was the biggest created groups that included more than half the samples (56%) was more complex because it consists of samples of limes and lemons that intended in investigation as control and samples of Darab and Manojan. This main group at level of 0.38 divided into three sub-groups, at first, Lisbon lemon was separated from other samples and next Manojan samples created their special sub-group and finally third sub-group which consist of 11 members (samples of Darab, MA6 from Manojan and three commercial cultivars, Persian lime, Mexican lime and Rough lemon). Principle Component Analysis (PCA) showed the first five principal components, which contributed 59.01% of the total variability of investigated samples. Maximum variability was contributed by the first component (22.77%) followed by the second component (12.54%), and the third component (9.85%). A two‐dimensional plot (2D plot) generated from PCA showed three groups. This grouping was roughly in line with the distribution of the samples in the resulting cluster analysis based on SM coefficient and UPGMA algorithm. Principle Component Analysis using 41 descriptors showed that 26 of 41 descriptors were informative and contributes significantly to the variation present in the germplasm.
This study described and estimated the extent of phenotypic variation present among the samples of limes from Iran germplasm. Morphological analyses among 23 genotypes and seven commercial cultivars from four regions of Iran were successfully used to calculate genetic diversity and genetic relationships. According to our results, it was confirmed that morphological analyses in limes and other Citrus species were exploit to determine genetic diversity and relationship, successfully. Characterization by using morphological descriptors based on 41 characters, revealed significant diversity in traits of leaf, fruit and seed. This investigation display the use of morphological characters to study genetic diversity of Iranian lime genotypes from four different regions that their relationships were somewhat clarified. The results of this study also opened a door to tackle the long standing problem of citrus classification and identification in Iran. But, we suggest that this type of study needs to be continued due to Iran has a very large and numerous citrus germplasm. In south and central regions of Iran, it is being propagated by seed which gives researchers a chance to find new genotypes that need to be classified, investigated and introduced as a new cultivar.