QTL Analysis Of Characters Related To Watermelon Rind Hardness

Fruit cracking is a complex physiological disease occurring at any stage of growth and ripening.It is mainly manifested as rind cracking and seriously restricts the quality and yield of fruits such as watermelon.The outer layer of rind as an important organizational structure,in the process of fruit harvest and stored the most susceptible to mechanical damage.In the previous studies,it has been found that high hardness rind tend to have better ability of crack resistance and leakage.Watermelon fruit rind also provides one of the most direct evaluation criteria for consumers acceptability during in commercial market.The scientific criteria for evaluation of watermelon rind hardness and its closely associated trait is less known.In this study,an F₂population of watermelon was obtained from selfing of F₁generation which was derived from crossing of“Little Watermelon-4”（female parent having hard rind fruit）and“Short Shoots-812”（male parent having soft rind fruit）.The quality,structure and data of reliable rind hardness and toughness was obtained through P/2 probe instrument by applying the puncture force on fruits of parental lines,F₁,and F₂generation.From the field investigation,we obtained data which affected the rind hardness and its related traits,and further analysis of correlation and QTL mapping was carried out.The obtained results are as follows:（1）The paraffin embedded section disclosed that P₁（“Little Watermelon-4”）had thinner and closer cell structure and P₂（“Short Shoots-812”）had thicker and looser arrangement of cell structures,while F₁hybrid exhibited medium type cell structure（neither wider nor closer）.The F₂samples disclosed different size of cell structure which also explained the rind hardness trait by distinct skin rupturing forces（g/cm²）.Mainly,the cell structure size and arrangement of fruit rind affected the all related phenotypes（rind-hardness,rind toughness,rind thickness,and fruit weight）.（2）The Correlation analysis showed that the rind hardness of F₂material was significantly positively correlated with rind toughness and single fruit weight（correlation coefficient was 0.24and 0.33,respectively）;the rind hardness was positively correlated with rind thickness（correlation coefficient was 0.06,not significant level）;the rind thickness was significantly negatively correlated with rind toughness（correlation coefficient was-0.23）.（3）BSA-seq method was used for preliminarily mapping of genetic region controlling rind hardness within the range of 1,792,001 bp to 4,036,000 bp（about 2.24 Mb）on chromosome 10.（4）Genetic linkage mapping was done by genotyping of 133-polymorphic CAPS markers within 133-F₂mapping individuals.The constructed linkage map covered 11 chromosomes which spanned total 2,606.38 c M genetic length and averaged 19.60 c M genetic distance between the whole genome adjacent CAPS markers.（5）Two QTLs of rind hardness（major“RH10”and minor“RH9”）were located at chromosome 10 and chromosome 9.The major QTL“RH10”was consistent with the location mapped by BSA-seq method and successfully proved the reliability of“RH10”site.For the first time,one QTL of watermelon rind toughness“RTO10”was mapped and closely located with RH10site.The QTL“RH9”was located near the genetic position of fruit weight QTL“FW9”at same chromosome 9.However,a single QTL of rind thickness“RTH2”affecting rind thickness was situated on chromosome 2.（6）We annotated the genes within the main genetic region of rind hardness“RH10”in a range of 292.76k bp,and found some genes that might be related to the formation of rind structure and cell wall development.In conclusion,the results of this study enrich the study of watermelon rind hardness and provide more ways to promote molecular improvement of watermelon with high hardness rind.