Inheritance And Heterosis For Boll Weight In Upland Cotton

Cotton is an important economic crop in China.Cotton fiber is one of the main sources of daily textiles,and it plays an important role in national economy and people's life.In the Yangtze River Basin,hybrid cotton has been used for nearly 20 years and planting proportion has reached more than 95%.The utilization of heterosis has made great contribution to the yield increase of cotton.The harvested organ of cotton is boll.In cotton breeding,however,the studies on this trait need to be addressed,for example,finding out the relationship among boll traits,selecting the combinations which are expected to have excellent performance on boll weight according to parental boll type,understanding the genetic mechanism of strong heterosis,and mapping quantitative tarit loci(QTL)for general combining ability and heterosis of boll weight.If doing so,it is helpful to improve the selection efficiency for boll weight traits,and also beneficial to combine many excellent boll traits together.Further,we may improve the breeding efficiency of yield traits in upland cotton.Therefore,we carried out the below studies:1)using diallel cross design ? and NC ? design,33 hybrid combinations between large and large bolls,between large and medium bolls,and between large and small bolls were made,and measured in 2012 and 2013 for the traits related to boll weight.The measured phenotypes were used to understand their genetic foundations using the ADE model approach;2)among all the 33 F1 hybrids,three over-parent heterosis combinations were used to construct the P1,P2,F1 and F2 populations,and these populations were jointly analyzed by mixed major genes plus polygenes inheritance analysis in order to dissect the genetic basis for heterosis;and 3)all the F1 hybrids along their parents were used as association population so as to detect QTL for boll weight,general combining ability,special combining ability and mid parent heterosis,the purpose was to mine the genes for heterosis.The main results were as follows:1.The genetic mechanism of boll weightUsing diallel cross design IV,15 hybrid crosses were made from six upland cotton lines with above medium boll,meanwhile using NC II design,18 crosses between these six lines and other three lines with small boll were made.These two experiments were carried in 2012 and 2013 in order to measure the phenotypic values for boll length,boll width,boll shell weight,boll shell ratio,seed index,seed number per boll and boll weight.These traits were analyzed by the ADE model so as to dissect the genetic foundations and herosis of these traits and to understand the relationship between these traits and boll weight.As a result,these traits were significantly positively related to boll weight,being the indirectly selected traits for boll weight;the boll weight was controlled by additive and dominance effects,and the additive effect was larger than the dominance effect;and maximum super parent heterosis was derived from the crosses between two big boll parents,such as the boll weight of 7.05 g and super parent heterosis of 16.69%for L178 × L029,40%crosses between big and medium boll parents have super parent heterosis,and super parent heterosis is unlikely to occur between big and small boll parents2.Genetic basis of the super parent heterosis for boll weightIn 2012,three cross combinations,L178 × L029(I),L178 × L057(II)and L029 ×GP72(?),had super parent heterosis.In each of the three cross combinations,P1,P2,F1 and F2 population were constructed and used to carry out mixed major genes plus polygenes inheritance analysis.The results showed that the boll weight and boll shell ratio were controlled by two major genes and polygene,although their main gene action modes were different in crosses.The super parent heterosis for boll weight was derived from the dominance effect of polygenes or the dominance-by-dominance major gene interaction,and the negative mid-parent heterosis for boll shell ratio was from the effects of polygenes or major genes.3.Mapping the QTL of boll weightThe above 33 F1 hybrids along with their 9 parents were used as a association population in order to perform genome-wide association studies for the boll weight phenotype,general combining ability,special combining ability and mid-parent heterosis using multi-locus method.As a result,two additive QTL linked to markers NAU3309 and NAU5702 and three dominance QTL close to markers dPL0613,NAU3309 and NAU3541 were significantly associated with boll weight;one general combining ability additive QTL associated with NAU3309 was detected;and two special combing ability additive QTL associated with NAU3309 and NAU5702,and two special combing ability dominance QTL associated with NAU3309 and NAU3541 were detectded;and one mid-parent heterosis additive QTL associated with NAU3309 and one mid-parent heterosis dominance QTL associated with dPL0613 was detected.Among these QTL,NAU3309 was associated with all the above four indicators.The results can provide theoretical support for the heterosis of reasonable choice with parents and molecular marker assisted breeding.