| Non-heading Chinese cabbage(Brassica campestris syn. rapa ssp. chinensis Makino) is one of the most important vegetable crops in the Brassica genus. Non-heading Chinese cabbage, known as Pak Choi, originated in China, is among the most popular vegetables crops in eastern Asia like China, Korea and Japan and is now common in Europe and America. Like the other Brassicas, non-heading Chinese cabbage is a good source of nutrients and has the characteristics of extensive adaptability and fast growth. Many important agronomic traits are quantitative in nature and have a complex genetic basis. The identification of quantitative trait loci (QTL) represents a first step towards dissecting the molecular basis of such complex traits. A pre-requisite for QTL mapping studies is the availability of genetic maps. Its molecular marker-based linkage map construction and QTL mapping will provide an available reference to genomic structural study and genetic breeding in non-heading Chinese cabbage. In this study, several doubled haploid (DH) populations of non-heading Chinese cabbage were obtained through isolated microspore culture, and then used for molecular genetic linkage map construction, and mapping QTL for several important agronomic traits. The results are as follows:1. Construction of a Genetic Linkage Map Using DH Population in Non-heading Chinese CabbageIn this chapter, a DH population including 164 lines was used to construct a detailed genetic map to establish the identity of linkage groups corresponding to reference map of Brassica A genome. A set of simple sequence repeats (SSR) markers provided anchors to previously published linkage maps for B. campestris and B. napus, and was used to designate linkage groups. Our selection of primer pairs corresponded to 231 genetic loci that we were able to map. The resulting consensus map presented 10 linkage groups ranging from 39.44 to 126.66 cM, including 99 SSR markers,47 sequence-related amplified polymorphism (SRAP) markers. The total length of the map was 678.25 cM and the average interval distance of the linkage map was 4.65 cM. The number of markers on the 10 linkage groups ranged from 5 (A10) to 50 (A02), with an average interval distance from 2.53 cM (A02) to 10.98 cM (A07). A map interval with the markers separated more than 20 cM was observed on A02 and A05, respectively. Accordingly, these linkage groups were named following the A01 to A10 of reference map, and were assigned to corresponding chromosomes. This map could be used to identify more markers, which would eventually be linked to genes controlling important agronomic characters in non-heading Chinese cabbage. Furthermore, considering the good genome coverage we obtained, together with an observed homogenous distribution of the loci across the genome, this map is a powerful tool to be used in marker assisted breeding.2. Quantitative Trait Loci for Photosynthetic Pigment ConcentrationQuantitative trait loci (QTL) mapping and marker development are essential steps in any molecular breeding programme. In this study, QTL for photosynthetic pigment concentration were identified using a DH population of 112 lines derived from an Fi hybrid of non-heading Chinese cabbage,’Shulv’, obtained from a cross between Brassica campestris lines SW-13 and SU-124 through microspore culture. QTL mapping was carried out using a modification to the map of’Shulv’. Alignment of the linkage groups was carried out with those of reference B. campestris maps, allowing the assignment of these groups as A01-A10. QTLs underlying photosynthetic pigment concentration in leaf were mapped with composite interval mapping (CIM) method, the numbers, location, explained variation and additive effect of QTLs were determined. In total,11 QTL were associated with photosynthetic pigment concentration and identified on linkage groups A02, A08, and AlO.The maximum phenotypic variance for a single QTL was 38.0%. Markers closely linked to these QTL could assist in the development of non-heading Chinese cabbage cultivars with increased yield, due to having improved photosynthetic capabilities. The objectives in this study were to investigate QTL underlying photosynthetic pigments concentration and to facilitate breeding for high photosynthetic efficiency through marker assisted selection (MAS) in non-heading Chinese cabbage and, through homology based approaches other Brassica crops.3. Mapping QTL for Bolting and Flowering Time TraitsEarly bolting of non-heading Chinese cabbage during spring cultivation often has detrimental effects on the yield and quality of the harvested products. Breeding late bolting or flowering is a major objective to enable year-round production for the non-heading Chinese cabbage. With CIM method, a genetic linkage map of non-heading Chinese cabbage were adopted to map and analyse QTL controlling the days of bolting and flowering. This study was based on a DH population with 164 lines in two years. The results as follows:19 putative QTLs, including 5 for days of squaring,6 for days of bolting, 6 for days of flowering,2 for bolting index were major mapped on A02, A04 and A05 linkage groups. There were unequal gene effects on the traits and unequal variation explained on the expression of the bolting and flowering time traits. These QTLs individually explained between 10.26% and 21.19% of the phenotypic variation. The QTL of associated traits often located on the same loci or near region of a linkage group. These mapped QTL could be used to marker assisted selection program for the bolting and flowering time traits in non-heading Chinese cabbage in the future.4. Mapping and Analysis of QTL Controlling Yield ComponentsA SSR and SRAP genetic linkage map with 146 markers and a DH population with 164 lines were employed in mapping and analysis quantitative trait loci. The number, location, variation explained and additive effect of QTL underlying yield trait were determined by using CIM method with software Win QTL Cartographer 2.5. The results as follows:26 putative QTLs, including 4 for ratio of blade and petiole,5 for petiole thickness, 3 for blade weight,2 for petiole weight,2 for plant weight were major mapped on A02, A05 and A07 linkage groups. There were unequal gene effects on the traits and unequal variation explained on the expression of the yield trait. These QTLs individually explained between 5.28% and 22.26% of the phenotypic variation. Co-location of QTL for different traits was found in many cases, which might suggest pleiotropy or tight linkage. These mapped QTL could be used to marker assisted selection programme for the yield trait in non-heading Chinese cabbage in the future. |
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