| Heterosis is that the hybrid shows better performance than its homozygous parents do. It is widely exploited in crop plants; however, the molecular genetic mechanisms remain enigmatic. The development of molecular quantitative genetics has facilitated the study of the genetic basis of heterosis in crops based on molecular analysis. Significant heterosis exists in Upland cotton(Gossypium hirsutum) for yield, as well as yield components. With the development of molecular quantitative genetics, new methods and ideas were employed to study the genetic basis of heterosis. To elucidate the genetic basis of Upland cotton heterosis, we developed two recombinant inbred lines (RILs) populations originated from two hybrids, one is XZ hybrid (Xinza No.1) and the other one is XZV hybrid, which shared common female parent, and two corresponding backcross populations based on two recombinant inbred lines (RILs) populations were backcrossed with common female parent. These four populations were planted in three environments in two replicates, respectively and conducted quantitative trait loci (QTL) genetic analysis for yield traits and heterosis performance, boll number per plant (BNP), fiber quality traits under multiple environmental conditions. The genetic effects of both the homozygous and heterozygous genotypes were explored using composite interval mappingand inclusive composite interval mapping. This study will provide new insights into our understanding of the genetic basis of heterosis and dynamic heterosis in Upland cotton. The main results are as follows:1. Genetic maps for the two hybrids were constructed based on the polymorphic loci. For XZ hybrid, the genetic map with 623 loci spanned 3889.9 cM. For XZV hybrid, the genetic map with 308 loci spanned 3048.4 cM. The yield and yield components of these populations were evaluated in three environments. At the single-locus level, a total of 78 and 66 QTL were respectivelydetected using composite interval mapping in RIL populations and BC populations, and 29 QTL were identified based on mid-parental heterosis (MPH) data. Considering all traits together, a total of 50 (64.9%) QTL with partial dominance effect and 27 (35.1%) QTL for overdominance effect were identified in two BC populations. At two-locus level,120 and 88 QTL with main effects (M-QTL), and 335 and 99 QTL involved in digenic interactions (E-QTL) were detected by inclusive composite interval mapping in RIL populations and BC populations, respectively. A large number of QTL by environment interactions (QEs) for M-QTL and E-QTL were detected in three environments. For most of traits, E-QTL averagely explained a larger portion of phenotypic variation than what M-QTL did in two RIL populations and two BC populations. It was concluded that partial dominance, overdominance, epistasis and QEs contributed to the heterosis in Upland cotton, in which partial dominance of single-locus level and epistasis of two-locus level played a relatively more important role than other genetic effects in the heterosis in Upland cotton.2. Three obvious features of heterotic QTL (hQTL) were observed. These were that the hQTL are sensitive to the environmental conditions, the effect of hQTL is pleiotropic, and hQTL in the BC population were not independent, which suggested that phenotypes and heterotic traits might be jointly controlled by multiple shared loci in Upland cotton.3. Dynamic QTL analysis for BNP and heterosis performance was mapped at multiple developmental stages and environments. By the single-locus analysis,23 QTL were identified at final maturity, while 99 QTL were identified across other three developmental stages other than maturity stage. Furthermore, QTL detected at later stage mainly existed in the partial dominance to dominance range and QTL identified at early stage mostly showed effects with the dominance to overdominance range during plant development.4. For most traits, a poor relationship was observed between heterozygosity of whole-genome and the performance of BC and MPH data in terms of yield and yield components in two hybrids. The result suggested that overall genome heterozygosity alone had little effect on trait performance.5. A host of epistatic interactions and QEs were observed with the three data sets in two hybrids. Universal epistasis played an important role not only in the variation of the performance of the RIL population but also in the expression of heterosis in BC population. Most digenic interactions that occurred were between complementary loci, with a few detectable main effects.6. Plant height served as model trait to analyse dynamic development. The results showed that the expression of QTL controlling plant height showed temporal characteristics during plant development. Dynamic single-locus effects are the main contributors to heterosis for plant height in Upland cotton. QTL identified at early stage mainly showed partial dominance effects, however, QTL detected at final stage mostly showed overdominance effects. And root served as another dynamic and development trait during plant development in recombinant inbred line populations of XZ hybrid.7. Additive effect of single-locus and epistasis with few detectable main effects played an important role in controlling fiber quality traits in Upland cotton. We detected some stable major QTL in multiple populations and environments, and these QTL provided reference for molecular marker assisted breeding. |
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