научная статья по теме THE GENETIC BASIS OF FLOWERING TIME AND PHOTOPERIOD SENSITIVITY IN RAPESEED (BRASSICA NAPUS L.) Биология

Текст научной статьи на тему «THE GENETIC BASIS OF FLOWERING TIME AND PHOTOPERIOD SENSITIVITY IN RAPESEED (BRASSICA NAPUS L.)»

ГЕНЕТИКА, 2008, том 44, № 3, с. 381-388

ГЕНЕТИКА РАСТЕНИЙ

УДК 575:582.683.2

THE GENETIC BASIS OF FLOWERING TIME AND PHOTOPERIOD SENSITIVITY IN RAPESEED (Brassica napus L.) © 2007 r. C. C. Cai, J. X. Tu, T. D. Fu, B. Y. Chen

Ntaional Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University, Wuhan, Hubei Province, 430070, China; e-mail: Tujx@mail.hzau.edu.cn Received September, 2006

The objective of this study was to dissect the genetic control of days to flowering (DTF) and photoperiod sensitivity (PS) into the various components including the main-effect quantitative trait loci (QTLs), epistatic QTLs and QTL-by-environment interactions (QEs). Doubled haploid (Dh) fines were produced from an F1 between two spring Brassica napus cultivars Hyola 401 and Q2. DTF of the DH lines and parents were investigated in two locations, one location with a short and the other with a long photoperiod regime over two years. PS was calculated by the delay in DTF under long day as compared to that under short day. A genetic linkage map was constructed that comprised 248 marker loci including SSR, SRAP and AFLP markers. Further QTL analysis resolved the genetic components of flowering time and PS into the main-effect QTLs, epistatic QTLs and QEs. A total of 7 main-effect QTLs and 11 digenic interactions involving 21 loci located on 13 out of the 19 linkage groups were detected for the two traits. 3 main-effect QTLs and 4 pairs of epistatic QTLs were involved in QEs conferring DTF. One QTL on linkage group (LG) 18 was revealed to simultaneously affect DTF and PS and explain for the highest percentage of the phenotypic variation. The implications of the results for B. napus breeding have been discussed.

Flowering is an important and complex adaptive trait for oilseed production conditioned by environmental factors (photoperiod and temperature), and internal genes and the interactions of genes [1]. Brassica napus is a long-day plant, indicating that it can flower and be harvested earlier under long day than short day. Besides temperature, photoperiod is another key factor which decides whe ther a plant flowers and sets seed or not. To our knowledge, most spring B. napus varieties from Canada and Europe are sensitive to photoperiod and flower early in the long-day condition but become very late under the short-day regime, Q2 is a Canadian spring canola cultivar with a high PS thereby limiting its cultivation in the traditional areas. But Hyola 401, one spring canola cultivar, exhibiting a low PS, is grown not only in the traditional areas as a summer crop under the LD condition but also in the tropical areas as a winter crop under the SD condition. As a result, in order to ensure optimal pollination and seed production, it is essential that flowering takes place at an optimal time of the year.

Photoperiod sensitivity is controlled genetically and interacts with other flowering genes to condition flowering time, thus limiting geographic adaptation of plants. The genetic control of photoperiod sensitivity have been studied successfully in modern plants rice and Arabidopsis thaliana [1-3], but as for the number of genes and type of their interactions, it is inconclusive so far.

The rapid development of molecular marker technology has facilitated the mapping of QTLs associated

with DTF in B. napus and other Brassica species. Many main-effect QTLs controlling DTF in B. napus and other Brassica have been identified using DH and other populations [4-15].

The quantitative nature of DTF in Brassica species has been summarized by Osborn and Lukens [16]. But how to dissect the genetic components, especially to consider epistasis and QEs which play an important role in affecting DTF in rice and Arabidopsis [17-21] was not described in detail for a lack of appropriate analysis tools. Fortunately, the QTLMAPPER 1.6 developed by Wang et al. [22] succeeded in dividing the total effects into three effects: main, epistatic and QE effects, and quantifying these effects. The objectives of this study were to identify QTLs controlling DTF and PS in one Brassica napus DH population derived from Hyola401 x Q2.

MATERIALS AND METHODS

Plant materials and field experiments. The materials used in this study were two spring canola B. napus cultivars Hyola 401 and Q2, exhibiting a low and high photoperiod sensitivity, respectively. Doubled haploid (DH) plants produced from F1 plants of the cross Hyola401 x Q2 using the microspore culture developed by Shi and Liu [23]. DH population and their parents, were grown in the autumn-spring growing season in Hezheng (35°20'N, 103°21'E, China, 2003 and 2004) with a 14.3 h day length and a 15.1°C average temperature and in the summer-autumn growing season in

Table 1. Descriptive statistics of DTF and PS for the parents and the DH population observed in the two locations (Hezheng and Zhaoqing)

Trait Locatoin Parent (mean ± SD) DH population

Hyola 401 Q2 mean ± SD Range Skewness Kurtosis

DTF Hezheng 47.5 ± 0.5 54.5 ± 0.7 54.3 ± 5.4 44.0-70.0 0.49 -0.38

PS Zhaoqing 89.5 ± 2.1 105.5 ± 0.7 98.9 ± 14.7 55.0-121.0 -1.57 1.65

42.0 ± 1.4 51.0 ± 1.4 44.6 ± 12.2 -7.0-64.0 -1.63 1.86

Note: DTF, days to flowering; PS, photoperiod sensitivity; SD, standard deviation.

Zhaoqing (23°16'N, 112°56'E, China, 2003 and 2004) with a 10.1 h day length and a 15.5°C average temperature. At each location a randomized complete block design was used with two replications. Each DH line and parent comprising 40-45 individuals planted in two rows with 15 cm between plants and 30 cm between rows.

Traits measurements. DTF was recorded as the number of days from the sowing date to the date when 50% of the plants in one DH line or parent had at least one open flower. The mean DTF of each DH line was used for QTL analysis. The degree of PS of each DH line or parent was calculated by the delay in DTF in Zhaoqing as compared to DTF in Hezheng.

DNA markers analysis. Total genomic DNA was isolated using a modified SDS method [24]. A total of 248 marker loci that comprised 82 SSR (simple sequence repeat), 72 AFLP (amplified fragment lengt polymorphism) and 94 SRAP (sequence-related amplified polymorphism) loci were detected. The SSR, AFLP and SRAP markers were designed according to http//ukcrop.net/perl/ace/search/BrassicaDB, Vos et al. [25] and Li, Quiros [26], respectively. The protocols of SSR and AFLP were followed as described by Piquemal et al. [27] and Liu et al. [28], respectively. The SRAP system was based on the procedure of AFLP selective amplification described by Vos et al. [25] with only one modification, 100 ng of genomic dNa. Amplifications were carried out in a MJ Research PTC-225 thermocycler ("MJ Research", Waltham, Mass.) using the cycling parameters described by Li et al. [26]. The PCR products were separated on a 6% denaturing poly-acrylamide gel at 85W for about 2.5 h and visualized by the silver staining system ("Promega", Madison, Wis.).

Data analysis. A genetic linkage map comprising 248 marker loci was constructed using MAPMAKER 3.0 [29]. QTLMAPPER 1.6 [22] based on a mixed linear model approach [30], wich estimates QTL main effects, epistasis, as well as predicting QE interaction effects, treating the locations as two environments, was employed to assess QTLs controlling the PS and DTF. In the analysis, the likelihood ratio (LR) and t-test were combined to test the significance of the single-locus QTL additive effects, epistatic effects and the QTL by environment (QE) effects. The LR value corresponding to P = 0.005 (equivalent to LOD = 3.2 for df = 4) was

used as the threshold for claiming the putative main-effect, epistatic QTLs or QEs. The peak points of the LR in the linkage map were taken as the putative positions of the QTLs. When a QTL was involved in more than one epistasis, its position and additive effect were taken from the point showing the largest effect. The relative contribution of a genetic component was calculated as the proportion of phenotypic variance explained by that component in the selected model.

RESULTS

Measurements of DTF and PS

Table 1 shows a summary of the descriptive statistics of DTF and PS for the two parents and DH lines. Highly significant differences between the parents were detected using the least significant difference (LSD) test at the 0.01 probability level for the two traits. Q2 had always a greater DTF and PS than Hyola 401 at both locations, implying that Q2 was more sensitive to photoperiod than Hyola 401. Furthermore, the DTF difference between two parents was less in Hezheng (7 days) than that in Zhaoqing (16 days), indicating that photoperiod sensitivity difference of two parents can be displayed more significantly in the short day condition. Figure 1 shows the distribution of the two traits in parents and DH population. A certain number of DH lines showed transgressive segregations in both directions for the two traits at the two locations, showing flowering time and PS as typical quantitative traits. In addition, the LSD (least significance difference) test detected significantly different DTF of the DH lines (P < 0.01) between Hezheng and Zhaoqing.

A two-way ANOVA revealed that there were highly significant differences between the two locations (environments) and also highly significant genotype-by-environment interactions for DTF in addition to the major genotype effects (Table 2).

Linkage map

A total of 248 loci covered all 19 chromosomes with a total genetic distance of 1634.7 cM and an average genetic distance of 6.6 cM between adjacent marker loci. The 82 SSR marker loci from Piquemal et al. [27] correspondes well with his map in the order (Fig. 2).

THE GENETIC BASIS OF FLOWERING TIME AND PHOTOPERIOD

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1 10

45 50 55 60 65 70 DTF in Hezheng

60 70 80 90 100110120 DTF in Zhaoqing

10 20 30 40 50 60 PS

P1: Hyola 401, P2: Q2

Fig. 1. The frequency distribution for DTF and PS in the DH population deri

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