Molecular characterization of oilseed rape accessions collectedfrom multi continents for exploitation of potential heterotic groupthrough SSR markers

Muhammad Younas • Yingjie Xiao •

Dongfang Cai • Wei Yang • Wei Ye •

Jiangsheng Wu • Kede Liu

Received: 15 May 2011 / Accepted: 30 November 2011 / Published online: 28 January 2012

� Springer Science+Business Media B.V. 2012

Abstract Evaluation of the genetic diversity in conven-

tional and modern rapeseed cultivars is essential for con-

servation, management and utilization of these genetic

resources for high yielding hybrid production. The objec-

tive of this research was to evaluate a collection of 86

oilseed rape cultivars with 188 simple sequence repeat

(SSR) markers to assess the genetic variability, heterotic

group identity and relationships within and between the

groups identified among the genotypes. A total of 631

alleles at 188 SSR markers were detected including 53 and

84 unique and private alleles respectively, which indicated

great richness and uniqueness of genetic variation in these

selected cultivars. The mean number of alleles per locus

was 3.3 and the average polymorphic information content

was 0.35 for all microsatellite loci. Unweighted Pair Group

Method with Arithmetic Mean clustering and principal

component analysis consistently divided all the cultivars

into four distinct groups (I, II, III and IV) which largely

coincided with their geographical distributions. The Chi-

nese origin cultivars are predominantly assembled in

Group II and showed wide genetic base because of its high

allelic abundance at SSR loci while most of the exotic

cultivars grouped into Group I and were highly distinct

owing to the abundant private and unique alleles. The

highest genetic distance was found between Group I and

IV, which mainly comprised of exotic and newly synthe-

sized yellow seeded (1728-1 and G1087) breeding lines,

respectively. Our study provides important insights into

further utilization of exotic Brassica napus accessions in

Chinese rapeseed breeding and vice versa.

Keywords Brassica napus L. � Allelic variation � Private

allele � Genetic distance � SSRs markers

Introduction

Rapeseed (Brassica napus L.; genome AACC, 2n = 38) is

today the most extensively cultivated crop species in the

crucifer family (Brassicaceae) and a major oil crop grown

in temperate, tropical and subtropical climates for high

quality vegetable oil, feed protein and biofuel purposes. It

covers about 30.2 Mha area along with other related oil-

seed Brassicas (e.g. mustards) in the whole world with a

current production of around 61 Mt (FAOSTAT 2009,

http://faostat.fao.org) and ranks the third among the oil

seed crops after soybean and palm in production of vege-

table oils [1], while fifth in the production of oil seed

proteins [2]. B. napus was first introduced to China in the

1930’ to 1940’ from Europe and Japan independently [3].

More or less, it is a winter oil crop and expanding rapidly

as a rotation crop following rice [4]. The local rapeseed

breeders developed most of the oilseed rape cultivars

through various classical breeding methods and interspe-

cific hybridization between European B. napus and the

indigenous B. rapa varieties. The ‘double low’ elite vari-

eties called ‘‘canola’’ were also introduced to China from

Europe, Canada and Australia and are used as parents by

M. Younas and Y. Xiao contributed equally to this work.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11033-011-1306-0) contains supplementarymaterial, which is available to authorized users.

M. Younas � Y. Xiao � D. Cai � W. Yang � W. Ye �J. Wu � K. Liu (&)

National Key Laboratory of Crop Genetic Improvement,

National Center of Plant Gene Research (Wuhan), Huazhong

Agricultural University, Wuhan 430070, China

e-mail: kdliu@mail.hzau.edu.cn

123

Mol Biol Rep (2012) 39:5105–5113

DOI 10.1007/s11033-011-1306-0

breeder in China to develop a series of double zero rape-

seed varieties, which are adapted to local conditions. Now

B. napus is the most important oil crop in China, which

occupied about 85% of total production area of oilseed rape

[5].

Brassica napus is predominantly self-pollinating but has

an open flower that promotes cross-pollination. In some

situations, up to 36% outcrossing has been observed among

plants in close proximity [6]. Most oilseed rape cultivars

are developed through pedigree breeding methods and

released as open-pollinated populations derived from

inbred plants [7]. It is well established that almost all the

current oilseed rape cultivars passed through a series of

bottlenecks selection processes during its domestication

since its origination [8]. Most of the modern B. napus

cultivars are difficult to be differentiated mainly due to

their narrow genetic background [4] because of breeding

for low erucic acid content [9] as well as the introgression

of low glucosinolates (GSL) genes [10].

Estimation of genetic diversity is very important in

designing crop improvement programs, management of

germplasm and most importantly, in assisting breeders to

understand the structure of B. napus germplasm for pre-

diction of maximal variation in heterotic crosses. As the

emphasis on specific quality traits has considerably nar-

rowed the gene pool of oilseed rape breeding materials

compared to its parental species, genetic variability is

restricted with regard to many valuable characters [11].

The selection of genitors and characterization of the

existing genetic variability is indispensable to improve

efficiency of breeding programs. Therefore, the classifica-

tion of breeding line into heterotic groups and the crossing

between distinct genetic types contribute to widen genetic

variances [12]. Significant heterosis for seed yield exists in

oilseed rape crop. Yield increases of 30 ± 60% over mid-

parent value have been observed in F1 hybrids [13, 14],

which have motivated the development of hybrid cultivars.

Positive correlation between heterosis and parental genetic

diversity was demonstrated for hybrid combinations within

the same ecotype group like spring by spring [15, 16],

winter by winter [17] and semi-winter by semi-winter

rapeseed hybrids [18].

There are various techniques available for evaluation of

crop genetic variability, such as morphological, biochem-

ical and molecular markers. Molecular markers have been

increasingly employed for analysis of genetic diversity

[19]. A variety of molecular markers have been used to

gauge the genetic variation among diverse group of

important crops in the genus Brassica, such as restriction

fragment length polymorphisms (RFLPs) [20], amplified

fragment length polymorphisms (AFLPs) [11, 21], random

amplified polymorphic DNA (RAPDs) [22] and simple

sequence repeats (SSRs) [23, 24] etc. However, it is

generally recognized that the best candidates for rapeseed

characterization are the SSR or microsatellites markers,

which are highly informative [25]. Their advantages for

diversity studies include uniform genome coverage, high

levels of polymorphism, co-dominance inheritance fashion,

easy-to-implement and highly reproducible compared to

other markers [26–29].

The co-dominant nature of microsatellite polymor-

phisms makes them particularly useful for map alignment

among different crosses. A potentially more important use

in genetic dissection of breeding lines through molecular

markers would be the allocation of genotypes to specific

heterotic groups, which would reduce both cost and labor

by eliminating intra-group crossings. Charcosset et al. [30]

suggested that genetic distance can not accurately predict

hybrid performance unless the DNA markers used in the

analysis were linked to the genes affecting the trait. The

frequent occurrence of simple repeat sequence in coding

DNA regions has rendered them suitable for marker-

assisted selection (MAS) of simple traits in oilseed rape

and genetic distance determination.

In the present study, SSR markers were evaluated in

rapeseed with the ultimate purpose to: (i) quantify the allelic

diversity to determine the genetic relationship among the

selected 86 accessions of conventional and modern Chinese

and exotic rapeseed cultivars, (ii) compare the polymor-

phism and allele frequencies in each of the identified group

and the introgression between the identified groups and (iii)

suggest potential heterotic groups among the genotypes

included in the study using genetic distance as measured by

the SSR markers.

Materials and methods

Plant materials and DNA extraction

The collection consists of 86 conventional and modern

cultivars, of which 59 are from across the China while ten,

nine, five and three from Europe, Canada, Australia and

Japan, respectively. All the cultivars and their origins are

presented in Table S1. Seeds of each cultivar were grown in

the Experimental Farm of Huazhong Agriculture University

Wuhan during 2008 and 2009 growing seasons. Total

genomic DNA was isolated using cetyltrimethylammonium

bromide (CTAB) from 200 to 500-mg samples of fresh

leaves collected from a single plant of each accession

according to the previous procedure [31]. The purity and

integrity of the DNA was confirmed by electrophoresis in

1% agarose gel with visualization under UV light after

staining with ethidium bromide and then adjusted to a uni-

form concentration (25 ng/ll) with double distilled water

(ddH2O) for SSR analysis.

5106 Mol Biol Rep (2012) 39:5105–5113

123

SSR genotyping and PCR amplification

The 86 cultivars were genotyped using 188 SSR markers

randomly distributed across the 19 chromosomes of rapeseed.

Because B. napus is an amphidiploid species with the AACC

genome originated from two diploid ancestral species which

have large-scale segmental or whole-genome duplication

events [32] and SSR markers usually detect multiple loci,

which make it difficult to assign alleles to specific loci. In this

study, only single-locus SSR markers from different resources

were used to eliminate ambiguous genotyping as described by

Chen et al. [33]. Markers prefixed with BnGMS [34], BnEMS

[35], BrGMS [36] and BoGMS [37] were developed in our

laboratory while markers prefixed with BRAS and CB were

developed by Piquemal et al. [38], markers prefixed with Ol,

Na, and BRMS were obtained from http://www.brassica.info

/resources/markers.php, and markers prefixed with sN, sR and

sS were developed by Agriculture and Agri-Food Canada

(http://brassica.agr.gc.ca/index_e.shtml). The chromosomal

identity and positions of these selected 188 SSR markers were

noted from the published genetic maps derived from the

double haploid population, BnZNDH (B. napus) [34, 36, 37]

and B. rapa physical map (http://www.brassica.info/resource

/sequencing/status.php) [36]. Their names, linkage group and

locations are summarized in Table S2. Marker assay followed

the protocol described by Cheng et al. [34].

Data collection and analysis

Individual alleles at each SSR marker across all the cultivars

were scored in ascending order of the amplified fragment

size. The number of alleles per locus, polymorphic infor-

mation content (PIC) value and average gene diversity at

each locus were calculated through PowerMarker software

version 3.25 [39]. The allelic data was also scored as ‘‘1’’ for

the presence or ‘‘0’’ for the absence of ones fragments as

described by Hasan et al. [24] to measure genetic relatedness

among genotypes using Unweighted Pair Group Method

with Arithmetic Averages (UPGMA) cluster analysis.

Principal component analysis (PCA) was done to show the

distribution of the genotypes in scatter-plot based on their

similarity matrix generated with Dice’s method using the

software NTSYS-PC 2.1 [40]. Genetic differentiation

between pairs of groups was calculated with pairwise Fst, a

measure of heterozygosity within subpopulations relative to

the total population [41].

Unique and private allele analysis

The importance of private allele in genetic diversity and

breeding has been thoroughly discussed by Chen et al. [33].

But still the criteria for classification of unique and private

alleles are not unified. Here we attempted to clarify the

difference between these terms. Unique alleles are specific

to single accession across the whole population under

study, and private alleles can be those which prevail in

more than one accession but only in single cluster/group.

We calculated unique and private alleles in our population

according to the above mentioned criteria from allelic

frequency determined by PowerMarker software version

3.25 [39].

Results

Allelic variation at SSR loci

All of the 188 analyzed markers were polymorphic and

produced a total of 631 alleles across the 86 cultivars

(Table 2). The number of alleles at each locus ranged from

2 to 13, corresponding to an average of 3.37 per locus thus

revealing a high level of genetic diversity in the selected

oilseed rape accessions. PIC values ranged from 0.04

(BoGMS1467) to 0.73 (BRMS008 markers). PIC is

regarded as one of the important features of the molecular

markers and can be used to evaluate the differentiation

ability of the markers [42]. Average PIC of all the SSR

markers was 0.35, indicating the ability of utilized markers

to differentiate the rapeseed genotypes. BoGMS1515 and

Na12-G12 markers have PIC values higher than 0.7. The

majority of polymorphic SSR loci generated two alleles

(36.6%) followed by three alleles (28.7%) (Table 1), and a

large proportion of markers exhibited high discrimination

power.

The mean value of gene diversity at over all loci in the

whole collection was 0.41, and a large variation in gene

diversity existed among different loci. The highest diver-

sity (0.77) was observed at the locus BoGMS1515 while

the lowest diversity (0.045) at the locus BoGMS1467. The

mean values of the four identified groups (I, II, III, and IV)

was 0.34, 0.35, 0.33 and 0.31, respectively. Gene diversity

Table 1 Allelic variation among polymorphic SSR loci

Number of alleles Number of SSR loci Polymorphic loci (%)

2 70 36.64

3 55 28.75

4 27 14.13

5 18 9.42

6 15 7.85

7 1 0.52

8 1 0.52

9 1 0.52

11 1 0.52

13 1 0.52

Mol Biol Rep (2012) 39:5105–5113 5107

123

is equivalent to the expected heterozygosity for diploid

data, and it is defined as the probability that two randomly

chosen alleles are different in the sample.

Clustering and principal component analysis

In order to explore genetic relationships of the selected 86

accessions of domestic and imported oilseed rape cultivars,

the genetic similarity coefficients between accessions were

calculated and a dendrogram was constructed depicting

relationships among the accessions (Fig. 1). All the 86

rapeseed cultivars were clearly discriminated at a genetic

similarity level of about 0.93 and broadly classified into four

groups (I, II, III and IV). Group I consists of 19 cultivars

mostly exotic except two Chinese cultivars (Zhongza-H8002

and SC-UG6) that are nested together at the genetic simi-

larity level of 0.68. Group II comprises of 58 accessions,

predominantly all are from China and clustered at the genetic

similarity level of 0.73. Six cultivars united together to form

Group III at the genetic similarity levels ranging from 0.70 to

0.72, of which four are released by different research insti-

tutes of China and two imported from Europe. Whereas,

interestingly two newly synthesized yellow seeded Chinese

cultivars (1728-1 and G1087) clustered with one European

cultivar (Naleo) to form Group IV at the genetic similarity

level of 0.60, representing quite diverse group. Although all

the clusters are very discrete and well differentiated from

each other, but none of these clusters completely represent

cultivars from one region, thus indicating a constant intro-

gression between local and exotic varieties.

It needs to mention that one-dimension clustering meth-

ods in UPGMA would be difficult to capture adequate

information if complicated genetic relationship exists

among cultivars. The UPGMA clustering sometimes results

in discrepancies depending on the choice of similarity index

and can be biased by rare alleles. Therefore the general

pattern of genetic diversity was further verified with PCA

which confirmed the UPGMA clustering positions of the

cultivars described above (Fig. 2).

Unique and private SSR alleles in populations

Overall 53 unique alleles were identified across all the

accessions (Table 2). The highest number of five unique

alleles was found in Rexi followed by Surpass-400 which

has four unique alleles (Table S3). Group III and IV have

10 and 5 unique alleles respectively, which reflects a bit

higher ratio than Group II (26) and I (12) with respect to

their number of genotypes. Among the analyzed SSR loci,

BRMS008 and BRAS087 revealed relatively higher num-

ber of unique alleles (5 and 3, respectively). This suggests

that these particular SSR markers can be useful in cultivar

identification and registration. Presence of unique alleles

has been previously reported for soybean [43], barley [44]

and rapeseed [45].

Genetic distinctiveness of the B. napus germplasm in

different populations was described by the prevalence of

private alleles. A total of 84 private alleles were found in all

the four groups, of which Group II has the highest number of

private allele (43) while Group III is the only population

having no private allele (Table 2). Fifty-two out of 84 pri-

vate alleles were detected in only two cultivars in different

groups (Table S3). Some of the private alleles were highly

represented within populations. For example, Allele-3 of

BrGMS4027, Allele-1 of BnEMS525 and Allele-3 of

BrGMS4027 marker were shared by 20, 10 and 10 genotypes

of Group II respectively, while Allele-2 of both BoG-

MS1118 and of BoGMS2468 were shared by 8 cultivars of

Group I and II separately. The richness of private alleles was

also evaluated in each group. The number of private alleles

observed in each group varied significantly from each other,

with 9 alleles per cultivar in Group IV, 0.73 in Group I and

0.63 in Group II (Table 2). Surprisingly Group III does not

have any private allele although it is highly diverse with

respect to unique alleles. Interestingly, 27 private alleles

were found in Group IV and all of these existed only in two

cultivars (1728-1 and G1087).

Genetic diversity of groups

Overall allelic diversity within the groups was estimated

based on the number of alleles per group. As a result, the

level of genetic diversity was highest in Group II (0.35)

followed by Group I (0.34), Group III (0.33), and Group IV

(0.31) (Table 2). The Group II also contained the most

alleles per locus and had the highest total number of alleles

of the four identified groups. Group I and II appeared to be

more diverse (within the groups) in terms of gene diversity,

total number of alleles, alleles per locus, number of unique

and private alleles (Table 2). Additionally, the genetic

Table 2 The genetic diversity index for all oilseed rape accessions

and groups identified in this study based on 188 SSR markers

Statistic Overall Cluster groups

I II III IV

Sample size 86 19 58 6 3

Allele no. 631 446 548 388 337

Allele/locus 3.37 2.37 2.91 2.06 1.79

Gene diversity 0.41 0.34 0.35 0.33 0.31

Major allele frequency 0.68 0.73 0.73 0.74 0.75

Private allele no. 84 14 43 0 27

Private allele richness 0.97 0.73 0.63 0 9

Unique allele no. 53 12 26 10 5

PIC value mean 0.35 0.29 0.30 0.27 0.25

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distance among the groups measured by Nei’s minimum

distance and pairwise Fst were consistent. The largest

genetic distance (0.26) was found between Group I and IV

(Table 3). It is worth to mention that two of the three

cultivars in Group IV are newly synthesized oilseed rape

inbred line (1728-1, G1087). They have white flower with

yellow seed and were produced through repeated hybrid-

ization between B. napus and Brassica species including

Raphanus sativus L., B. alboglabra Baily by the breeding

research group in our laboratory. So on the basis of high

genetic distance, we postulate that crossings between the

accessions of these two groups can produce high hybrid

vigor. The second highest distance was found between

Groups II and IV and Group III and IV; and slightly

smaller distance was seen between Group I and III and

Group I and II. The lowest distance was found between

Group II and III. The identified heterotic patterns should be

field tested between these groups to confirm what appears

to be promising alternative heterotic patterns based on the

SSR markers.

Fig. 1 Dendrogram for 86

domestic and imported

cultivated rapeseed accessions

based on cluster analysis

(UPGMA) of similar

coefficients

Mol Biol Rep (2012) 39:5105–5113 5109

123

Discussion

The earlier studies have established that SSRs show very

high level of polymorphism in plants [25]. In this study, 86

accessions representing a wide range of conventional and

modern rapeseed cultivars originating from different geo-

graphical locations across China and others rapeseed

growing countries were surveyed using a total of 188 SSR

markers.

Our results make it obvious that the data generated from

a set of 188 SSR markers were highly informative and the

86 cultivars were distinguished successfully. The average

number of alleles per SSR marker in this study was 3.3,

lower than the 7.3 alleles per primer pair obtained from

SSR marker analysis of 96 accessions of the European gene

pool, which included a broader range of varieties [24] and

higher than the genetic survey of Chinese and Swedish

oilseed rape, generating 2.7 per SSR marker [46]. This

relatively small number is most likely due to three factors:

the polymorphism of SSR markers, the diversity of germ-

plasm accessions, and the sensitivity of DNA fragment

separation systems. However, since the SSR markers

selected were evenly distributed along the rapeseed gen-

ome, the genetic relationships revealed by this study within

the investigated groups of rapeseed cultivars are repre-

sentative and meaningful. The association between number

of alleles per locus and PIC means that either estimator is

useful for determining the value of a marker for diversity

studies. Allelic abundance at SSR loci reflects the overall

genetic diversity inside a population and will influence the

genetic distance from other populations, based on dissim-

ilarity matrices. As the Group II had more alleles compared

to Group I which reflects that Chinese cultivars are geneti-

cally more diverse than the imported varieties of rapeseed. It

corroborates the findings of Zhou et al. [46] who mentioned

that genetic diversity within Chinese genotypes was broad

compared to that of Swedish material.

Cultivar relationships as revealed by UPGMA clustering

(Fig. 1) generally reflected the tendency of cultivars to

associate with breeding institutes and geographic location.

For instance, cultivars assembling in the Group I were pre-

dominantly introduced from different rapeseed growing

countries. The highest number of alleles (548) was identified

in Group II, the largest group comprising 58 cultivars which

are mostly released by various rapeseed breeding institutes

Fig. 2 Plot of the first and second principal components calculated from the correlation matrix of 631 polymorphic fragments. The letterscorrespond to the identified groups in Table 2

Table 3 Genetic distance between identified oilseed rape groups

I II III IV

I 0 0.13 0.15 0.26

II 0.25 0 0.04 0.25

III 0.25 0.17 0 0.18

IV 0.33 0.54 0.308 0

The top diagonal is Nei minimum distance and the bottom diagonal is

pairwise Fst

5110 Mol Biol Rep (2012) 39:5105–5113

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of China except for three Japanese (G1178, Nonglin22,

Nonglin40), three Canadian (Ienvenu, Jiaoyou3, SV-pyriter)

and one cultivar from Europe (Dac-chosen). The clustering

pattern shows that all the clusters are not completely

homogeneous which reinforces the fact that the breeders

have integrated cultivars released from other companies into

their germplasm pool and also verified that breeding origin

had a substantial impact on cultivar heterogeneity. An earlier

study also accounted that a Chinese B. napus population

frequently experienced interspecific hybridization such as

with B. rapa [33].

Among the total of 631 alleles, 53 were unique across all

the four groups. Group II has the highest number of unique

alleles (26) which might be due to its large number of cul-

tivars (56). Rexi, Surpass-400 and 1728-1 have more unique

alleles (5, 4 and 3 respectively) compared to other genotypes,

demonstrating that these cultivars have had limited genetic

exchange with other cultivars and therefore may have unique

alleles for various (functional) traits as well. So, these cul-

tivars could be used to enhance the diversity of other elite

breeding material. Unique alleles are useful not only in

specific categorization of genotypes, but also for their sub-

sequent utilization in breeding and plant development as

unique markers. Furthermore, cultivars in Group III were

found to be genetically divergent from each other due their

low level of genetic similarity. The cultivars belonging to

Group IV carry plenty of private alleles, indicating that the

origins of these cultivars are different from other and

genetically distinct. Our result demonstrated that most of the

imported cultivars clustering in Group I are rich in private

alleles (0.73) as compared to cultivars of Group II (0.63)

which are mainly from China. But in contrast, Chen et al.

[33] found that the richness of private SSR alleles in Chinese

rapeseed is greater than those of Australia, Europe or Can-

ada. The abundant private alleles detected in Group IV

collection in the present study demonstrated that it could

serve as an efficient and timely avenue to broaden the genetic

base of elite breeding material of rapeseed in China.

Assessment of the amount of genetic variation in oilseed

rape accession is direly needed for its sustainable production

and breeding. It provides a general blue print for choosing

inbreed lines to make suitable cross combinations for par-

ticular breeding purpose. The diverse and unique oilseed

rape genotypes identified in this study may therefore repre-

sent a useful resource for widening the genetic base and

improving heterotic potential in oilseed rape accessions

cultivated in China. Butruille et al. [47] described significant

yield increase in spring oilseed rape hybrids through intro-

gression of winter germplasm. However, this also requires

backcrossing to re-establish the desired seasonality. But all

the diverse genotypes in our study are semi-winter cultivars,

therefore, the backcrossing for season adoptability is not

required.

Selection of desirable parents is an important task for the

production of high yielding hybrid because heterosis is

associated with the interaction of different alleles at a locus

[48]. It has been suggested that molecular diversity can be

used to select parents for hybridization. Genetic distances

between populations, individuals or lines estimated through

molecular markers have been widely used for descriptive

analyses in crop plants, e.g., reconstructing breeding histo-

ries, describing patterns of genetic diversity, and assigning

lines to heterotic or other biologically or economically

important groups. We found that considerable high genetic

distance exists between Group I and IV (Table 3) which may

render them the ultimate candidate for obtaining significant

heterosis in rapeseed hybrid breeding programs. Genetic

distance is useful for predicting yield potential and heterosis

of intra-subspecific hybrids [49] and a positive correlation

has been found between genetic distances determined by

molecular markers and heterosis in rapeseed [15–18].

Almost all the cultivars in Group I are exotic and Group IV

have two distinct newly resynthesised lines (1728-1 and

G1087), rich in unique and private alleles with a wide

genetic base. So it would be the best choice to cross the

accessions between these groups to develop productive and

adaptable hybrids. Seyis et al. [50] produced high yielding

hybrid from crosses between genetically diverse resynthes-

ised rapeseed and adapted oilseed types.

It has been reported that not all polymorphic DNA

fragments contribute to heterosis due to the considerable

number of fragments that are either located in non-encoded

regions or have no association with agronomically impor-

tant traits [30]. It is well established that favorable alleles at

a loci controlling agronomic traits are rapidly fixed in any

genotype through artificial selection. Many reports have

demonstrated that a large number of SSR markers occur in

well-characterised genome regions containing quantitative

trait loci, thus increasing their potential relevance for

allele–trait association analysis [51]. Therefore the finding

of this study provided significant information to better

understand the current situation of heterotic groups and

diversity patterns at the molecular level in the selected

cultivars of rapeseed.

Our study evaluated the genetic diversity within and

between selected local and imported rapeseed accessions

and gauges the relationships among these cultivars. These

informations would help researchers and breeders to select

highly distinct crossing parents for the development of

mapping population and breeding of B. napus. The diver-

sity could be maximized for new crosses that would reduce

the breeding cost by eliminating the evaluation of intra-

group hybrids. The diverse groups we found are useful as

preliminary heterotic groups (following field crossing for

confirmation), and hybrid rapeseed breeding can be greatly

boost up by use of this information.

Mol Biol Rep (2012) 39:5105–5113 5111

123

Acknowledgments The research was supported by the National

Natural Science Foundation of China (No. 31071452) and the Doc-

toral Fund of Ministry of Education of China (No. 20100146110019).

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