2006 Guang chinacottonGD

8/2/2019 2006 Guang chinacottonGD

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Acta Genetica Sinica, August 2006, 33 (8)733745 ISSN 0379-4172

Genetic Diversity of Source Germplasm of Upland Cotton in

China as Determined by SSR Marker Analysis

CHEN Guang, DU Xiong-Ming

Key Laboratory of Cotton Genetic Improvement of Agricultural Ministry, Cotton Research Institute, Chinese Academy of Agricul-

tural Sciences, Anyang455004,China

Abstract: The genetic diversity of 43 sources of Upland cotton germplasm with different parental origins, breeding periods, and

ecological growing areas in China were studied on the basis of simple sequence repeat (SSR) markers. A total of 130 gene alleles

with 80% polymorphism were detected from 36 SSR primers. The number of alleles per primer ranged from two to eight with an

average of 3.6. The polymorphism information content (PIC) range was 0.278-0.865, with an average of 0.62. The average geno-

type diversity index (H) was 1.102, the highest was 2.039 and the lowest was 0.451. The average coefficient of the genetic similar-

ity of SSR markers among source germplasm was 0.610, ranging from 0.409 to 0.865. These indicated that the genetic diversity at

the genomic level of the selected source germplasm was rich, and was representative of the diversity of the germplasms, in general.

The diversity at the genome level of the base germplasm from the second and third breeding periods was decreased compared to

that of the first period, indicating that the cotton genetic background in China became narrow gradually. The diversity of SSR

markers among the base germplasm from early maturity cotton growing areas in the north was higher than those from the Huanghe

and Yangtze growing areas. The molecular marker genetic similarity index of the domestic varieties was higher than that in the

introduced varieties, which indicates that the genetic diversity in domestic cultivars was lower than that in the introduced varieties.

This study gives an overview of the genetic diversity of the cotton germplasm base in China, and provides a guide for breeders to

develop new cultivars efficiently.

Key words: source germplasm; SSR; genetic diversity

Received: 2005-09-07; Accepted: 2005-12-19

This work was supported by the 10th Five Years Key Programs for Science and Technology Development of China (No.

2004BA525B05). Corresponding author. E-mail: [email protected]; Tel: +86-372-2525 352

Source germplasm for cotton (Gossypium hirsu-

tum L.) breeding in China includes germplasms with

stable genetic characters, excellent yield, adaptability,

and better general combining ability, and have been

used frequently as parents in many breeding pro-

grams. There are few reports on source germplasm

research. Huang[1]

described in the book Cotton

Variety and Pedigree in China 36 source germplasms

from the G. hirsutum varieties, comprising 25 foreignvarieties, eight Chinese varieties and three varieties

with low gossypol. Du and Liu[2]

described a new

method of classification of source germplasm by

emphasizing the number of derived varieties in addi-

tion to their pedigrees. The authors defined the source

germplasm as being lines or varieties from which 20

or more applied varieties have been derived. Accord-

ing to this criterion, 47 germplasm sources of Upland

cotton used in China were described, which include

16 foreign varieties, 29 Chinese varieties, and two

varieties with low gossypol.

Genetic diversity is the basic portion of biological

diversity and is the base of biological polymorphism and

species diversity. Genetic diversity and parenthood ofgermplasm play an important role in cotton breeding. The

precise evaluation of the genetic diversity of excellent

germplasm will provide a guide for choosing parents and

predicting the degree of inheritance, variation, and level

of heterosis, which are essential for realizing the breeding


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734 Acta Genetica Sinica Vol.33 No.8 2006

goal. Molecular marker analysis is a modern technique,

which discloses genetic differences at the DNA level in

plants and is an effective tool for testing genetic diversity

of germplasm in breeding programs[3, 4]

. The study of

cotton germplasm diversity has expanded from the phe-

notypic, cellular, and biochemical levels to the DNA level.

Modern molecular marker techniques can illuminate the

individual differences and relationships among species at

the DNA level. Most cotton varieties planted in China

were derived from a few sources of germplasm such as

DPL, Stoneville, King, Uganda, Foster, and Trice, all of

which were introduced from abroad. These varieties were

the foundation of Chinese cotton breeding programs and

played a decisive role in the self-breeding varieties of

China. It has been indicated that the genetic base was

narrow and the genetic diversity was low in Upland cot-

ton in China. This was caused by a limited quantity of

source germplasms. We will present evidence from mo-

lecular marker analysisshowing the correlation among

source germplasms genetic diversity. The method using

simple sequence repeat (SSR) markers is a fast and con-

venient molecular marker method with good reproduci-

bility and high veracity, which can be used to evaluate

cotton varieties. Genetic similarity and clustering analyses

of source germplasm at the molecular level were carried

out in this study to provide some data for parent selection,

germplasm enhancement, and application in our cot-

ton-breeding program. The collection, conservation, and

utilization of large collections of crop germplasms

have aroused peoples interest for a long time. To

solve the problem of the difficulty for finding source

germplasm, research of core collections of wheat

(Triticum aestivum L.), rice (Oryza sativa L.), and

soybean (Glycine max L.) crop germplasms was done,

and it is necessary to construct a core collection for

cotton in China as there are more than 7 000 acces-

sions in the Chinese cotton collection. The study on

source Upland cotton germplasm will provide a base

for identifying a core collection of Chinese cotton.

1 Materials and Methods

1. 1 Plant materials

Forty-three Upland cotton source germplasms

were used in this experiment (Table 1). They were

grouped using three methods. One method divided the

germplasm according to their three breeding periods

the first period (before 1950s), the second period

(1950s-1960s), and the third period (1970s-1980s).

The second method grouped germplasm on the basis of

three Chinese growing areasYangtze river valley

area, Huanghe river valley area, and the area in north-

ern China. The final method divided germplasm into

two groups on the basis of their origins those bred

in China and those introduced from abroad.

1. 2 SSR molecular marker analysis

Cotton genomic DNA was extracted on the basis of

Zhangs[5]

CTAB method with some modifications[6,7]

.

Three hundreds and ninety-eight SSR primers, which

were chosen on the basis of previous studies and po-

tential polymorphism, were used to make a primary

survey among eight varieties. From this survey, 78

polymorphic primers were identified. Thirty-six

primers with good amplification, clear gel bands,

strong signal, and clear background were used in the

PCR reactions. A 10 L volume of PCR reaction

mixture contained 1.0 L 10 PCR buffer (contain-

ing 20 mmol/L Mg2+

), 0.2 L dNTP (10 mmol/L), 0.3

L Taq enzyme (2 U/L), 6.2 L ddH2O, 0.65 L

forward primer (5 mol/L), 0.65 L reverse primer (5

mol/L), and 1 L DNA template (50 ng/L). The

PCR reaction was carried out with a PTC-100 ther-

mocycler (MJ Research, Waltham, USA). Amplifica-

tion was programmed for pre-denaturing at 94 for

3 min, 30 cycles of denaturng at 94 for 30 s, an-

nealing at 57 for 30 s, and extension at 72 for

45 s, followed by a final extension at 72 for 7 min.

PCR amplification products were separated by elec-

trophoresis using 8% nondenaturing polyacrylamide

gels and were visualized following silver staining[8]

.

1. 3 Data analysis

After observing the PCR electrophoresis results,

the bands of DNA fragments were scored as present (1)

or absent (0). Genetic diversity analyses were madeon the basis of these scores. The statistical methods


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CHEN Guang et al.: Genetic Diversity of Source Germplasm of Upland Cotton in China as Determined by SSR Marker Analysis 735

Table 1 Source germplasm and relative information

No. Name Ecological area Origin Parent name Breeding period[2]

1 DPL14A America America DPL14A 1

2 DPL15 America America DPL15 1

3 Stoneville 4 America America Stoneville 14A 1

4 Stoneville 4B America America Stoneville 4B 1

5 Stoneville 2B America America Stoneville 2B 1

6 Coker100 America America Coker100 1

7 Delfos531 America America Delfos531 1

8 Empire America America Empire 1

9 Foster 6 America America Foster 6 1

10 Guan nong 1 North China Liaoning King 1

11 Trice America America Trice 1

12 Jiangsu mian 1 Yangtze Jiangsu DPL14A 2



15 57-681 Yangtze Sichuan DPL15A 2

16 Dong ting 1 Yangtze Hunan DPL15A 2

17 Guang ye dai zi mian America America DPL15A 2

18 DPL16 America America DPL15A 2

19 Zhong mian suo 2 Huanghe Henan DPL15A 2



22 Shan mian 4 Huanghe Shanxi DPL15A 2

23 Shan mian 5 Huanghe Shanxi Stoneville 4 2

24 Liao mian 3 North China Liaoning Stoneville 4 2

25 Xu zhou 209 Yangtze Jiangsu Stoneville 2B 2

26 Xu zhou 1818 Yangtze Jiangsu Stoneville 2B 2

27 Uganda 4 Uganda Uganda Uganda 2

28 Zhong mian suo 7 Huanghe Henan Uganda 2

29 52-128 Yangtze Sichuan Delfos531 2

30 Gan mian 1 Yangtze Jiangxi Foster 6 2

31 Shan mian 3 Huanghe Shanxi Foster 6 2

32 Chao yang mian 1 North China Liaoning Chao Yang Mian 1 2

33 Jin mian 2 North China Liaoning King 234 Jin mian 1 North China Liaoning King 2

35 Ke ke 1543 Russia Russia Ke ke 1543 2

36 Yi shu hong Yangtze Hubei Trice 2

37 86-1 Huanghe Henan Stoneville 4 3

38 Ji mian 1 Huanghe Hebei Stoneville 2B 3

39 Zhong mian suo 12 Huanghe Henan Uganda 3

40 Hei shan mian 1 North China Liaoning King 3

41 Zhong mian suo 10 Huanghe Henan King 3

42 Lambright GL-5 America America Lambright GL-5 3

43 Micnarie 210 America America Micnarie 210 3


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and formulae used are shown below:

(1) Simpson diversity index, also known as

Polymorphism Information Content: PIC= 1-Pi2,

wherePi represents the variation in frequency of the

ith allele.

(2) Shannon-weaver diversity index, also re-

ferred to as genotype diversity (H):H = -PiLnPi ,

where Pi indicates the variation frequency of the ith

allele[9]

.

(3)Ne is the effective number of alleles for each

locus:Ne = 1/Pi.

(4) Genetic similarity coefficients (Jaccards co-

efficients) among varieties were calculated using the

Qualitative Date program of NTSYSpc 2.1 software

(Biostatistics Inc., New York, USA).

(5) Clustering analyses were performed using

NTSYS-pc (version 2.11a) to calculate the genetic

similarity matrices, and the dendrograms were con-

structed by the unweighted pair-group method of

arithmetic averages (UPGMA).

2 Results

2. 1 SSR marker multiple analyses

One hundred and thirty alleles with 80% poly-

morphism among 43 source germplasms were de-

tected. The average number of alleles for each SSR

locus was 3.6 (Table 2). The effective number of al-

leles for each SSR locus ranged from 1.385 to 7.405,

with an average of 3.066. ThePICvalue for the SSR

loci ranged from 0.278 to 0.865, with an average of

0.62. The genotype diversity was 0.451 to 2.039, with

an average of 1.102. A total of 157 unique genotypes

were detected by 36 pairs of polymorphic primers

from a combination of 43 source germplasms. The

average number of unique genotypes for each locus

was 4.36. The polymorphic loci were distributed on

cotton chromosomes 3, 4, 5, 8, 9, 10, 16, 18, 20, and

23, indicating that the variations of SSR alleles were

dispersed in the whole cotton genome.

Among the source germplasm, two to eight SSR

loci were obtained from each primer pair that wasanalyzed. The primers that amplified more than one

allele were BNL530, BNL3031, NBL1672, BNL2590,

and BNL3259 having 8, 7, 6, 6, and 6 alleles, respec-

tively. These primers possessed a strong potential for

germplasm evaluation. Primers BNL3254 and

TMB04 had only one polymorphic locus with less

distinguishing potential among germplasms.

2. 2 Genetic similarity and clustering analyses

2. 2. 1 Source germplasm

A similarity matrix was built using genetic simi-

larity between all possible combinations of two

germplasms. Genetic similarity was calculated with

NTSYS 2.1 using the Jaccard coefficient according to

0-1 data from the SSR markers. The genetic similar-

ity coefficients among source germplasm ranged from

0.409 to 0.865, with an average of 0.610. The largest

similarity was 0.865 between Stoneville 4 and Stone-

ville 2B, which became source germplasm in China

after they were introduced from Stoneville, USA. The

similarity between Jiangsu mian 2 and DPL15 was

0.841. Jiangsu mian 2 had consanguinity with DPL14

and DPL15 as it was bred from a cross between

DPL14 offspring and DPL15. Also, Uganda 4 had

high similarity with Zhong mian suo 12 (0.816) andZhong mian suo 7 (0.818). The common origin of

these three varieties, which were derived from proge-

nies of Uganda, had been validated by the SSR

marker analysis. The lowest similarities were be-

tween Foster 6 and Jin mian 2 (0.409), Jin mian 2 and

DPL14 (0.413), Jin mian 2 and DPL15 (0.423),

DPL15 and Lambrigh GL-5 (0.425), and Jiangsu

mian 2 and Lambrigh GL-5 (0.421). The lower simi-

larities between these varieties indicated that they

were more distant from each other. In summary, the

genetic similarity between source germplasms was

lower, indicating that there were differences among

the source germplasms. The low genetic similarity

also showed that the source germplasms selected for

analysis were representative of a range of cotton

germplasm.

The cotton source germplasm could be divided

into five groups on the basis of the average similarity

coefficient (0.610) among the source germplasm (Fig.1). The first group contained DPL14, DPL15, Uganda,


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Table 2 Allelic variation of 36 SSR loci among 43 source germplasms

PrimerNo. of

alleles

No. of polymorphic

alleles

Polymorphism

(%)

No. of

genotypesChromosome PIC H Ne

BNL530 8 8 1 12 4 0.865 2.039 7.405BNL830 4 3 0.75 5 - 0.696 1.266 3.285

BNL1053 4 3 0.75 4 5 0.673 1.227 3.056

BNL1231 2 2 1 3 20 0.397 0.586 1.658

BNL1317 3 2 0.667 3 923 0.614 1.008 2.588

BNL1414 3 2 0.667 3 923 0.635 1.054 2.742

BNL1421 2 2 1 3 10 0.489 0.682 1.957

BNL1495 4 4 1 11 10 0.722 1.327 3.600

BNL1672 6 5 0.833 6 9 0.800 1.680 4.990

BNL1694 4 3 0.75 5 16 0.682 1.240 3.141

BNL2449 3 2 0.667 3 10 0.622 1.027 2.647

BNL2590 6 4 0.667 4 9 0.784 1.607 4.634

BNL2634 3 2 0.667 4 - 0.590 0.961 2.439

BNL2960 2 2 1 3 - 0.310 0.488 1.449

BNL3031 7 6 0.857 7 923 0.834 1.867 6.019

BNL3254 2 1 0.5 2 18 0.341 0.525 1.518

BNL3255 2 2 1 3 10 0.436 0.628 1.774

BNL3259 6 5 0.833 7 3 0.780 1.588 4.536

BNL3383 5 4 0.8 6 23 0.718 1.358 3.550

BNL3442 4 3 0.75 5 18 0.737 1.360 3.804

BNL3474 5 4 0.8 5 26 0.778 1.558 4.512

BNL3482 3 1 0.333 2 - 0.665 1.096 2.986

BNL3806 3 2 0.667 3 25 0.602 0.989 2.513

BNL3948 2 2 1 3 20 0.477 0.670 1.911

BNL3976 3 2 0.667 3 5 0.551 0.873 2.229

BNL4030 2 2 1 3 22 0.496 0.689 1.982

JESPR65 4 3 0.75 5 - 0.681 1.229 3.139

JESPR101 3 2 0.667 3 - 0.590 0.962 2.441

JESPR114 2 2 1 3 - 0.499 0.692 1.996

JESPR152 2 2 1 3 - 0.278 0.451 1.385

TMG10 3 3 1 4 - 0.593 0.967 2.458

TMK19 3 2 0.667 3 - 0.605 1.000 2.535

TMP02 4 4 1 6 - 0.708 1.305 3.422

TMB04 3 1 0.333 2 - 0.651 1.074 2.868

TME12 5 5 1 7 - 0.779 1.548 4.523

TMH08 3 2 0.667 3 - 0.625 1.037 2.669

Average 3.6 2.889 4.361 0.620 1.102 3.066

Note:PIC= polymorphic information content,H= Shannon-weaver diversity index,Ne = effective number of alleles.


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and the source germplasms derived from them. The

second group was mainly composed of Stoneville and

their derived source germplasms. Lambright GL-5

and Micnarie 210 with very low gossypol content

also belonged to this group. The third group included

three source germplasms: Foster 6, Trice, and Ji mian

1. Jian mian 2 and Delfos531 formed the fourth and

fifth group, respectively.

2. 2. 2 The source germplasm from different breed-

ing periods

There were 11 source germplasms (Table 3) from

an early breeding period, most of which were intro-

duced from the USA except Guan nong 1. According

to the similarity matrix, Stoneville 2B and Stoneville

4 had the greatest similarity (0.865), whereas DPL15

and Stoneville 4 had the least similarity (0.446). The

average similarity coefficient of these source germ-

plasms was 0.587. Guan nong 1 was derived from

King of USA, and was an early maturity variety bred

in 1930 in China. It was popular in the early maturity

cotton growing area of northern China, including the

Liaoning Province, and had become a source germ-

plasm of the early period. More than 100 varieties

were derived from Guan nong 1. According to the

Table 3 SSR genetic similarity coefficient of source germplasms from different breeding periods

Breeding period Similarity coefficient Range No. of samples

First period (before the 1950s) 0.587 0.446-0.865 11

Second period (1950s-1960s) 0.630 0.445-0.818 25

Third period (1970s-1980s) 0.630 0.525-0.782 7

Overall 0.610 0.409-0.865 43

Fig. 1 Dendrogram of 43 source germplasms based on SSR similarity coefficient


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similarity coefficient, Guan nong 1 has a close rela-

tionship with DPL cotton. The early breeding periodsource germplasms were classified into three groups

by the average similarity coefficient (0.578) (Fig. 2).

The first group was DPL type including DPL15,

DPL14, Guan nong 1, Coker 100, and empire cotton.

The second group was Stoneville type containing

Stoneville 2B, Stoneville 4, Stoneville 4B, Foster 6,

and Trice. Delfos531 itself became the third group.

Tracking the breeding history of these three groups,

the ancestors of Delfos, DPL, and Trice had some

relationship with Foster cotton. Foster with some

specific characters was bred by the Foster farm, in

USA, in 1904. This common ancestor shows that the

genetic background of cotton was very narrow.

The second period source germplasm included

25 germplasms, 22 of which were derived from nine

of the first-period source germplasms. The sources of

the other three germplasms were Ke ke 1543, Zhong

mian suo 7, and Uganda 4. The average similarity

coefficient of these 25 source germplasms was 0.630.The similarity between Uganda 3 and Zhong mian

suo 7 was the greatest (0.818). The lowest similarity

was between Jin mian 2 and Jiangsu mian 2 (0.445),which were derived from Guan nong 1 and DPL14,

respectively. The second period source germplasms

were divided into three groups on the basis of the

average similarity coefficient (0.63) (Fig. 3). The first

group included varieties derived from DPL, Uganda 4,

Zhong mian suo 7, 52-128, and Guan nong 1. The

second group contained Foster offspring (Gan mian 1

and Shan mian 3), a DPL15-derived line (57-681),

and a Stoneville-derived line (Liao mian 3). The third

group contained four source germplasms, Xu zhou

1818, Yi shu hong, Chao yang 1, and Jin mian 2.

The third-period source germplasm included

seven varieties, and their average similarity coeffi-

cient was 0.630. The two closest basic germplasms

were Ji mian 1 and Zhong mian suo 12 with a simi-

larity coefficient of 0.728. The two farthest basic

germplasms were Zhong mian suo 12 and Lambright

GL-5 (0.525). Zhong mian suo 12 was developed

from the cross of Uganda 4 and Ji mian 1. Zhongmian suo 12 and Ji mian 1 were classified into one

Fig. 2 Dendrogram of first period source germplasm originating before the 1950s based on SSR similarity coefficient


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group by SSR cluster analysis, indicating that the

derived variety had a close relationship with its par-

ents at the DNA level.

The third-period source germplasms were classi-

fied into two groups by average similarity coefficient

(0.63) (Fig. 4). The first group was Lambright GL-5

and Micnarie 210 with lower gossypol content and

86-1 with resistance to the Fusarium wilt. The second

group contained Ji mian 1, Zhong mian suo 12, and

two early maturity cottons, Hei shan mian 1 and

Zhong mian suo 10, which were derived from Guan

nong 1. The comparison of SSR genetic similarity

coefficients of different breeding periods is shown in

Table 3. The average similarity coefficient of the

first-period source germplasm was the lowest (0.587)

with a broad range (0.446-0.865), indicating that this

germplasm had many differences at the genome level.

The similarity coefficients of the second- and the

third-period source germplasms were greater than that

of the first period, and the range was also smaller. This

indicates that the similarity in the second- and thethird-period source germplasms increased. The main

reason was that most of the source germplasms of the

second and the third periods were derived from those

of the first period source germplasms. Moreover, some

genetic differences among the varieties were lost in the

process of selection of high-quality and high-yield

traits in the breeding program, and this resulted in the

genetic base for breeding becoming narrower.

2. 2. 3 Source germplasm from different cotton

growing areas

From the similarity coefficient of source germplasms

from different cotton areas (Table 4), we found the fol-

lowing results. The similarity among the introduced

American varieties was the lowest, which indicated sig-

nificant genetic differences among the American varieties.

The similarity among the source germplasm from the

Huanghe River valley cotton area was the highest. The

similarity among the source germplasm of Yangtze River

valley area was also high, as their average similarity coef-

ficient reached 0.610. These coefficients showed that the

variation of source germplasms in the two main cot-

ton-growing areas of China declined compared with thatof the germplasms introduced originally. However, the

Fig. 3 Dendrogram of second period source germplasm originating between 1951 and 1970 based on SSR similarity coeffi-

cient


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similarity among the source germplasm in the north early

maturity cotton area was lower than that in the Huang-he

and Yangtze River areas. The breeders usually focused on

using the varieties with high quality, high yield, and

stronger adaptability, and this likely made the genetic

background among the varieties in the Huang-he and

Yangtze River areas narrower.

2. 2. 4 The domestic and introduced source germ-

plasm

Many of the Chinese breeding source germplasms

had been based on the introduction, selection, and do-

mestication of germplasms from other countries. As a

result, the average genetic similarity coef ficient ofdomestic source germplasm (0.624) was higher than

that of the introduced germplasm (0.585, Table 5).

This indicated that it would be difficult for the ge-

netic base of domestic germplasms to exceed foreign

germplasms, which could be the result of the limited

genetic resources used in China.

It was necessary to analyze the introduced source

germplasm as the source germplasms in China were all

derived from introduced varieties. There were four

groups of foreign source germplasms measured with the

average similarity coefficient of source germplasm. The

first group was composed of DPL type, Empire,

Coker100, and Uganda cotton. The second group was

composed of Stoneville type, Foster, and Trice. Thethird group was composed of those with lower gossypol

content and Ke ke 1543 introduced from Russia. The

Delfos cotton formed a group by itself (Fig. 5).

Fig. 4 Dendrogram of third period source germplasm originating between 1971 and 1990 based on SSR similarity coeffi-

cient

Table 4 SSR genetic similarity coefficient of source

germplasm from different cotton growing areas

Cotton-growing areas Similarity coefficient No. of samples

Yangtze River areas 0.610 10

Huanghe areas 0.651 11

Northern China 0.586 6

American 0.576 14

Overall 0.610 43

Table 5 SSR genetic similarity coefficient of domestic and

introduced germplasm

Source germplasm Similarity coefficient No. of samples

Domestic 0.624 27

Introduced 0.585 16


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3 Discussion

The genetic relationships among source germ-plasms of Upland cotton in China have been analyzed

on the basis of SSR molecular markers. The main

results are summarized below:

1. The average genetic similarity coefficient of

source germplasms was 0.610 and ranged from 0.409

to 0.865. This suggested that the analyzed source

germplasms possessed vast diversity, a large extent of

variation, and were a general representative of the

diversity of source germplasms.

2. The average genetic diversity coefficients of

the SSR markers for the source Upland germplasms

of the first, second, and third breeding periods were

0.587, 0.630, and 0.630, respectively. This implied

that the genomic difference among the modern source

germplasm had gradually decreased compared to that

of the early source germplasm. This was probably a

result of the narrow breeding base as the breeders

used a limited number of varieties with high quality

and high yield.3. The similarity coefficients of the SSR markers

among the source germplasms in different Chinese

cotton growing areas were different. The diversity of

the source germplasm in the northern early maturitycotton area was largely preserved. However, the di-

versity declined distinctly in the Huanghe and Yang-

tze River areas compared to the diversity of the in-

troduced source germplasm. The varieties with high

quality, high yield, and stronger adaptability were

preserved, whereas those with poor adaptability were

eliminated through selection, and this lessened the

genetic difference among the varieties in Huanghe

and Yangtze River areas.

4. The SSR similarity coefficient of domestic

source germplasm (0.624) was higher than that of the

introduced germplasm (0.585). This suggests that the

introduced source germplasm had adapted to the en-

vironment and climate in China, and many varieties

were derived from them. However, the limited ge-

netic diversity of domestic source germplasm could

never exceed the diversity of the introduced germ-

plasms.

China is not a native cotton growing area; there-fore, its cotton breeding and production was based on

Fig. 5 Dendrogram of introduced source germplasm based on SSR similarity coefficient


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the introduced germplasms. Source germplasm is the

foundation of cotton breeding, the basis of derived

varieties, and plays an important role in the cotton

production in China. Therefore, it is necessary to

study the source germplasm. The genetic diversity of

43 source germplasms of Upland cotton, classified

according to the quantity of derived varieties and

their pedigrees, have been assessed in this study. The

large differences among the source germplasms cov-

ered 95% of the diversity of varieties bred in China

and proved that the source germplasms selected were

a representative sample of Chinese cotton[2]

.

The source germplasms contributed considerably

to breeding programs in China, although their typesand amounts were limited. This increased the similar-

ity among their derived varieties, and made the ge-

netic base to narrower. This is explained by the fol-

lowing experimental results: the genetic diversity of

the source germplasm of the second and the third

breeding period was lower than that of the first period;

the diversity of domestic source germplasm declined

compared to that of the introduced germplasm; and

the diversity among the derived varieties decreased

compared to those of their source parents. The main

reasons were that the source germplasms of the latter

two breeding periods were mostly derived from the

first period and the domestic source germplasms were

mostly derived from introduced germplasm sources.

Other research[10-14]

on cottons genetic diversity us-

ing RAPD, ISSR, and SSR markers also showed that

the genetic base of cotton breeding was narrow in

China. For instance, Bie et al.[10]

clustered 30 varie-

ties from three main cotton-growing areas in China

into three groups and showed that their genetic base

was narrow. Their diversity also had some relation-

ship with their pedigrees on the basis of RAPD

marker and phenotype analyses of the varieties. Xu et

al.[15]

reported that the genetic diversity of the varie-

ties bred by CRICAAS from the end of the 1970s to

the middle of the 1990s was higher than that of the

varieties from the Hebei Province. The 19 varieties

bred in the Hebei Province also had a much narrower

base. Xu et al.

[16]

analyzed the genetic diversity of thevarieties resistant to Fusarium wilt using RAPD

markers. This research pointed out that the genetic

diversity of Upland cotton was lower in China, but it

was vital for introducing Fusarium wilt resistance

genes from G. arboreum, G. barbadense, and other

species into Chinese G. hirsutum. Wu et al.[17] studied

36 domestic and introduced cultivars of Upland cot-

ton with SSR markers and morphological characters

and showed that there were some genetic differences

among these materials, but the genetic diversity of

these cotton cultivars was low. This again suggested

that the genetic base of Upland cotton was narrow.

When Xu et al.[18]

compared Upland cotton in Yang-

tze River and Huang-he River areas by RAPD marker

analysis, the genetic diversity of the varieties in both

areas were similar. This further supported the view

that common germplasm sources and same breeding

targets, similar breeding methods, and policies could

be the most important factors to influence the genetic

diversity of these two cotton-growing areas in China.

Molecular markers can show genetic diversity at

the genome level, whereas the phenotypes express the

interactions of genes and environments. The majority

of domestic source germplasm and their derived va-

rieties have been bred in China by selecting and do-mesticating introduced varieties. The domestic varie-

ties have higher yield and improved quality, but the

selection pressure and limited source germplasm has

narrowed the genetic base in domestic cultivars.

The development of a core collection, which

identifies a small number of unique germplasms to

represent the genetic diversity present in a larger col-

lection of germplasm, is a very important research

area. On the basis of this study, it is obvious that

source germplasms are the most diverse part of the

cotton collection in China. This research of source

germplasms by SSR marker analyses will provide

important methods and technologies for the construc-

tion of a Chinese cotton core collection. It will also

provide data for variety improvement, and updating

and enhancing the diversity of germplasms. Further-

more, both source germplasms and their derived va-

rieties are studied systematically to understand the

genetic base of cotton breeding more clearly in ourprogram.


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Acknowledgments: We thank Prof. Liu Guo-Qiang

and deputy Prof. Sun Junling, who provided valuable

suggestions for the paper , and helped review this

article. We also acknowledge Dr. Lori Hinze,

USDA-ARS, College Station, TX 77845, for a critical

review of this article.

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