Animal Breeding and Genomics Centre, Wageningen University, Wageningen, The Netherlands

Animal Breeding and Genomics Centre, Animal Science Group, Lelystad, The Netherlands

Abstract

Background

Partial least square regression (PLSR) was used to analyze the data of the QTLMAS 2010 workshop to identify genomic regions affecting either one of the two traits and to estimate breeding values. PLSR was appropriate for these data because it enabled to simultaneously fit several traits to the markers.

Results

A preliminary analysis showed phenotypic and genetic correlations between the two traits. Consequently, the data were analyzed jointly in a PLSR model for each chromosome independently. Regression coefficients for the markers were used to calculate the variance of each marker and inference of quantitative trait loci (QTL) was based on local maxima of a smoothed line traced through these variances. In this way, 25 QTL for the continuous trait and 22 for the discrete trait were found. There was evidence for pleiotropic QTL on chromosome 1. The 2000 most important markers were fitted in a second PLSR model to calculate breeding values of the individuals. The accuracies of these estimated breeding values ranged between 0.56 and 0.92.

Conclusions

Results showed the viability of PLSR for QTL analysis and estimating breeding values using markers.

Background

Detection of genomic regions affecting traits is a goal in many genetic studies. Studies applying distinct methods for detection of these regions, called quantitative trait loci (QTL), have been described, ranging from single marker regression

Partial least square regression (PLSR) is one method for simultaneously fitting multiple markers and was applied by Bjornstad et al. for detection of QTL

The objectives of this study were to use PLSR to search for QTL and to estimate breeding values in the dataset of the QTLMAS 2010 workshop.

Material and methods

Initial analyses

The data were analyzed to identify generation and gender effects. Furthermore, a bivariate animal model was fitted in ASREML

Marker based analyses

Let **X** be the genotype matrix; the number of rows is the number of individuals and the number of columns is the number of markers. Elements of **X** are 0, 1, or 2, according to the number of one of the two alleles for that marker in that individual. Let **Y** be the matrix of phenotypes; the number of rows is the number of individuals and the number of columns is the number of traits in the data (two in these data).

We describe our regression methods in the following sections, beginning with an analysis where each trait was regressed on each marker independently and continuing with PLSR.

Single marker regression

We used function lm of R

**y**_{t}**x**_{m}b**e**; (1)

**e** ~ **I**σ^{2} );

where **y**_{t}**x**_{m}**y**_{t}**x**_{m}**e** is the vector of residuals. Inference was based on ANOVA applied to the fitted models.

Partial least square regression

In PLSR, matrices **X** and **Y** are decomposed into principal components and loadings:

**X** = **TW**′

**Y** = **UQ**′ (2)

where **T** and **U** are the matrices of scores and **W** and **Q** are the matrices of loadings **X** and **Y**. The first requires orthogonality of **W** and **Q** and the second requires maximal correlation between the columns of **T** and **U****U** is regressed on **T** :

**U** = **TB** + **E**, (3)

where **B** is an unknown matrix of regression coefficients and **E** is a matrix of residuals. We used matrix **B** to calculate the matrix of regression coefficients of the individual markers,

Fitting

We treated the data of both traits equally, without accounting for the non-normal nature of the discrete trait. Since PLSR using all marker loci (~ 10000) was impossible, we calculated the regression coefficients in two steps. First, we regressed the phenotype data on the markers in each chromosome using PLSR and obtained the empirical distributions of these regression coefficients by bootstrapping. Second, we selected the 1000 most significant markers for each trait, defining significance of a marker as the absolute value of its regression coefficient divided by its empirical standard error. Subsequently, we regressed the phenotype data on the selected markers using PLSR and recalculated their standard errors using bootstrapping.

We used the R-package pls

Detecting QTL

Our method assumed that the variance explained by markers reaches a maximum in the neighborhood of a QTL. We used locally weighted regression

Calculating EBV

We estimated breeding values for all individuals in the data using the regression coefficients for the markers in the second PLSR model. Estimated breeding values (EBV) were calculated as

Results

Initial analyses

The initial analysis revealed a positive correlation between the traits. No signals of selection nor sex effects were detected in the data.

The results showed that both traits were heritable and genetically correlated. Heritability of the first trait was 0.53 (s.e. 0.06) and heritability of the second trait was 0.22 (s.e. 0.04). The phenotypic correlation was 0.25 (s.e. 0.03) and the genetic correlation was 0.66 (s.e. 0.09).

Single marker regression

Figure

Smoothed curve of the -log(P) of the marker effects for the two traits, estimated using locally weighted regression.

Smoothed curve of the -log(P) of the marker effects for the two traits, estimated using locally weighted regression.

Partial least square regression with bootstrapping

Figure

Smoothed curve of standardized regression coefficients of individual markers estimated with PLSR, estimated using locally weighted regression.

Smoothed curve of standardized regression coefficients of individual markers estimated with PLSR, estimated using locally weighted regression.

Local maxima identified in Figure

Estimated regression coefficients and approximate standard error for the most significant markers in of the second PLSR model. The highlighted cell contain a regression coefficient which was considered most significant, the other cells contain the less significant regression coefficients.

**marker**

**Cont. trait**

**Discr. trait**

MAF

**Chrom. 1**

156

0.46

2.9920

0.0817

**0.0612**

**0.0405**

159

0.39

**-2.3459**

**0.0479**

-0.0654

0.0442

494

0.48

**0.3242**

**0.0010**

-0.0369

0.0148

495

0.06

-1.4611

0.0046

**0.0672**

**0.0116**

1058

0.31

**4.1433**

**0.1355**

0.0426

0.0170

1087

0.45

-0.4640

0.0020

**-0.0397**

**0.0170**

1621

0.41

0.2338

0.0005

**0.0108**

**0.0012**

1976

0.29

**-0.0818**

**0.0001**

**0.0336**

**0.0102**

**Chrom. 2**

1977

0.37

**-0.3898**

**0.0013**

**-0.0924**

**0.0866**

2340

0.42

2.8491

0.0724

**0.0598**

**0.0378**

2481

0.31

**1.0574**

**0.0088**

0.0434

0.0175

2864

0.27

1.2254

0.0107

**0.0320**

**0.0087**

3242

0.09

**0.3485**

**0.0004**

-0.0043

0.0001

3274

0.31

1.3028

0.0134

**0.0867**

**0.0703**

4034

0.17

**1.5167**

**0.0119**

**0.0572**

**0.0200**

**Chrom. 3**

4035

0.31

**-1.6002**

**0.0200**

**-0.0150**

**0.0021**

4384

0.37

**-1.3395**

**0.0153**

-0.0650

0.0427

4519

0.40

-0.1875

0.0003

**-0.0489**

**0.0249**

4832

0.47

**-1.7533**

**0.0281**

-0.0576

0.0360

5447

0.45

**1.9137**

**0.0333**

0.0062

0.0004

5695

0.46

-1.2392

0.0140

**0.0278**

**0.0084**

5811

0.38

**-2.1743**

**0.0407**

-0.0930

0.0883

6082

0.28

**0.6369**

**0.0030**

0.0123

0.0013

**Chrom. 4**

6083

0.02

**3.1419**

**0.0078**

**0.0192**

**0.0003**

6671

0.46

**-1.4765**

**0.0199**

0.0126

0.0017

6995

0.06

5.3177

0.0585

**0.1009**

**0.0250**

7099

0.41

**-0.9055**

**0.0073**

-0.0352

0.0131

7209

0.46

**-0.3541**

**0.0011**

0.0010

0.0000

7386

0.39

**-0.4365**

**0.0017**

0.0109

0.0012

7433

0.46

0.4793

0.0021

**0.0590**

**0.0377**

7697

0.37

**-0.1112**

**0.0001**

0.0500

0.0252

8073

0.22

**2.1894**

**0.0300**

**0.0117**

**0.0010**

**Chrom. 5**

8324

0.44

1.5840

0.0226

**-0.0076**

**0.0006**

8528

0.39

1.1567

0.0117

**-0.0035**

**0.0001**

8538

0.42

**1.0233**

**0.0093**

0.0027

0.0001

8890

0.35

**0.4145**

**0.0014**

0.0404

0.0162

8920

0.21

-0.1872

0.0002

**0.0565**

**0.0233**

9514

0.13

**-2.6520**

**0.0283**

0.0186

0.0017

9523

0.44

1.2587

0.0143

**0.0274**

**0.0080**

9744

0.21

**-1.1246**

**0.0078**

0.0487

0.0173

9754

0.16

-0.805510

0.0032

**-0.0094**

**0.0005**

The correlations between the EBV and the phenotypes and between the EBV and the true breeding values (accuracies of EBV) are displayed in Table

Correlations between estimated breeding values (EBV) and phenotypes (P) or true breeding values (TBV) of the continuous and discrete trait of individuals in simulated generations zero to three. TBV of all individuals and phenotypes of individuals in generation 4 were only used to evaluate the correlations but were not used to fit the models.

Trait

0

1

2

3

4

**Cont. trait**

r(P,EBV)

0.70

0.70

0.68

0.69

**0.52**

r(TBV,EBV)

0.79

0.73

0.66

0.69

**0.56**

**Discr. trait**

r(P,EBV)

0.64

0.63

0.55

0.55

**0.37**

r(TBV,EBV)

0.92

0.90

0.82

0.79

**0.72**

Discussion and conclusions

We used a non conventional method to infer QTL, based on finding local maxima of a smoothed curves traced through the QTL probabilities (in the single marker regression) and through the standardized regression coefficients (in PLSR). This method assumed that a QTL will appear as a local maximum in the smoothed curves. An advantage of this method over methods that concentrate on profiles of single markers is that it combines evidence provided by a series of markers in the proximity of QTL. A disadvantage is that it does not provide a quantitative test statistic to statistically test for the presence of QTL.

Comparing the QTL detected with our method to true QTL locations revealed differences and similarities. We detected QTL on chromosome 5 while no QTL were simulated on this chromosome. This false detection is inherent to our method since detection was only based on local maxima in the curves. The method suggested many pleiotropic QTL and agreed with the truth, because the majority of the QTL were pleotropic.

We used single marker regression to estimate the variance of individual QTL because we expected that PLSR would underestimate the regression coefficients of QTL in LD with many markers. A disadvantage of this could be biased regression coefficients of QTL in LD with other QTL

The correlations between EBV and true breeding values of individuals in generations 0 to 3 agreed with the correlations of EBV calculated in the studies of Meuwissen et al.

Competing interests

The authors declare no competing interests.

Authors’ contributions

AC analysed the data and wrote the manuscript. MPLC was involved in the discussion and conclusion of the manuscript.

Acknowledgements

This article has been published as part of