Investigation of indirect selection criteria and DNA markers for clean wool colour in sheep

Abstract

In Experiment 1, progeny test data for 53 Corriedale sires and 1492 offspring were used to estimate heritabilities for and phenotypic and genetic correlations between yellow predictive colour (YPC), greasy (GFW) and clean fleece weight (CFW), Yield, mean fibre diameter (MFD), and Visual Score. Results showed a low heritability for YPC (0.28±0.06), and positive genetic correlations of YPC with CFW or MFD. Direct selection against YPC was expected to decrease CFW and to decrease MFD. Although reductions in MFD are economically beneficial, reductions in CFW would translate in large net losses in wool income. Visual Score and YPC were strongly genetically correlated, however using Visual Score as an indirect selection criterium for YPC would be expected to produce even greater correlated responses in CFW, resulting in bigger economic losses, than when YPC was used.

The same flock was used for Experiment 2, where 440 offspring from 19 Corriedale sires were included in the analyses of heritability for and phenotypic and genetic correlations between wool colour traits: clean wool colour (CWC), brightness (Y), YPC, and Visual Score; wool production traits: GFW, CFW, Yield, MFD; suint traits: suint percentage and potassium (K) and sodium (Na) concentrations in suint, and physiological traits: potassium and sodium concentrations in plasma, red blood cells, and skin. The main objective of this study was to find suitable indirect selection criteria for clean wool colour.

The heritability of CWC was estimated at 0.27±0.13. Clean wool colour showed strong positive genetic correlations with CFW and MFD, thus direct selection against CWC was also expected to reduce CFW and MFD. A high genetic correlation between YPC and CWC was observed, indicating that YPC could be a suitable indirect selection criterium for CWC. However, the heritability for YPC in this experiment was lower than that observed in Experiment I and direct selection against CWC was predicted to produce faster genetic improvements in CWC than that expected under indirect selection. Brightness, skin K, Visual Score, YPC and suint K were amongst the best indirect selection criteria for clean wool colour. However, selection using these traits was expected to reduce CWC from 58% to 49% of that estimated for direct selection.

Suint traits were highly genetically correlated to YPC. Suint K, but not suint percentage, was found to have a high genetic correlation with CWe. Plasma K was strongly genetically correlated to YPC and suint K, which suggests that plasma is the pool of potassium that is excreted through the sweat glands. Skin K was considered an unreliable predictor of suint K.

A selection index was calculated, with CFW, MFD and CWC being both selection criteria and breeding objective traits. The index estimated was I₁ = +1.21CFW +0.11MFD +0.42CWC. A second selection index having CFW, MFD and CWC as breeding objective traits and CFW, MFD and YPC as the selection criteria was derived as I₂ = +1.22CFW +0.14MFD +0.19YPC. The high relative economic value (REV's) of CFW compared to the other traits fitted in these indices caused positive expected correlated responses for all traits, therefore undesirable genetic gains were predicted for CWC. A third index having CFW, MFD and CWC as both selection criteria and breeding objective traits, where CFW was restricted to nil genetic change, was estimated at: I₃ = +0.23CFW -0.04MFD -0.01CWC. This index was expected to cause a negligible genetic gain for CWC (-0.04 Y-Z units/year).

The results showed that the most effective way to genetically improve CWC was through direct selection, however, the premiums for CWC in the Corrledale breed at the time of this study were not sufficient to justify the expected reductions in CFW. The best practical recommendation would be to use the selection index (I₃) with CFW restriction in order to improve clean wool colour without having economic losses.

In Experiment 3 an Australian Merino flock was screened for low (resistant) and high (susceptible) YPC breeding values in order to compare extreme individuals using the differential display of mRNA technique. One differentially expressed cDNA band was visualised only in the resistant group and another was present only in the susceptible group. These bands showed no identity with the DNA sequences of public databases, however they showed short homologies with a number of database sequences. The use of these candidate genes as DNA markers need to be confirmed against sheep with a wide range of susceptibility to wool yellowing to verify the results. The fact that the candidate genes for YPC resistance and susceptibility were present in 4 and 3 individuals out of 5, respectively, indicates that there would be some animals that will not have their genotypes identified and more studies are needed to establish more reliable markers and screening criteria.