Fibre shedding and fibre-follicle relationships in the fleeces of Wiltshire Horn × Scottish Blackface sheep crosses

Extract

1. Changes in fleece structure produced by crossing the Wiltshire Horn and Scottish Blackface breeds of sheep have been analysed. Skin and fleece samples were available from lambs and adults of both parental breeds, and from the F₁ F₂ and back-cross genotypes. For the analysis, three types of fibre: kemps, hairs (both primaries) and secondary fibres were distinguished.

2. The frequency of kemp fibres was high in the Wiltshire and relatively low in the Blackface. The frequency of hair fibres was nil in the Wiltshire and fairly high in the Blackface. The crosses were intermediate between the parents. Despite these fibre type changes among the primary fibres there was no difference between the breeds or crosses in follicle population densities or in the relative proportions of primary and secondary follicles.

3. Kemp fibres were shorter and finer in the Wiltshire than in the Blackface. In the crosses they were intermediate. Hairs, when present in the crosses, were shorter and finer than in the Blackface. Secondary fibres were shorter in the Wiltshire than in the Blackface but in some of the crosses they were at least as long as in the Blackface. Wiltshire secondary fibres were remarkably coarse and strongly medullated and this notable characteristic was carried over to some extent in the crosses. Throughout the study comparison in these terms was always complicated by fibre shedding.

4. Fibre shedding was universal for all types of follicle in the Wiltshire. In the Blackface, primary central follicles appeared to shed their kemp fibres two or three times yearly. For hairs and secondary fibres shedding did not occur more than once per year and the frequency of shedding amongst these fibres was low. In the crosses the frequency of shedding in kemps and the incidence of shedding in hairs and secondaries were higher than in the Blackface. Increases in the incidence and frequency of shedding probably had some effect in decreasing the measurable fibre length in the crosses.

5. The mean length and diameters of the different fibre types were usually positively correlated within sheep. In this and other ways there was not much evidence of competition for fibre substrate between the different types of mature follicles, in this material.

6. Birth-coat classifications distinguished between the typically smooth Wiltshire and the hairy Blackface birth-coats. Birth-coats in the crosses were intermediate but showed partial dominance of the hairy type.

7. The most important of the observed changes in fibre and follicle morphology can be briefly summed up by considering how the Blackface phenotype was modified in the crosses by the introduction of the Wiltshire genotype, viz.

(a) The proportion of kemps was increased at the expense of hair fibres. This was brought about by kemp fibres tending to replace hairs in all primary central and in some primary lateral follicles.

(b) All primary fibres were diminished in length and thickness; secondary fibres increased in thickness. The general fibre population therefore became more uniform.

(c) The shedding frequency of kemps and the shedding incidence among hairs and secondaries were both increased.

(d) Follicle population density, follicle group structure and the S/P follicle ratio were unaltered.

(e) The degree to which changes occurred in the different crosses was generally proportional to the percentage of Wiltshire-derived genes present.

8. The fact that the relative frequencies of kemp and hair fibres could be altered without any concomitant changes in the follicle group pattern showed that a particular follicle type was not a priori bound to produce a particular fibre type. It seems clear therefore that the physiological activity of follicles can be altered genetically whilst their organization within the follicle group pattern remains invariant.