Analysis of the genetic and phenotypic expression of a variant Romney in the progeny of three breeds of sheep : the Tukidale, a carpet wool mutant

Abstract

In two successive lambing seasons two rams exhibiting double structured carpet-wool type fleeces, well developed horns and heterozygous for the gene T, assumed responsible for this primary pleiotropic phenotype and considered a more powerful allele of the commercially established N-type sheep, were mated to cull stud ewes of Border Leicester, Romney and Corriedale heritage.

The progeny of these crosses were rigorously examined from the day of birth until 12 months of age. A fibre by fibre analysis was made of birthcoat wool samples taken at 14 and 100 days of age. Skin samples taken at this time provided details of the corresponding follicle populations. Phenotypic expression was monitored in terms of carpet-wool type and potential, between breed differences, within breed differences and variation in expression with increasing age.

An experiment was designed to generate quantitative data on the foetal and early post-natal growth rates of the specific birthcoat fibre-types characteristically produced by the T gene, thereby elucidating the time and mode of action of the frequently described prenatal check.

A monogenic ratio of hairy to normal or T/+ to +/+ lambs was established. All hairy lambs produced Grade VIIf birthcoats. Heterozygous variability was absent except for a slight tightness or damping in the apical curls of some staples of the neck and shoulder regions. No shoulder patch was observed.

Normal (non-hairy) lambs produced birthcoats of Grades I-II. No intermediates were observed. The T gene proved completely dominant, although sexlinked for horn production. No breed cross differences were detected.

No breed Cross or sex differences were expressed in the gross analysis of fibre arrays within the respective genotypes.

For T/+, based on fibre-type morphology, an additional halo-hair type (HHmod) was described. The chalkiness of medullation observed in bulk is sustained by the super-sickle and hairy-tip-curly-tip fibre-types in detail.

Analysis of follicle populations indicate that central primary follicles grow the largest diameter fibres, HHmod and super-sickles. Lateral primaries have a limited capacity to grow super-sickle-types, but grow predominantly HTCT.

The precipice point, marked as an abrupt length reduction, and change of fibre-type class within an array delineates the advent of secondary follicle fibre production.

Maintenance of staple length and medullation is borne by lateral primary HTCT fibres. Central primary fibres are subject to the most dynamic state of change in growth.

The primary action of the T gene is directed at the growth and enhancement of productivity of primary follicles, specifically central primary follicles. Approaching mature production little difference is found in mean minimum fibre diameter, secondary fibre diameter and S:P follicle ratios between T/+ and +/+ lambs irrespective of breed cross-type.

Wool weights at 12 months age indicate no significant differences between genotypes. The individual constituents of greasy-wool weight show a similar equivalence.

Quantitative estimates of specific fibre-type growth rate obtained from in situ auto radiographic labelling recorded a strong prenatal growth rate check in all Pre-curly-tip and HTCT fibres. The check exerted its maximum expression between 105-111 days of foetal age. Halo-hairs recorded the maximum growth-rate depression, but showed an equally rapid post-check increase to highest absolute rates. Primary fibre types were all growing before 93 days of foetal age.

A second depression in foetal fibre growth rates was recorded in primary fibre-types immediately prior to birth.

The T gene is deemed responsible for the vigour and specific activity of primary follicles, thereby creating and maintaining the production of a double structured carpet wool type fleece: In this respect a single dose of T produces a fleece in bulk and in detail at least the equivalent of the biological features of N/N homozygous N-type New Zealand Romney.