Recessive and dominant inheritance patterns associated with synonymous differences in wool fibre curvature and medullation

ABSTRACT

Segregation into lustrous and wild-type birth coats suggested both dominant and recessive inheritance patterns associated with birth coat and subsequently with low wool fibre curvature at lamb and yearling shearing. The hypothesis that these phenotypes also differ in the proportion of medullated fibres (%) was tested here. Progeny expressing dominant segregation of lustre at birth showed significant differences in medullation of lambs wool (P < 0.001), where 42.3% (SEM ± 6.5%) of fibres were medullated in the lustrous phenotype, and 4.8% (± 1.0%) in the wild-type. Medullation at yearling shearing had declined to 19.7% (± 5.3%) and 1.8% (± 0.3%) respectively but the significant difference prevailed (P < 0.001). Progeny expressing recessive segregation of lustre also exhibited more medullation as lambs (54.6% ± 6.0%) with only 9.5% (± 2.6%) in the wild-type (P < 0.001). Again, medullation declined in yearlings but the difference (P < 0.001) between phenotypes remained (22.6 ± 8.2 vs. 4.0 ± 1.7%). For both inheritance patterns medullation increased as fibre diameter increased in lambs wool (P < 0.001), but across the range of fibre diameter observed, more fibres were medullated in those lustrous at birth (P = 0.001).

KEYWORDS:

• Wool bulk
• hairy
• woolly
• crimp
• lustre

Introduction

Wool fibres exhibit variation in physical dimensions like length, strength, diameter and curvature (or crimp) and visual properties like pigmentation, medullation and lustre and many of these parameters correlate or interact with each other. Scobie et al. (2023) reported segregation that suggested a single recessive gene mutation possibly causes lustre in the coat of the newborn and low fibre curvature in the fleece of lambs and yearlings without differences in fibre diameter. Scobie et al. (2021) had previously reported segregation that suggested a single dominant gene, or perhaps genes, that caused a very similar phenotype with lustrous newborns and low fibre curvature in the lamb and yearling fleeces but similar fibre diameter. Dominant inheritance patterns had previously been reported (Short 1958; McGuirk and Short 1967; Blair 1990) and phenotypically similar animals, assumed to carry dominant mutations, were also identified by Li et al. (2009). Scobie et al. (2021, 2023) have described the phenotypes in both types of inheritance pattern as lustrous or a shiny highly reflective birth coat, which then go on to grow wool that may lose this lustrous appearance but exhibit very low fibre curvature without differences in fibre diameter in comparison to wild type siblings. A single generation of the experiment reported by Scobie et al. (2023) was found to exhibit considerably more medullated fibres in the offspring that had low fibre curvature as lambs and again as yearlings. Specifically, this was the proportion of the total number of fibres carrying a medulla. These findings led to a new hypothesis, that the segregation of lustre of the neonatal birth coat and low fibre curvature in lambs and yearlings was also associated with segregation of medullation.

Materials and methods

Wool samples from all animals in the previous studies (Scobie et al. 2021, 2023) were available to test their levels of medullation. The first generation of matings reported in Scobie et al. (2023) was conducted alongside those of Scobie et al. (2021) such that both experiments commenced as one. The animals reported in Scobie et al. (2021) were from mating of two unrelated sires named ‘Razzle' and ‘Dazzle' whose progeny segregated within sire groups to exhibit a wild type birth coat (n = 12 Dazzle, n = 15 Razzle) or a lustrous birth coat as neonates (n = 4 Dazzle, n = 9 Razzle). No lustrous lambs were born from Basil (n = 17) or Fazil (n = 18) from an initial mating. Due to a paucity of ewe lambs (Basil n = 4, Fazil n = 6) and death of Basil, a second generation was created by mating the first-generation ewes back to Fazil which segregated into lustrous (n = 9) and wild-type (n = 8) birth coats in the newborn (Scobie et al. 2023). Segregation in the progeny of the second generation suggested a recessive mode of inheritance of lustre at birth.

Briefly with respect to fibre curvature, the four sires Razzle, Dazzle, Basil and Fazil who themselves had been born with lustrous birth coats, had low fibre curvature as yearlings (22.3, 24.4, 16.1, 26.7°/mm respectively) and were each mated to (n = 12) unrelated ewes that had moderate fibre curvature (37.2°/mm). The segregation of birth coat type was associated with segregation of fibre curvature of lambs and yearlings among the progeny from Razzle and Dazzle. Testing of the wool fibres from the final cohort sired by Fazil demonstrated the presence of a high proportion of medullated wool fibres in those that were lustrous as lambs (Scobie et al. 2023). Wool samples had been stored from all animals and all cohorts and these were all subsampled and sent to a laboratory for measurement.

Wool fibre measurements

Wool samples were measured at a commercial wool-testing laboratory on an Optically-based Fibre Diameter Analyser (OFDA 2000) (BSC Electronics, 13 Willcock Street, Ardross, Western Australia 6153, Australia) for average fibre curvature, average fibre diameter and for medullation. These measurements were undertaken in laboratory conditions of 20°C and 65%RH following International Wool Textile Organisation test standard TM 47.

Statistical analysis

Each medullation variable (lamb and yearling fleeces) was first compared in a 1-way analysis of variance (ANOVA) with a group factor, defined by a combination of generation, sire and lustre factors. This Group factor had eight levels: 1st generation from Basil with wild type, 1st generation from Fazil with wild type, 1st generation from Dazzle with wild type or lustre, 1st generation from Razzle with wild type or lustre, and 2nd generation from Fazil with wild type or lustre.

Next, each medullation variable was compared in another 1-way ANOVA with a different group factor, defined by a combination of generation, gene and lustre factors. The gene factor combined Dazzle and Razzle sires into a single level called ‘dominant' and Basil and Fazil into the other level ‘recessive'. Hence, in these ANOVAs the Group factor had five levels: 1st generation recessive with wild type, 1st generation dominant with wild type or lustre, and 2nd generation recessive with wild type or lustre.

Within each ANOVA, group comparisons were made using the Fisher’s least significant difference (LSD) method.

The relationship of each medullation variable with fibre diameter was investigated with linear regression analysis, where data were pooled from lustre and wild type groups. Then, group-specific straight lines were fitted by regarding medullation as the dependent variable and the diameter as the predictor. The strength of the relationships was statistically tested and compared between the groups. This analysis was first undertaken on the measurements from all lamb samples, and repeated on all yearling samples. Figure 1 depicts each sire group and lustre phenotype separately for lambs, but since there were no noticeable differences observed between sires, the lustre and wild-type phenotypes were pooled for lambs and again for yearlings. All statistical analyses were carried out with software SAS version 9.4.

Figure 1. The relationship between fibre diameter (µm) and medullation (%) in the fleeces of lambs at shearing that were judged lustrous (open symbols) or wild-type (closed symbols) at birth. The progeny of Razzle (diamonds), Fazil (circles), Dazzle (squares) and Basil (crosses).

Results

Wool sample measurements reported in Scobie et al. (2021, 2023) are summarised in Table 1 for fibre curvature and fibre diameter of lambs and yearlings. The observations are reported here for the reader. Lambs that were declared lustrous at birth went on to produce fleeces of low fibre curvature when shorn as lambs and again as yearlings. This was evident in the progeny of Razzle and Dazzle and also for the second-generation progeny of Fazil where progeny that were born lustrous expressed lower fibre curvature than the wild-type. There was no sign of segregation of lustre at birth in the first-generation progeny of Basil or Fazil (Table 1). Fibre diameter generally increased between lamb and yearling samples. Fibre curvature was much higher in wild-type progeny of Razzle as a consequence of their part merino genotype but changed very little between lamb and yearling samples in any animal judged wild-type at birth. In the progeny recorded as lustrous from the two sire groups that suggested dominant inheritance, fibre curvature increased slightly between lamb and yearling samples. In contrast, fibre curvature almost doubled in the second-generation progeny of Fazil that were recorded as lustrous at birth. The latter change was one of the very few phenotypic differences between the two ways of inheriting lustrous birth coats.

Table 1. Summary of average fibre diameter and average fibre curvature of samples from lambs and yearlings that were the progeny of two sires (‘Razzle' and ‘Dazzle') exhibiting phenotypic segregation that suggested dominant inheritance of lustre in the birth coat from Scobie et al. (2021) and the progeny of two sires (‘Basil' and ‘Fazil') exhibiting segregation that suggested recessive inheritance of lustre in a second mating from Scobie et al. (2023). The second-generation progeny of one sire (‘Fazil') are indicated (*).

Sire group	Fibre curvature (°/mm)		Fibre diameter (µm)
	Wild-type	Lustre	Wild-type	Lustre
Lamb shearing
Razzle	67.5	22.8	23.9	24.2
Dazzle	56.2	24.5	27.9	25.7
Basil	43.1		27.5
Fazil	43.8		27.9
Fazil*	46.5	21.1	26.3	29.2
Yearling shearing
Razzle	74.1	29.8	27.2	25.4
Dazzle	55.0	28.1	32.0	30.0
Basil	43.4		30.7
Fazil	45.4		30.3
Fazil *	56.9	41.6	29.6	30.5

Lambs classified as lustrous at birth were found to have a very high proportion of medullated fibres in their fleece at lamb shearing and again as yearlings (Table 2). This was evident (P < 0.001) for lambs sired by Razzle and Dazzle expressing segregation for lustre at birth (44.2 vs 4.8% and 38.1 vs 4.9%, lustrous vs wild-type, respectively). Likewise, at lamb shearing the second-generation progeny of Fazil judged to be lustrous at birth had significantly more (P < 0.001) medullated fibres (54.6 ± 6.0%) than their wild type siblings (9.5 ± 2.6%). There was no significant difference in medullation between the sire groups as either lambs or yearlings within those judged to have a lustrous birth coat or within those judged to have a wild-type birth coat. Where the first-generation progeny of Basil and Fazil showed no segregation of lustre at birth, the levels of medullation in their fleece were 14.7 ± 3.0% and 17.4 ± 3.2% respectively as lambs, which was a significantly greater than in fleeces of the wild-type progeny of Razzle (4.8 ± 1.7%) and Dazzle (4.9 ± 0.8%) (P = 0.002). This difference was much smaller and not significantly different when measured again as yearlings (P = 0.276).

Table 2. The percentage of fibres carrying a medulla (medullation %) in samples of wool from lambs and yearlings from the first mating of four different sires and one sire (*) mated back to ewe progeny from the first mating. Progeny of ‘Razzle' and ‘Dazzle' suggested dominant inheritance of lustre at birth, while the progeny of ‘Basil' and ‘Fazil' suggested recessive inheritance. Standard error of the mean is in parentheses. Various comparisons within groups and the associated P values are listed.

Sire	medullation (%)		comparison	group	P value
	Wild-type	Lustre
Lamb shearing
Razzle	4.8 (±1.7)	44.2 (±8.4)	Razzle vs Dazzle	Wild-type	P = 0.988
Dazzle	4.9 (±0.8)	38.1 (±11.1)	Razzle vs Dazzle	Lustre	P = 0.459
			Lustre vs wild-type	First generation	P < 0.001
Basil	14.7 (±3.0)		Basil vs Fazil	Wild-type	P = 0.572
Fazil	17.4 (±3.2)		Dominant vs recessive	First generation	P = 0.002
Fazil *	9.5 (±2.6)	54.6 (±6.0)	Lustre vs Wild-type	Second generation	P < 0.001
			First vs second generation	Wild type	P = 0.219
Yearling shearing
Razzle	1.8 (±0.5)	20.0 (±7.3)	Razzle vs Dazzle	Wild-type	P = 0.987
Dazzle	1.8 (±0.3)	19.1 (±7.3)	Razzle vs Dazzle	Lustre	P = 0.885
			Lustre vs wild-type	First generation	P < 0.001
Basil	4.0 (±1.3)		Basil vs Fazil	Wild-type	P = 0.692
Fazil	5.5 (±1.9)		Dominant vs recessive	First generation	P = 0.276
Fazil *	4.0 (±1.7)	22.6 (±8.2)	Lustre vs Wild-type	Second generation	P < 0.001
			First vs second generation	Wild type	P = 0.858

In all sire groups, the amount of medullation decreased between lamb shearing and yearling shearing. There was more than double the amount of medullation in lamb fleece samples compared with that in yearling samples in lustrous progeny in the first generation (44.2 vs 20.0% and 38.1 vs 19.1%, for Razzle and Dazzle, respectively) and the second generation progeny of Fazil (54.6 vs 22.6%). The wild-type siblings of these animals in all three sire groups also exhibited more than twice as much medullation as lambs than when sampled as yearlings but at much lower levels. It was interesting that the proportion of medullated fibres at lamb shearing was greater in wool samples without a difference in average fibre diameter. The relationships between average fibre diameter and the proportion of fibres with a medulla were significantly different between those exhibiting lustrous and wild type birth coats (P = 0.001) (Figure 1). This remained evident when the animals were shorn again as yearlings although the overall level of medullation had declined, and the relationships were no longer significantly different (P = 0.074). Note that Figure 1 shows the sire groups separately, while the relationship analysis essentially compared phenotypes only because there was no significant difference between sire groups.

Discussion

Age, season and genotype have an important influence on the proportion of medullation in wool and will be discussed below. However, the notable difference between the wool samples from the sheep described here is that the fibre characteristics disobey generalised trends often reported for wool. Genotype has a strong influence on medullation (Holst et al. 1997), with sometimes profound differences between breeds and the N-type Romney produced a dramatic increase in both medullation and fibre diameter (Dry 1955). Medullation is generally found to increase with fibre diameter (Skarman and Nommera 1954) and yet, at similar diameters we observed two very different levels of medullation in the current experiment. Scobie et al. (1993; 1998) also reported exceptions to this generality. Another generality suggests that as fibre diameter increases, the amount of staple crimp tends to decrease which reflects decreasing fibre curvature. There are breed and selection line differences (Sumner 2007) which defy this generalisation, but staple crimp remains a rapid visual method to broadly separate raw wool into fibre diameter classifications. The differences in fibre curvature (24°/mm) between yearlings selected for and against loose wool bulk reported by Sumner (2007) were most likely due to the effect of a small number of genes. What may potentially be single genes studied here were associated with a similar difference in curvature, without a change in diameter comparing samples from yearlings that were judged wild-type or lustrous at birth. Breaking the nexus between fibre diameter, fibre curvature and medullation using these genotypes might enable us to understand how those traits are biochemically and also environmentally controlled. Other genes at the loci we assume are associated with the traits reported here may also control fibre curvature and medullation in other breeds.

A much greater proportion of the fibres of lambs were medullated compared with yearlings in all the sire groups and subgroups based on birth coat (Table 2). Lambs commonly carry greater levels of medullation than sheep of older ages in many breeds (Grandstaff and Wolf 1947; Goot et al. 1979; Holloway 2017) although the opposite has been reported (Tabbaa et al. 2001). Medullated fibres can disappear soon after birth in Australian Merinos (Gallagher 1970) and in New Zealand Romneys (Goot 1945). Scobie et al. (1993) reported a seasonal increase in medullation that lagged behind the annual summer peak in fibre diameter in Romney sheep. Since the samples collected here were only harvested on two occasions for each cohort, the decline in medullation between lamb shearing and yearling shearing cannot be separated in more detail from the effect of season. Furthermore, the fleeces from animals expressing recessive and dominant inheritance patterns were measured in different years, but with such dramatic differences, processing of these wools would have markedly different outcomes. The first-generation wild-type progeny of sires that showed no segregation of lustre at birth exhibited slightly more medullation as lambs (Table 2) and there is thus the potential that the genes involved in that pattern of inheritance might have some different influence on medullation. This had disappeared by yearling shearing and may be an effect of entirely different genes that affect medullation in lamb fleeces in the progeny of Fazil and Basil who we suspect may be related (Scobie et al. 2023).

The fibre diameter and curvature relationships were reported by Scobie et al. (2021, 2023) and are summarised in Table 1 for the reader. The data for fibre diameter reported earlier and for medullation reported here were used to examine the relationship between fibre diameter and medullation (Figure 1). Clearly medullation increases as fibre diameter increases in the samples from lambs, but when lambs had been judged lustrous at birth their lamb fleece carried many more medullated fibres than their wild type siblings irrespective of sire group. Genotype was found by Sumner (2007) to have a strong influence on fibre curvature measured as loose wool bulk, and to a lesser degree age of the animal affected bulk, but season tended to have little influence on this trait. Fibre diameter on the other hand is widely found to increase with age of the animal. The lambs from the second-generation progeny of Fazil that were judged to be lustrous at birth had fibres 3 µm greater in average diameter than the wild type (29.2 vs 26.3 µm) and again this suggests at that age some difference in these phenotypes. Further matings and larger numbers of progeny may be warranted to investigate the relationships reported from these small sire groups.

As noted by Scobie et al. (2023) it would be very interesting to further examine what is likely to be multigenic differences that cause the differences in loose wool bulk in the Perendales reported by Sumner (2007) and birth coat differences reported by (Allain et al. 2014). It would be interesting to know if the loci that lead to low fibre curvature in the sire groups here coincide with the loci that precipitate differences in loose wool bulk (Sumner 2007) and lamb birth coat (Demars et al. 2017). Now further questions arise with respect to medullation in those populations as well, particularly where the phenotypic differences were reported as ‘hairy' versus ‘woolly’, (Allain et al. 2014; Demars et al. 2017) because ‘hairy’ may encompass medullation. Reports by Plowman et al. (2022) showed there are marked differences in the keratin and keratin-associated proteins, along with differences in the cellular structure in the fibres of the progeny that showed segregation in the first generation studied here. Preliminary results from a study into the fibres from the second-generation progeny indicate that there is no difference in the relative abundancies of proteins, nor the internal fibre morphology, between these and the first-generation progeny (Plowman et al. unpublished). Clearly since the medulla is largely composed of voids, the differences in medullation found in the investigation reported here foretell that protein synthesis within the wool follicles is markedly affected and it would be interesting to examine the molecular basis of these genetic differences.

Acknowledgements

Hayley Lloyd identified three of the sires found on Lincoln Research farm, and it was her persistence that allowed us to bring these findings to light. We are also very grateful to Alison Waugh who brought to our attention the original lustrous sire carrying the recessive gene and Issi Anderson who bred Razzle and retained him as an entire male.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Ministry of Business, Innovation and Employment [C10X0710].