Effects of rapid inbreeding and of crossing of inbred lines on the body weight growth of sheep

Abstract

Sheep of three hill breeds, Scottish Blackface, Cheviot and Welsh Mountain and the reciprocal crosses among these breeds were inbred (mostly by parent × offspring mating) for four generations to five levels with inbreeding coefficients ofO, 0·25 (I1), 0·38 (I2), 0·50 (I3) and 0·59 (I4). Inbred females were also mated to unrelated inbred males of the same breed type to produce line crosses (LC). Each type of dam (except LC and I4) was mated to produce lambs of up to three different inbreeding levels thus allowing the effects of the individual's own inbreeding to be separately assessed from the effects of maternal inbreeding. This study examined body weight at a sequence of ages from birth to 4 years of age with 2369 animals (ages up to 78 weeks old) or 1062 animals (2 to 4 years old) contributing to the analyses.Inbreeding of the individual had a highly significant retarding effect on body weight and growth rate up to the I2 level for weights of lambs up to 24 weeks old, with a slight recovery in performance thereafter. For the later ages examined, the maximum depression was reached at the I3 stage. The partial recovery in performance at the higher levels of inbreeding for lambs less than 24 weeks of age was also noted within lines and did not therefore appear to arise only from a loss of lines as inbreeding proceeded. The effect of dam's inbreeding was to depress growth up to the I3 level and at most ages up to I4. Line-cross lambs were generally heavier (but not significantly so) than non-inbred (F2 and the pure equivalent, O2) and progeny of line-cross dams were similar in weight to those from other non-inbred (F1, or F2 and O1, or O2 dams) or slightly better. The more highly inbred the individuals the more, in general, they fell behind the weights of the corresponding non-inbreds as they grew older. Inbreeding may thus have permanently stunted the sheep.There were no significant differences between purebred and crossbred sheep in the rate of inbreeding depression. Only at the ages of 3 and 6 weeks was there a significant difference between the three pure breeds in the changes in weight attributable to inbreeding. Up to the age of weaning (15 weeks) the changes with inbreeding of individuals could not be explained statistically in terms of dominance effects alone and non-allelic interactions may therefore also be involved. At all ages the effect of inbreeding of the dam was consistent with dominance effects alone.

Understanding the relationship between the inbreeding coefficient and multilocus heterozygosity: theoretical expectations and empirical data

“…However, the observed inbreeding depression and/or heterosis in this population is not surprising or unprecedented. Inbreeding depression for morphological traits in other breeds has been described elsewhere (Falconer, 1989;Wiener et al, 1992). Furthermore, the two lines from which these sheep were derived showed an asymmetric response towards selection for backfat depth (Morris et al, 1997), an observation consistent with a trait having a relatively large dominance variance component (Falconer, 1989;Frankham, 1991;Merilä and Sheldon, 1999), and so being susceptible to inbreeding depression.…”

Effect of inbreeding on lean meat percentage and average daily gain in Hungarian Landrace pigs

“…Numerous authors experienced inbreeding depression in different domestic (MIGLIOR et al, 1995;WIENER et al, 1992;HORN and MELEG, 2000;CURIK et al, 2003;BRADE, 1991;FREYER et al, 2005;KOLK GEN SUNDAG et al, 2006;TRIEBLER et al, 1980;WOCAC, 2003;ZELLER and PIRCHNER, 1993) and experimental (ISSA and SEELAND, 2001) populations. Inbreeding depression is mainly apparent in traits that are closely related to fitness (FALCONER and MACKAY, 1996;PIRCHNER, 2004), but the reduction is not confined to reproductive traits.…”

Variance component analysis of skin and weight data for sheep subjected to rapid inbreeding

“…Another complication requiring consideration is the sensitivity of dominance components to epistasis. Epistasis has been observed for loci of large effect on bristle number [13] and the presence of epistasis in previous analyses of this data set were significant for live weight [25] but not for N S/ Np [30]. Such a sensitivity to at least some form of epistasis (which is a very general term covering all interactions among loci) might be expected.…”

“…Harris [9] and Cockerham [3] developed complete mathematical models for the genetic variance of non-random mating populations. These models were used to predict gene frequency changes in populations undergoing selection [4], and, while some attempts were made to apply them to agricultural populations [5] [29,30], weight [25], and measures of body size, reproduction, fertility and profitability [26][27][28]. However, a variance component analysis including dominance was not possible before now.…”