Low Selection Lines Research Articles

Divergent selection for natural antibodies in poultry in the presence of a major gene

BackgroundNatural antibodies (NAb) are antibodies that are present in a healthy individual without requiring previous exposure to an exogenous antigen. Selection for high NAb levels might contribute to improved general disease resistance. Our aim was to analyse the genetic background of NAb based on a divergent selection experiment in poultry, and in particular the effect of a polymorphism in the TLR1A gene.MethodsThe study population consisted of a base population from a commercial pure-bred elite white leghorn layer line and seven generations of birds from a High and Low selection line. Birds were selected for total KLH-binding NAb titer (IgTotal). An enzyme-linked immunosorbent assay was performed to determine NAb titers in blood plasma for IgTotal and the antibody isotypes IgM and IgG. NAb titers were available for 10,878 birds. Genotypes for a polymorphism in TLR1A were determined for chickens in generations 5, 6 and 7. The data were analysed using mixed linear animal models.ResultsThe heritability estimate for IgM was 0.30 and higher than that for IgG and IgTotal (0.12). Maternal environmental effects explained 2 to 3% of the phenotypic variation in NAb. Selection for IgTotal resulted in a genetic difference between the High and Low line of 2.4 titer points (5.1 genetic standard deviation) in generation 7. For IgM, the selection response was asymmetrical and higher in the Low than the High line. The frequency of the TLR1A C allele was 0.45 in the base population and 0.66 and 0.04 in generation 7 of the High and Low line, respectively. The TLR1A polymorphism had large and significant effects on IgTotal and IgM. Estimated genotypic effects suggest full dominance of the TLR1A C allele. Significant TLR1A by generation interactions were detected for IgM and IgTotal.ConclusionsThe effect of a polymorphism in the TLR1A gene on IgTotal and IgM NAb was confirmed. Furthermore, we provide experimental verification of changes in allele frequencies at a major gene with dominant gene action on a quantitative trait that is subjected to mass selection. TLR1A by generation interactions indicate sensitivity to environmental factors.

High and low feather pecking selection lines of laying hens differ in response to a judgment bias test

Feather pecking represents a serious problem in the poultry industry that can negatively affect production as well as the welfare of laying hens. Although feather pecking has been studied from many different angles, there are only a few studies of the relationship between feather pecking and cognition. This study aims to compare the cognitive performance of hens from the high feather pecking (HFP) and the low feather pecking (LFP) lines in a visual discrimination (Go/No-Go) task and to study their decision making under ambiguity using the judgment bias test.Twenty HFP and 20 LFP hens were trained in a visual discrimination task to approach a coloured feeder (white for half of the hens and black for the other half) containing a reward (one mealworm) and to refrain from approaching a feeder with a different colour (colour opposite to positive, i.e. black or white) to avoid punishment (water spraying). During the subsequent judgment bias tests hens were tested in the presence of the positive, negative or ambiguous stimulus (grey coloured feeder), always one type of stimulus at a time. The latencies to reach each of the stimuli were recorded. At the end of the visual discrimination training phase, 36 out of 40 hens successfully discriminated the positive and the negative coloured feeder. There was a slower association of the coloured feeder with the reward in the HFP line and HFP hens did not suppress the response to the negative stimulus as effective as HFP hens, which could be a sign of their high motor impulsiveness. However, in the judgment bias test HFP hens approached the ambiguous feeder significantly faster than LFP hens (HFP 13.59 ± 1.11 s, LFP 16.68 ± 1.10 s, P < 0.05), that can be interpreted as evidence that hens from the HFP line are more optimistic, i.e. that they are in a more positive affective state. The high motor impulsiveness of HFP hens provides another possible explanation for their response to the ambiguous stimulus. However, higher motor activity of the HFP line did not affect the results of the judgment bias test. Furthermore, there were no significant differences in plasma corticosterone levels between the lines, suggesting that differences in stress levels might not explain the results of the judgment bias test.

Excreta emissions in progeny of low and high enteric methane yield selection line sheep fed pasture of different qualities

Selection of sheep with low enteric methane (CH4) emissions is a greenhouse gas (GHG) mitigation option suitable for pastoral systems. However, the effect of breeding sheep with low enteric CH4 emissions on excreta output and associated CH4 and nitrous oxide (N2O) emissions and therefore total GHG emissions are not known. The objective of the current experiments were to determine excreta output, and estimate associated GHG emissions, from progeny of low and high enteric CH4 per unit of dry matter intake (DMI) selection line sheep (CH4/DMI). The animals were fed two qualities of cut perennial ryegrass-based pasture (very mature vs. vegetative, 12 animals per CH4/DMI line) in Exp. 1 and cut pasture in two repeated seasons (autumn and winter; 15 animals per CH4/DMI line × 2 seasons) in Exp. 2. Total faecal and urine output was determined on individual animals, followed by enteric CH4 emission measurements in respiration chambers. GHG emissions from urine (N2O) and faeces (CH4 and N2O) were estimated based on New Zealand Agricultural GHG Inventory methodology. There was no interaction between CH4/DMI selection line and diet quality in Exp. 1 or seasons in Exp.2. Total daily faecal output of DM, organic matter (OM) and neutral detergent fibre (NDF; all g/d) and associated calculated faecal CH4 emissions were greater for low compared to high CH4/DMI sheep in Exp. 1 (P < 0.05), while being similar between CH4/DMI selection lines in Exp. 2. Nitrogen (N) excretion and N partitioning into urine, faeces and body retention, and calculated excreta N emissions, were mostly similar between CH4/DMI selection line sheep in both experiments. Except, faecal N output (g/d and per unit of N intake) and associated calculated direct faecal N2O-N emissions (g/d) were greater in low compared to high CH4/DMI sheep in Exp. 1 (P < 0.05). Enteric CH4 emissions were numerically 8% less (P = 0.15) in Exp.1 and 10% less (P = 0.004) in Exp. 2 and total animal level GHG emissions (CH4 and N2O) were numerically 7% less (P = 0.21) in Exp. 1 and 8% less (P = 0.006) in Exp.2 for progeny of the low compared to the high CH4/DMI line sheep. In conclusion, the magnitude of difference in enteric CH4 (expressed as CO2-equivalent) between low and high CH4/DMI selection line sheep were still present when CH4 from faeces and N2O emissions from urine and faeces were also accounted for. The animal genetic traits were expressed independent of environmental factors, i.e. pasture quality and season.

Ruminal fermentation characteristics of goats selectively bred to have greater consumption of juniper foliage

The objective of this study was to determine if ruminal fermentation characteristics differed in goat lines selectively bred for high or low consumption of J. pinchotii foliage. Ten Boer × Spanish-composite bucks (age = 2 yr; initial body weight (BW) = 67.2 ± 4.3 kg) previously selected for high (High; n = 5) or low (LOW; n = 5) juniper consumption selected from each line were subjected to three periods: Period 1, d 0, grazing native range predominated by Juniperus spp; Period 2, d 1 to 26, group fed for ad libitum intake of sorghum × sudan hay; and Period 3, d 27 to 63, indivually fed sorghum × sudan hay at 3% of body weight (BW) and offered fresh J. pinchotii foliage for ad libitum intake. During each period, rumen fluid was evaluated for pH, VFA, ammonia N (NH3-N), and juniper foliage was evaluated for 48-hr true IVDMD (tIVDMD). There was a Selection line × Period interaction observed for total VFA (P < 0.01) explained by a reranking of Total VFA observed in HIGH compared to LOW selection lines in Periods 1 and 3, but the effect was reversed in Period 2. However, total VFA did not differ within any period between HIGH and LOW. During Period 1, rumen fluid from HIGH contained greater (P < 0.05) ruminal isovalerate and tended to have greater ruminal isobutyrate (P = 0.09) and NH3-N (P = 0.07) than LOW. There was a period effect (P < 0.02) for all response variables. When transitioned to a sorghum × sudan hay diet (Period 2), total ruminal VFA concentrations declined 26 and 4% for HIGH and LOW, respectively, and total VFA (P = 0.08) and valerate (P = 0.09) tended to be greater for LOW compared with HIGH. In vitro digestibility of juniper did not differ (P ≥ 0.48), but numerically, digestibility declined 13 and 12% for HIGH and LOW, respectively, from Period 1 to 2. During Period 3, when transitioned to individual pens with ad libitum access to sorghum × sudan hay and juniper foliage, juniper intake did not differ (P = 0.16) with HIGH consuming 1.2 g/kg BW of juniper compared with 0.62 g/kg BW for LOW. No differences were detected (P > 0.05) for VFA, NH3-N, pH, and tIVDMD between HIGH and LOW goats during Period 3. Goats selectively bred for high or low juniper consumption exhibited minor differences in rumen fermentation parameters measured in the current study.

Selective breeding for high natural antibody level increases resistance to avian pathogenic Escherichia coli (APEC) in chickens

Keyhole limpet hemocyanin (KLH)-binding natural antibody (NAb) titers in chickens are heritable, and higher levels have previously been associated with a higher survival. This suggests that selective breeding for higher NAb levels might increase survival by means of improved general disease resistance. Chickens were divergently selected and bred for total NAb levels binding KLH at 16 weeks of age for six generations, resulting in a High NAb selection line and a Low NAb selection line. To for test differences in disease resistance, chickens were challenged with avian pathogenic Escherichia coli (APEC) in two separate experiments. Chickens at 8 days of age received one of four intratracheal inoculations of 0.2 mL phosphate buffered saline (PBS): 1) mock inoculate, 2) with 0.2 mL PBS containing 108.20 colony-forming units (CFU)/mL APEC, 3) with 0.2 mL PBS containing 106.64 CFU/mL APEC, and 4) with 0.2 mL PBS containing 107.55 CFU/mL APEC. Mortality was recorded during 7 days post inoculation. Overall, 50–60% reduced mortality was observed in the High line compared to the Low line for all APEC doses. In addition, morbidity was determined of the surviving chickens at 15 days of age. The High line had lower morbidity scores compared to the Low line. We conclude that selective breeding for high KLH-binding NAb levels at 16 weeks of age increase APEC resistance in early life. This study and previous studies support the hypothesis that KLH-binding NAb might be used as an indicator trait for to selective breed for general disease resistance in an antigen non-specific fashion.

Long-term selection of chickens for body weight alters muscle satellite cell behaviors

Muscle satellite cells (SC) are resident stem-like cells that play an integral role in skeletal muscle growth and repair. Understanding how SC maintain their identities and dynamic properties is critical to animal growth. However, the genetic and environmental factors governing SC behaviors and the underpinning mechanisms remain unknown. To explore whether genetic selection influences SC behaviors, we used 2 lines of chickens selected for over 50 generations with over a 10-fold difference in body weight at 56 d of age-the Virginia high weight selection (HWS) and low weight selection (LWS) lines. To study these 2 lines, we performed both in vivo and in vitro experiments. In vivo, we studied the abundance of SC in normal physiological settings and tested their functional roles in muscle regeneration using a muscle injury model. In vitro, we isolated SC from chicken skeletal muscle and assayed their ability to proliferate and differentiate under cultured conditions. Immunohistochemical staining of breast muscle (pectoralis major) revealed that muscle fibers from HWS chickens possessed more SC than those from LWS. Further analysis showed that the SC pool from HWS muscles contained a higher percentage of activated SC compared to that of LWS. When isolated SC from HWS and LWS muscles were cultured, HWS SC exhibited greater abilities to proliferate and differentiate than those SC from LWS. To test whether the observed in vitro differences in SC properties could be confirmed in vivo, we subjected chicken breast muscle to barium chloride to induce muscle injury and regeneration. Consistent with in vitro data, breast muscle in HWS chicken experienced a faster and more robust recovery than that of LWS, as evidenced by quicker regeneration and larger muscle fiber size. Taken together, these findings suggest divergent selection for body weight not only results in correlated responses in SC number, but also changes SC growth kinetics. Further dissection of the molecular mechanism will aid the identification of the target molecules for growth intervention in chickens.

Artificial selection on ant female caste ratio uncovers a link between female-biased sex ratios and infection by Wolbachia endosymbionts.

Social insect sex and caste ratios are well-studied targets of evolutionary conflicts, but the heritable factors affecting these traits remain unknown. To elucidate these factors, we carried out a short-term artificial selection study on female caste ratio in the ant Monomorium pharaonis. Across three generations of bidirectional selection, we observed no response for caste ratio, but sex ratios rapidly became more female-biased in the two replicate high selection lines and less female-biased in the two replicate low selection lines. We hypothesized that this rapid divergence for sex ratio was caused by changes in the frequency of infection by the heritable bacterial endosymbiont Wolbachia, because the initial breeding stock varied for Wolbachia infection, and Wolbachia is known to cause female-biased sex ratios in other insects. Consistent with this hypothesis, the proportions of Wolbachia-infected colonies in the selection lines changed rapidly, mirroring the sex ratio changes. Moreover, the estimated effect of Wolbachia on sex ratio (~13% female bias) was similar in colonies before and during artificial selection, indicating that this Wolbachia effect is likely independent of the effects of artificial selection on other heritable factors. Our study provides evidence for the first case of endosymbiont sex ratio manipulation in a social insect.

Genetic and Genomic Response to Selection for Food Consumption in Drosophila melanogaster.

Food consumption is an essential component of animal fitness; however, excessive food intake in humans increases risk for many diseases. The roles of neuroendocrine feedback loops, food sensing modalities, and physiological state in regulating food intake are well understood, but not the genetic basis underlying variation in food consumption. Here, we applied ten generations of artificial selection for high and low food consumption in replicate populations of Drosophila melanogaster. The phenotypic response to selection was highly asymmetric, with significant responses only for increased food consumption and minimal correlated responses in body mass and composition. We assessed the molecular correlates of selection responses by DNA and RNA sequencing of the selection lines. The high and low selection lines had variants with significantly divergent allele frequencies within or near 2081 genes and 3526 differentially expressed genes in one or both sexes. A total of 519 genes were both genetically divergent and differentially expressed between the divergent selection lines. We performed functional analyses of the effects of RNAi suppression of gene expression and induced mutations for 27 of these candidate genes that have human orthologs and the strongest statistical support, and confirmed that 25 (93%) affected the mean and/or variance of food consumption.

Growth, feed intake and maternal performance of Angus heifers from high and low feed efficiency selection lines

Feed conversion efficiency is an important factor affecting profitability for cow-calf production systems. One method of characterising feed conversion efficiency is by calculating residual feed intake (RFI), the difference between measured and expected feed intakes. A high RFI value indicates low feed efficiency while a low RFI value indicates high feed efficiency. The present experiment used 49 Angus heifers sired by Angus bulls selected for high or low RFI. The progeny were bred on commercial farms and then transferred to Massey University at weaning at ~200 days of age. The heifers were weighed at ~30-day intervals and were mated at 15 months, calved at 2 years old and reared their calf to ~160 days of age. Each heifer had recorded its own liveweight gain (from its weaning to weaning of its first calf), herbage intake at 350 and 450 days of age using the n-alkane technique and maternal performance (calf production). These records were analysed to estimate the RFI of each heifer, and to determine differences in RFI between selection lines. There were no significant differences (P &gt; 0.05) in liveweight or dry matter requirements between selection lines at Day 350, but differences were significant (P &lt; 0.05) at Day 450 such that the high feed efficiency line were 24.6 kg heavier and required an extra 0.32 kg/head.day of herbage. There were no significant differences (P &gt; 0.05) between the two lines in herbage intake or RFI measured at either Days 350 or 450. There were no differences (P &gt; 0.05) in pregnancy rates, calf birth dates or birthweights, estimated 24-h milk production or calf weaning weights between the two lines. This experiment shows that beef cattle selected for low RFI have higher growth rates and heavier liveweights than cattle selected for high RFI; both lines had similar calf production at first breeding.

Onset of puberty and early-life reproduction in Angus females divergently selected for post-weaning residual feed intake

Angus cattle that had been divergently selected for Low and High post-weaning residual feed intake (RFI) were used in two projects to evaluate early-life reproductive performance and onset of puberty in females. In the first project, data on the 1999-born females (n = 64) were evaluated for weight, subcutaneous fat (P8 fat depth) and reproductive performance over two breeding cycles. These females that were the result of 1.0–2.5 (mean of 1.8) generations of selection, had mean (±s.e.) post-weaning RFI of –0.82 ± 0.19 kg/day and 0.57 ± 0.18 kg/day for the Low and High RFI selection lines, respectively. In the second project, ultrasonography was used to scan the ovaries of the 2008-born heifers (n = 121) on four occasions following weaning. In these heifers the presence of corpus luteum provided evidence of ovulation, and hence the commencement of onset of puberty. The average of the estimated breeding value (EBV) for RFI of the parents of the Low RFI and the High RFI selection line heifers were –0.72 ± 0.05 and 0.37 ± 0.03 kg/day, respectively. In the first project, no significant selection line differences were evident for weight, age at first mating, pregnancy and calving rates (percentage of females that were pregnant, or the percentage that calved out of the total number used for mating), calf birthweight and weight of calf born per female exposed to bull. Females from the Low RFI line had significantly (P &lt; 0.05) lower P8 fat depth relative to their High RFI contemporaries at most of the measurement dates (e.g. 9.2 ± 0.5 versus 12.0 ± 0.5 mm at the start of first mating). Low RFI females also calved significantly (P &lt; 0.05) later in the calving season than High RFI females (35.7 ± 3.0 versus 27.6 ± 2.4 days). The results indicate that there is a delayed pregnancy date during the first mating season leading to a later calving date for the Low RFI heifers. The later first calving date was then maintained at subsequent calving. The later calving, however, did not impact on pregnancy and calving rates. In the second project, it was observed that irrespective of selection line, heifers that had attained onset of puberty had significantly (P &lt; 0.05) greater P8 fat depth than those that had not. Hence the expectation was that, relative to High RFI heifers, the Low RFI heifers with their lower P8 fat, will attain onset of puberty at a slightly older age. This expected trend was observed but the difference was not significant, and further investigations are recommended.

Natural variation in learning and memory dynamics studied by artificial selection on learning rate in parasitic wasps

Animals form memory types that differ in duration and stability. The initial anaesthesia-sensitive memory (ASM) can be replaced by anaesthesia-resistant memory (ARM), and/or by protein synthesis-dependent, long-term memory (LTM). We previously showed that two closely related parasitic wasp species differ in learning rate and memory consolidation. In Cotesia glomerata, LTM lasting at least 24 h was formed after single-trial conditioning, whereas single-trial conditioning led to ARM that waned before 24 h in Cotesia rubecula. This species formed LTM only after repeated conditioning trials spaced in time. Here, we used artificial selection on learning rate to investigate whether selection for a low learning rate in C. glomerata would result in C. rubecula-like memory dynamics. Memory consolidation was tested by using cold-shock anaesthesia and protein synthesis inhibitors. After single-trial conditioning, ARM was consolidated within hours in unselected C. rubecula, but directly, without an intermediate ARM phase, into LTM in unselected C. glomerata. We obtained low learning rate selection lines of C. glomerata wasps that, like C. rubecula, did not form LTM after single-trial conditioning, to see whether such wasps would then consolidate ARM instead of LTM. We showed that this was not the case. The selected wasps formed LTM after repeated, spaced conditioning trials, but formed only ASM without consolidation of ARM or LTM after single-trial learning. Ecological consequences of this type of memory formation are discussed.

Identification of genetic markers associated with residual feed intake and meat quality traits in the pig

Residual feed intake (RFI) has become increasingly important and is being considered as a more reasonable approach to evaluate feed efficiency in livestock. However, the cost and technical difficulties in measuring this trait restrict the extensive adoption of RFI selection, and this makes marker assisted selection (MAS) a feasible tool. In addition, the effects on meat quality caused by low RFI selection have yet to be clarified. In this study, 11 SNPs from eight candidate genes were evaluated in a Yorkshire pig experimental population (n=169) consisting of a low RFI selection line and a randomly selected control line. Associations of these SNPs with RFI, growth rate, carcass composition, and meat quality measures including water holding capacity, pH at 2d postmortem, meat color and sensory traits were analyzed. The SNPs FTO p.Ala198Ala and TCF7L2 c.646+514A>G showed significant (P<0.05) and suggestively significant (P<0.1) associations with RFI, respectively. The MC4R SNP p.Asp298Asn was associated with backfat but it was not with ADG and meat quality attributes. Both SNPs within HNF1A were associated with intramuscular lipid content and sensory juiciness. The SNPs ACC1 c(*)384C>T and TCF7L2 c.646+514A>G were significantly (P<0.05) associated with ADG. The SNPs CTSZ p.Arg64Lys and TCF7L2 c.646+514A>G were associated with both visual scoring of meat color and the objective L-value measure of meat color. This study has identified potential genetic markers suitable for MAS in improving RFI, ADG, and meat color traits, but these associations need to be validated in other larger populations.

Quantitative analysis of performance, carcass and meat quality traits in cattle from two Australian beef herds in which a null myostatin allele is segregating

Two Australian beef cattle herds, in which selection for muscularity was a primary objective, were used in this study to identify bovine myostatin (MSTN) mutations associated with increased muscling, and to assess associations between genotype and performance, carcass and meat quality traits. One was a research herd (herd A) established from Angus × Hereford cows, and comprised a high and low muscle selection line. The other (herd B) was a commercial beef enterprise with cattle of Angus and Charolais origin. Sequencing of the MSTN coding region and flanking splice junctions in an initial sample of 34 animals from both herds identified the 821 del11 mutation as well as six other polymorphic sites. The nucleotide 374–50C &gt; T polymorphism in intron 1 was found to be in linkage disequilibrium with the 821 del11 mutation, with both variants confined to the high muscle selection line in herd A. No other variants were exclusive to either of the two herd A selection lines. The effect of the 821 del11 mutation was further investigated in a total of 803 cattle from both herds. A relatively high prevalence of 821 del11 heterozygotes (herd A 16%; herd B 23%) was found and heterozygotes had significant advantages in eye muscle area and muscle score over their wildtype counterparts, and did not differ in meat quality. Retail beef yield from steers was higher for the 821 del11 heterozygotes from herd A (67.0 v. 63.5%) and herd B (71.8 v. 68.6%), relative to homozygous wildtype contemporaries, demonstrating the benefits of incorporating single null MSTN alleles into breeding programs.

Crawl-out behaviour in response to predation cues in an aquatic gastropod: insights from artificial selection

Local adaptation to predation often occurs in populations experiencing stable predator regimes. Under such conditions, prey species may respond by fine-tuning their behavioural defences towards a local optimum, although it is often difficult to ascertain whether such local adaptation is due to selection on fixed traits, developmental plasticity that is dependent on relatively long term exposure to environmental cues or phenotypic plasticity that can respond rapidly to a changing environment. Here we investigate whether anti-predator behaviour in two populations of the freshwater gastropod Lymnaea stagnalis responded to artificial selection. Previous work had shown that populations of this species showed a higher level of innate avoidance behaviour (crawling above the water line) in the presence of predatory fish compared with sites lacking this predation threat. By selectively breeding from high and low response selection lines, we demonstrated that this crawl-out behaviour responds rapidly to artificial selection: high response selection lines showed a significant increase and low response selection lines a significant decrease in avoidance compared with non-selected control lines. This suggests that the crawl out response in this species is heritable, and that there is potential for a response to selection in natural populations, which may produce the divergence in the plasticity of crawl out behaviour found between gastropod populations experiencing high and low predation intensity.

Effects of divergent selection for serum insulin-like growth factor-I concentration on performance, feed efficiency, and ultrasound measures of carcass composition traits in Angus bulls and heifers

Angus bulls and heifers from lines divergently selected for serum IGF-I concentration were used to evaluate the effects of IGF-I selection line on growth performance and feed efficiency in 2 studies. In study 1, bulls (low line, n = 9; high line, n = 8; initial BW = 367.1 +/- 22.9 kg) and heifers (low line, n = 9; high line, n = 13; initial BW = 286.4 +/- 28.6 kg) were adapted to a roughage-based diet (ME = 1.95 Mcal/kg of DM) for 24 d and fed individually for 77 d by using Calan gate feeders. In study 2, bulls (low line, n = 15; high line, n = 12; initial BW = 297.5 +/- 34.4 kg) and heifers (low line, n = 9; high line, n = 20; initial BW = 256.0 +/- 25.1 kg) were adapted to a grain-based diet (ME = 2.85 Mcal/kg of DM) for 32 d and fed individually for 70 d by using Calan gate feeders. Blood samples were collected at weaning and at the start and end of each study, and serum IGF-I concentration was determined. Residual feed intake (RFI) was calculated, within study, as the residual from the linear regression of DMI on midtest BW(0.75), ADG, sex, sex by midtest BW(0.75) and sex by ADG. In study 1, calves from the low IGF-I selection line had similar initial and final BW and ADG, compared with calves from the high IGF-I selection line. In addition, DMI and feed conversion ratio were similar between IGF-I selection lines; however, calves from the low IGF-I selection line tended (P < 0.10) to have lesser RFI than calves from the high IGF-I selection line (-0.26 vs. 0.24 +/- 0.31 kg/d). In study 2, IGF-I selection line had no influence on performance or feed efficiency traits. However, there was a tendency (P = 0.15) for an IGF-I selection line x sex interaction for RFI. Bulls from the low IGF-I selection line had numerically lesser RFI than those from the high IGF-I selection line, whereas in heifers, the IGF-I selection line had no effect on RFI. In studies 1 and 2, weaning and initial IGF-I concentrations were not correlated with either feed conversion ratio or RFI. However, regression analysis revealed a sex x IGF-I concentration interaction for initial IGF-I concentration in study 1 and weaning IGF-I concentration in study 2 such that the regression coefficient was positive for bulls and negative for heifers. These data suggest that genetic selection for postweaning serum IGF-I concentration had a minimal effect on RFI in beef cattle.

Selection for salt tolerance and its genetic basis in perennial ryegrass (Lolium perenne L.)

Salt tolerance of two selected lines of Lolium perenne L. (high selection and low selection) was assessed in comparison with control unselected line after three weeks growth in solution culture at 0, 50, 100, 150, 200, 250, and 300 mol m-3 NaCl. The high selection line had significiantly higher root growth compared with the low selection and unselected lines at all salt concentrations. In order to assess the usefulness of a second cycle of selection, 8000 seeds of each of the high selection and unselected (control) lines were screened after three weeks growth in 325 mol m-3 NaCl. A large number of individuals in the high selection line produced remarkably longer roots than the control lines. Pair crosses were made between the high selection and the low selection in order to investigate additive genetic component at two NaCl treatments, 150 and 250 mol m-3, Additive genetic component was quite high at both the salt treatments.

Allelic recombination in the his-I locus of Neurospora crassa in isolates showing high and low frequency of intergenic recombination

From lines selected for high and low frequency of second division segregation (SDS) for asco in linkage group (LG) VI, one wild-type isolate of mating type (mt) A was picked out from the high and the low selection line, respectively. Both isolates were chosen on the basis of showing extreme SDS-frequencies, and both belonged to the last generation of selection. The two wild-type isolates were crossed with a strain carrying marker genes suitable for the analysis of allelic recombination in the his-I locus in LG V. The progeny of mt a was analysed for the influence of high and low line background on allelic recombination. The frequency of his-I prototrophs seems to be related to the high and low line background of the isolates. The two backgrounds showed no different influence on the recombination of markers flanking the his-I locus among his-I prototrophs. Variation of the frequency of his-I prototrophs not attributable to the rec-I locus is suggested. The data are interpreted according to Holliday's model of genetic recombination.

Recombination in other chromosomal regions than the interval subjected to selection, in lines of Neurospora crassa selected for high and for low recombination frequency

Recombination studies involving regions of the linkage groups (LG:s) I, II, III, IV, V and VI were performed in wild-type isolates of Neurospora crassa picked out from lines selected for high and for low frequency of second division segregation (SDS) for asco in the left arm of LG VI. Significant differences in recombination frequencies between high and low selection lines were obtained when proximal regions in the LG:s I, IV, V and the left arm of LG VI were scored, while no difference between lines could be established in the regions located in the LG:s II, III and in the right arm of LG VI. The correlation between SDS-frequencies for asco obtained with different asco tester strains was much stronger than the correlation between SDS-frequencies for asco and the recombination frequency in any of the other regions tested. This suggests that some of the genetic factors separating the recombination frequencies of the two selection lines operate mainly within the asco-centromere interval.

Variation of high and low recombination frequencies in Neurospora crassa owing to genetic and environmental conditions

From lines selected for high and low frequency of second division segregation (SDS) for asco in linkage group (LG) VI, one wild-type isolate of mating type (mt) a showing extreme SDS-frequency was picked out from the high and the low selection line, respectively. Both wild types were crossed with a strain carrying a met-8 (LG III) and a his-1 (LG V) allele. The met-8′ progeny of mt A was analysed with reference to SDS-frequency for asco. As all crosses were performed in repeats, the influence of environmental conditions could be analysed. No significant influence of the regions surrounding the his-I locus could be established, neither for high, nor for low SDS-frequencies. Environmental conditions significantly influenced the SDS-frequencies of isolates which had a high line background. The isolates with a low line background were less affected in this respect. The SDS-frequencies of isolates with a low line background showed a bimodal-like distribution. This suggests the influence of a major genetic factor. If this is located in any of the LG:s I, III and V, it would not be closely linked to the mt, the met-8 or the his-I locus.

Different effects of temperature on recombination in lines of Neurospora crassa selected for high and for low recombination frequency

Wild-type isolates of Neurospora crassa, picked out from lines selected for high and for low frequency of second division segregation (SDS) at 25oC for asco in linkage group (LG) VI, were scored for temperature effects on SDS-frequency for asco and on recombination frequencies in two adjacent intervals in LG II. Results obtained at 25oC and 17oC were compared. In the high selection line, the SDS-frequency for asco in both mating types (mt:s) decreased at 17oC compared to results obtained at 25oC. In the low selection line the SDS-frequency for asco in mt A increased at 17oC compared to results obtained at 25oC, while no significant effect of temperature on SDS-frequency for asco could be established in mt a. In LG II, temperature effects on recombination frequencies could only be established in the interval spanning the centromere, in one of the crosses involving an isolate from the high selection line. The recombination frequency increased at 17oC compared to results obtained at 25oC. The results are interpreted in terms of varying access in different genotypes, to gene products promoting and obstructing meiotic recombination, respectively.