EFFECT OF SEX-LINKED DWARF GENE ON EXTERIOR APPEARANCE , PRODUCTIVE PERFORMANCE AND EGG CHARACTERISTICS IN A COLORED BROILER DAM LINE

The effect of sex-linked dwarf gene was investigated through comparison of dwarf hens with their full-sib normal sisters obtained by mating heterozygous males (DW/dw) to normal females (DW/_) from line F (used as maternal form for production of slow-growing colored chickens) with respect to the following traits: body weight, shank and keel length at 40 weeks of age, age of sexual maturity (at 50 % production), egg production, egg weight, feed intake, feed utilization, livability, fertility, hatchability and egg quality characteristics. The results demonstrated that the dw gene caused statistically significant reduction of body weight by 29.15 %, shank length by 20.17 %, keel length by 7 % and egg weight by 5.72 % (p<0.001). The hens with normal genotype attained sexual maturity 7 days earlier (p<0.001), but nevertheless, rate of lay was similar to that of mini forms. There were no considerable differences between both genetic groups with respect to livability percentage over the production cycle. Dwarf hens consumed by 23.38 % less feed (p<0.01) than normal sized hens and converted nutrients more efficiently by 12.69 % (p<0.05). The presence of dw gene in hen genotype increased the eggshell percentage, reduced egg yolk and albumen weights and had no effect on their quality. The positive effect of the sex-linked dwarf gene on economically important traits – feed intake and feed conversion, hatchability of eggs set, is a prerequisite for the development of more efficient broiler breeder hens for production of slow-growing chickens.


Introduction
Efficiency is a key factor in poultry breeding.One of the possible ways for its improvement is the utilization of mini-hens, carriers of a recessive gene located in the sex chromosome.On the basis of extensive literature data, Merat (1990) outlines that the so called dwarf gene, described by Hutt (1959), results in reduction of body weight of birds by 33 % and feed consumption by 20-25 %.The existing interest towards the practical application of that gene is due to several advantages of mini hens: higher stocking density per square meter (Charpentier, 2009), better utilization of dietary nutrients for egg production (Galal and Younis, 2006;Galal et al. 2007), no need from application of restricted feeding programmes (Decuypere et al., 2006;Dawkins and Layton, 2012), higher survival rate (Garcês et al., 2001), better reproduction ability (Decuypere et al., 2012), better resistance to heat stress (Gowe and Fairfull, 1995;Rashid et al., 2005;Islam, 2005).These advantages resulted in using the dwarf gene in some broiler breeder maternal lines for production of standard broilers, chickens with medium growth or slow growth such as the Label rouge type.Data reported by EFSA (2010) showed that in Europe, female dwarf broiler breeders are 18-20 %, whereas in France, constitute the major part of broiler hens.
The production of broilers from mini maternal forms is based on the acknowledged fact that when the sire is from the standard DW/DW genotype, the progeny is of normal body size.
The most commonly used breeding scheme for production of dwarf maternal form is through crossing homozygous recessive dwarf male dw/dw with a standard female (DW/_).A second alternative is to use heterozygous DW/dw cocks, but this is related to extra costs and time.
The economically relevant differences between normal and mini-hens vary depending on the production type and genetic diversity of the population, in which the dw gene is introduced.The introduction of the gene in light populations leads to delayed sexual maturity and lower egg production (Merat, 1990;Horst and Becker, 1991;Merat and Bordas, 1991;Merat et al., 1994;Garces et al., 2001;Missohou et al., 2003), whereas in broiler hens, this negative effect is less pronounced or absent (Marks, 1981;Kousiakis et al., 1985).Anonymous (2003) reports about relatively higher egg production and higher number of eggs fit for incubation in dwarf breeders consequently to lower number of double-yolk eggs, eggs with soft or thin shell, irregular shape or other defect.After introduction of dw gene in broiler lines selected for high egg production, Leenstra et al. (1986) established that hens with normal genotype laid about 17 % of all ovulated follicles in defective eggs while for dwarf hens this percentage was only 2 %.The economic analysis of results from using a mini-maternal form for production of broiler chickens showed that financial costs for rearing of the breeder flock were reduced by 15 %, and for production of one day-old chickby 20 % (Charpentier, 2009;Tudik et al., 2011).
The objective of the present study was to evaluate the effect of sex-linked dwarf gene on exterior appearance and productive performance of hens of a colored broiler dam line used for production of slow-growing chickens.

Stock, Husbandry and Traits measured
The experiment was performed in the Selection Base of the Poultry Breeding Unit, Agriculture Institute -Stara Zagora from December 2013 to June 2014.Dwarf hens and their normal sibs were used for the current study.They were obtained by mating heterozygous cocks (DW/dw) and hemizygous normal hens (DW/_) from line F which was developed in the Selection Base and used as maternal form for production of colored slow-growing broiler chickens.Dwarf and normal genotype birds were separated by visual appraisal on the basis of body size and shank length.At 20 weeks of age, two groups were formed -with normal (DW/_) and dwarf genotype (dw/_).Each group consisted of five replicates of 20 hens and 2 cocks, housed in floor pens.The birds were reared until 50 weeks of age under identical conditions, in the same laying house, on wooden shavings litter as required by the production system used in the Selection base, with free access to feed and water.They were fed a diet containing 16 % crude protein and 2750 kcal/kg metabolizable energy.
During the experiment the following parameters were observed: Body weight and body measurements: Body weight was determined individually at 40 weeks of age for each genotype using a technical balance with precision of 5 g.At the same age, keel length (distance between the anterior and posterior end of the sternum) and shank length (from the top of hock joint to the foot pad) were determined with a measuring tape.
Egg production traits: The number of eggs laid and dead birds was recorded daily for each replicate.On this basis, the hen day egg production (%) was calculated for the entire experimental period.Sexual maturity was determined when 50 % egg production has been reached.Mean egg weight was monitored at 2-week intervals from the beginning of lay to the end of the trial via weighing the daily yield of all replicates of both genotypes.Egg mass was calculated on the basis of hen day egg production and mean egg weight.
Feed intake was recorded at the end of each week.Feed conversion ratio (FCR) was expressed as kg of feed consumed per kg of egg produced.
Egg quality traits were evaluated at 40 weeks of age.For this purpose, 30 eggs from each group (six eggs per replicate), laid within a day were examined.
The weights of the whole egg, albumen, yolk and eggshell (together with membranes) were determined with precision of 0.01 g.Eggshell weight, albumen weight and yolk weight were expressed as a percentage of the egg weight.The length and breadth of eggs were measured using a digital caliper with precision of 0.01 mm.Egg volume and egg surface area were calculated on the basis of exterior egg dimensions (Narushin, 2005).Albumen and yolk heights were measured with Ames micrometer (precision 0.01 mm), albumen and yolk diameterswith digital caliper (precision 0.01 mm), eggshell thickness together with the inner membranewith micrometer (precision 0.001 mm) at three different points (top, middle, and bottom) and presented as average of the three measurements.Egg albumen quality was assessed through albumen index and Haugh units; yolk qualityvia the yolk index.
For determination of the reproduction potential of dwarf hens and their full-sib normal sisters, 400 eggs from each group collected over 7 consecutive days at the end of the experiment, were set for incubation.The fertility rate, hatchability of eggs set and hatchability of fertile eggs were calculated on the basis of obtained results.

Statistical Analyses
The analysis of data was performed with Statistica software (Stat Soft), using one-way analysis of variance and the following linear model: Yij = μ + g i + e ij , where Yijj th observation of the respective trait µ -grand mean of the trait g ifixed effect of the ith genotype (i=1,2) e ijrandom error

Results and discussion
The data presented in Table 1 show that under the influence of the dw gene, the body weight of mini-forms was reduced by 29.15% as compared to those of their full-sib normal sisters.The dwarf gene does not reduce uniformly the growth of the different body parts.The length of metatarsus was most substantially reduced (20.17%), a reason for the so-called shortlegness of dwarf birds.Hussain et al. (1982) have established that the shank length in mini hens was 7-8 сm, while in those of normal size: 9.5-10.5 cm.This finding is in agreement with our results.The keel length changed less markedly (-7%) and the differences between both genotypes were proportional to the change in body weight.The reduction of body weight, shank length and keel length in the present study was confirmed by Missohou et al. (2003), Chen et al. (2004) Younis and Galal (2006).
Hens with normal genotype attained sexual maturity 7 days earlier than birds carrying the dwarfing gene (Table 2, p<0.001).This difference is limited by the reduction of body weight and feed intake (Renden and Marple, 1986), as well as by the body fat deposition level (Zelenka et al., 1986) and the related functional ovarian activity in maturing mini-hens.In their experiments with different polygenic combinations, Khan and Verma (1983) demonstrated ages of sexual maturity of 152 and 145 days for (dw) and (DW) broiler breeders, respectively.According to Sharifi et al. (2010), the sex-linked recessive gene prolonged this age by about two weeks.Opposite data are communicated by Marks (1981) and Sadjadi et al. (1983), which stated the lack of statistically significant difference between both genotypes.The advantage of mini-breeders vs normal breeders was also reported by Anonymous ( 2003), accounting for a difference of 14 days.There were no statistically significant differences between both genotypes with respect to the egg production rate and egg mass, which ranged between 57.23 -61.63 % and 32.08 -36.57g, respectively.The lack of differences in egg production between normal and dwarf sibs is supported by Marks (1981).Presented by Islam ( 2005) reports showed that laying rate decreased by about or more than 10% in strains with low body weight, while this parameter was unchanged or improved in heavy-type birds.
The data for egg weight (Table 2) showed that the presence of dw gene in the genotype of hens reduced considerably the weight of laid eggs by 3.4 g (-5.72%).This could be explained by the existing high positive correlation between body weight and egg weight, as well as the smaller reproductive tract of dwarf hens (Кatongole et al., 1990).The effect of sex-linked dwarf gene on this trait is confirmed by Missohou et al. (2003), Galal et al. (2007), whose results provide evidence for lower egg weight by -9% and -4.8%, respectively.According to Sadjadi et al. (1983) the reduction of egg weight in birds from the egg-laying type was greater and ranged between -9.6 to -12.6%.
Feed intake and conversion are economically important traits.Under the conditions of this experiment, the presence of the dw gene, determining the dwarfism expression in birds, reduced substantially feed consumption by 23.38 % (p<0.01) as compared to their normal sibs (Table 2).Mini forms converted feed energy by 12.7% more efficiently (p<0.05) for egg mass synthesis.The effect of the gene with respect to these parameters is explained not only by the lower body weight, but also by reduced biochemical activity of the basic metabolism, the higher ratio between anabolic and catabolic processes compared to those in normal sibs, resulting in lower energy demands and more efficient dietary energy utilisation by mini-hens (Kiselev and Nadalyak, 1985).Our results are comparable to those reported by Missohou et al. (2003), Galal and Younis (2006) and Galal et al. (2007), demonstrating lower feed intake and better feed utilisation under the influence of the dwarf gene.The presence of the dw gene in hens' genotype did not have a statistically significant effect on the livability within the duration of the present experiment, as the differences were insignificant.A similar conclusion was drawn by Ipek et al. (1999).In the view of Garcês et al. (2001) birds carrying the dw gene exhibited higher livability percentages than their normal sibs, and Kousiakis et al. (1985) reported a high mortality in dwarf hens during the production period.
The data from Table 3 present the results related to reproductive performance in both genotypes.It showed a statistically significant difference (p<0.05)only in hatchability from eggs set, higher in mini-forms (18.03%), as confirmed by the studies of Ipek et al. (1999).The fertility and hatchability of eggs set percentages varied within similar ranges, 86-88 % and 88-91%, respectively.The results of Kousiakis et al. (1985) showed a superiority of hens with normal genotype with regard to the fertility and hatchability of eggs set percentages although the hatchability of fertile eggs between the groups was not generally different.On the other hand, Islam (2004) and Tahir et al. (2011) observed a positive effect of the dw gene on fertility and hatchability of eggs.Investigating the fertility rate between dwarf hens and their normal sibs, Marks (1983) established identical values of both genotypes.The hatchability of fertile eggs according to the author was by about 10% higher in dwarf hens compared to those of normal sized.The data about egg quality traits presented in Table 4 showed that the reduction of egg weight in mini-forms was accompanied by a proportional change in external egg quality characteristicsegg length and breadth, egg volume and egg surface area by -4.10% (p<0.001),-2.27% (p<0.01),-7.97% (p<0.001) and -5.53% (p<0.001)respectively.A marked and statistically significant tendency towards lower egg albumen and yolk weightsby -7.38 and -4.84 % respectively (p<0.05), was present.This was confirmed by Garcês and Casey (2003).The presence of dw gene had a substantial effect on eggshell weight and thickness, as well as on egg quality parameters related to yolk (yolk index) and albumen (Haugh units, albumen height and albumen index).Published data about the influence of the dw gene on internal egg quality traits are controversial.Comparing the eggs of hens of a normal genotype with eggs produced by dwarf dams Islam (2005) did not found differences in albumen height, similar to our results.Garcês and Casey (2003) reported about lower albumen height in eggs of dwarf hens, whereas Galal et al. (2007) affirmed that the egg albumen quality of mini-forms was higher.The analysis of data about egg part proportions in normal and mini-hens, showed similar values for albumen (58.94-59.44%) and yolk (30.47-31.55%) proportions, whereas eggshell percentage changed considerably under the influence of the dwarf gene by 7.74 % (p<0.01).Hussain et al. (1982) and Islam ( 2005) also outlined on influence of the dwarf gene on albumen and yolk percentages, while Galal et al. (2007) found out that the presence of the dw gene increased considerably the proportion of the yolk and decreased that of albumen.According to the data of authors, the dw gene resulted in higher eggshell percentage similar to our findings, whereas Kousiakis et al. (1985) established no statistically significant difference between hens with or without the dw gene with respect to eggshell quality.

Conclusion
The positive effect of the sex-linked dwarf gene on economically important traitsfeed intake and feed conversion, hatchability of eggs set, is a prerequisite for the development of more efficient broiler breeder hens for production of slowgrowing chickens.

Table 1 . Body weight, shank length and keel length of hens as affected by dwarf (dw) gene.
2 Significant difference between normal and dwarf hens: *** p<0.001