POLYMORPHISM OF SELECTED PROTEINS OF YOLK AND WHITE OF EGG IN RELATION TO PERFORMANCE TRAITS OF HENS

Magdalena Gryzińska

ABSTRACT

The studies were aimed to establish a connection between polymorphism of egg-yolk and egg-white proteins and selected performance traits of laying hens. Data on performance of the studied breeds of hens were collected from breeding records. The following traits were measured in the analysed hens: body weight at 8, 18, and 20 days of age, age of sexual maturity, initial egg-laying rate, mean egg weight at 33 weeks of age, number of eggs with a complete eggshell, number of eggs with a broken eggshell, number of eggs without an eggshell or with a soft one, and a mean eggshell strength at 33 weeks of age.

Key words: hens, proteins, eggs, polymorphism, performance..

INTRODUCTION

Identification of genes that are responsible for quantitative traits is difficult due to their large number and a weak additive effect each of them has. Therefore, a quest has begun to find genetic markers, i.e. traits of simple and clear mechanism of heritability, that would remain unchanged over the entire life of an individual. An analysis of the genetic background of quantitative traits is intended to find the genes of strong effect (so called major genes), or the genes that are coupled with them. O'Brien et al. [7] divided markers in two classes, where one class includes DNA coding sequences, and the other class represents non-coding nucleotide sequences. The markers of the previous class are mostly represented by erythrocytic antigens (blood groups), major histocompatibility complex (MHC) antigens, or polymorphic proteins of milk, or yolk or white of eggs.

An application of the markers belonging to this class depends on results of studies on biochemical polymorphism. Despite the fact that a new class of markers was distinguished, the genes encoding quantitative traits, such as polymorphic proteins of blood plasma or egg content, have still been in use. The facts that make us investigate on this group of markers are as follows: polymorphic variants of proteins are not influenced by environmental factors, they enable evaluating individuals lacking a phenotypic expression of a given trait, and they shorten the time needed for estimating the breeding value of an animal. According to Leenstra et al. [6], a marker a higher polymorphism is easier to identify, which also means better selection performance. Moreover, markers may enhance efficiency of selection for the traits low-heritability. Expected genetic progress is fast if markers are used, unlike that attained in progeny-based selection [4]. Together with the remaining markers, they may be used for creating new, improved genetic programmes.

The aim of this study was to determine a relationship between polymorphic forms of egg yolk and white proteins and selected performance traits of laying hens.

MATERIALS AND METHODS

The material included the yolk and white of eggs from hens of the following breeds: Green-Legged Partridge (GLP), 181 hens, from Agricultural Experimental Unit in Felin; Rhode Island White (RIW), 116 hens, Rhode Island Red (RIR-M), 106 hens, New Hampshire (NH), 119 hens, and Leghorn (Lg), 150 hens, from Mienia; and Sussex (Sx), 189 hens, and Rhode Island Red (RIR-J), 163 hens, from the Polish Academy of Sciences Institute of Genetics and Animal Breeding in Jastrzębiec.

The polymorphism of egg proteins was determined with thin-layer horizontal polyacrylamide (paa) gel electrophoresis, according to the methods given by Gahne et al. [1] and Głuchowski et al. [3]. For the electrophoresis, 10 and 12% paa gel was used.

Each of the studied breeds of hens was derived from a single hatching. The data on the performance of each breed were collected from breeding records. The following traits were determined in the examined hens:

• body weight at 8, 18, and 20 days of weeks, measured with precision to 0.1 g;

• hen's age in days from hatching until laying the first egg – sexual maturity;

• initial egg-laying rate, in percent;

• mean egg weight at 33 weeks of age, measured with precision to 0.1 g;

• number of eggs with cracked eggshell;

• mean strength of eggshell at 33 weeks of age, examined along the long axis using an Instron instrument, in N.

Moreover, those eggs were also weighed to 0.1 whose content was subjected to electrophoresis, studied at 28 weeks for RIW, RIR-M, ad RIR-J, at 33 weeks for NH and GLP, as well as at 47 weeks of age for Lg and Sx.

The hens were assigned to groups depending on polymorphic forms of yolk and white proteins. In so constructed groups, the mean vales of the traits were compared using the Duncan test, based on one-way analysis of variance.

The protein names were adapted according to the notation used by Gahne et al. [1]. The alleles belonging to the same range were denoted as A, B, C, and D, according to decreasing speed of migrating to the anode.

RESULTS AND DISCUSSION

An electrophoregram of hen egg yolk and white proteins, as well as an electrophoregram of blood serum proteins for a comparison, separated with polyacrylamide gel horizontal electrophoresis, are presented in Fig. 1. The paa gel electrophoresis allowed separation of egg-content proteins into 30-35 bands, the polymorphism being found in six subregions of yolk proteins and in five subregions of white proteins. Tables 1-7 present the levels of the examined performance traits in hens assigned into groups depending on the phenotypic form of egg-content proteins, for which statistical significance of differences was confirmed.

A number of significant relationships between the phenotypic forms of egg-content proteins and performance traits were found for each breed of hens; however, fast-migrating prealbumin (Pa-F) in yolk and ovoglobulin (G₄) in egg white can be considered the most versatile subregions.

The protein phenotypes of fast-migrating prealbumin Pa-F subregion were encoded by four genes, A, B, C, and D, in the Leghorn and New Hampshire hens. Three genes, B, C, and D, were found in the population of Rhode Island White, Rhode Island Red-J, and Sussex. The remaining breeds, Rhode Island Red-M and Green-Legged Partridge had two genes each, B and D.

Significant differences in the Pa-F subregion were demonstrated for Leghorn, Rhode Island White, Rhode Island Red-J, and Sussex. The Leghorn hens (Table 1) of the phenotypes BB and BD, had lower egg weights than the hens of the phenotype DD. For the Rhode Island White hens (Table 3), differences were found for the age of sexual maturity and initial egg-laying rate. The birds with the fast-migrating prealbumin phenotype DD attained maturity five days earlier than those with the phenotype BB, whereas the initial egg-laying rate was different in the hens with the BD phenotype than the DD phenotype of the subregion. For the Rhode Island Red-J (Table 5), differences in the Pa-F subregion were found in mean egg weight at 33 weeks of age, in initial egg-laying rate, and in the weight of the studied eggs. The recorded initial egg-laying rate was different among the birds with the BB, BD, CD, and DD phenotypes. The weight difference b etween eggs collected from the BB-phenotype hens and those with the BD phenotype was 1.6 g. The weight of the eggs whose content was analysed differed significantly between the BB and CD hens, and between BD and CD, by 1.1 g and 2.5 g, respectively. Statistically significant differences in the subregion of fast-migrating prealbumin in Sussex hens (Table 6) were found for the following six traits: sexual maturity age, initial egg-laying rate, and mean egg weight at 33 weeks of age. The hens with the BB phenotype of the subregion reached sexual maturity at 150 days of age. Those with the CD phenotype laid their first egg at the age of 154 days and had a lower initial egg-laying rate in comparison with the hens with BB and BC phenotypes. The hens with the BD phenotype laid heavier eggs than those of the phenotype BB did. What is more, eggs (whose content was subjected to electrophoresis) laid by BC and BD fast-migrating prealbumin phenotype hens were heavier than the eggs laid by the BB-phe notype hens.

In all the studied breeds, three phenotypes in the ovoglobulin G₄ subregion, AA, AB, and BB, were found to occur with varied frequency. The phenotypes of the subregion depended on two genes, A and B.

Significant differences in the subregion of ovoglobulin G₄ were demonstrated for Leghorn, New Hampshire, Rhode Island Red-M, and Green-Legged Partridge.

The age of reaching sexual maturity for Leghorn hens (Table 1) with the phenotypes AA and AB of ovoglobulin G₄ was 164 days. The BB homozygotes needed eight days more to reach maturity than the AA or AB hens. Significant differences in the weight of eggs were found between AB and BB phenotypes. New Hampshire hens (Table 2) of the phenotype AA were heavier than either AB or BB phenotypes in the G₄ subregion. Rhode Island Red-M hens (Table 4) differed in mean egg weight in this subregion. Eggs obtained from homozygous ovoglobulin G₄ AA hens had larger weight than eggs of hens with the AB and BB phenotypes. In the Green-Legged Partridge hens (Table 7), significant differences were found in egg weight. The layers of the BB phenotype laid eggs by 3.5 g lighter than the hens with the phenotype AA, and by 3.8 g lighter than the hens with AB phenotype of ovoglobulin G₄ subregion.