BILATERAL ASYMMETRY IN THE TRANSVERSE PROCESSES OF THE CERVICAL VERTEBRAE IN THE CHICKEN

Abstract

Bilateral asymmetry in the transverse processes (TP) of the cervical vertebrae in domestic chickens was investigated. The asymmetry was classified into two categories; directional and fluctuating. The directional asymmetry was measured by the difference between the lengths of the two TP, that is, left minus right. The fluctuating asymmetry, however, was defined as asymmetry due to chance fluctuation in the development of left and right TP and was measured by the bilateral asymmetry remaining after subtraction of the average directional asymmetry of each strain. If the degree of fluctuating asymmetry in a single bird is a, then a=|(left-right)-average(left-right)|. Since a was found not to distribute normally, it has been transformed to A=log10[(a+0.1)2]. The birds investigated were 16-month old White Leghorn females of 6 inbred, one closed and 5 F1 hybrid strains. The lengths of the left and right TP (4) of the sixth to ninth cervical vertebrae were measured.A comparison of the differences between the directional asymmetries of the four vertebrae was found to be statistically significant, but there were no differences among strains. The left TP was usually longer than the right, the mean difference being 0.126mm. It appears that this asymmetry is directional rather than random.Variation in fluctuating asymmetry among strains was highly significant: the F1 hybrids were less asymmetric than their inbred parents. If we assume that the fluctuating asymmetry in length of the TP's of the cervical vertebrae is a result of developmental instability in the bird, it is reasonable to expect that heterozygosity might reduce this instability. This was found in this study.Directional and fluctuating asymmetries are positively correlated with each other (r=0.696). In other words, those vertebrae which showed high directional asymmetry had simultaneosly high fluctuating asymmetry. Correlation between body weight and TP length was estimated to be 0.789, whereas body weight was not significantly correlated with either directional or fluctuating asymmetry. It should be mentioned that neither directional nor fluctuating asymmetry is significantly correlated with TP length or with the difference between the lengths of the TP of the F1 hybrids and the corresponding mid-parental values.