Cell number and sex ratio in unfertilized chicken eggs (Gallus gallus domesticus)

Abstract

In chickens and other birds, females have two different sex chromosomes (ZW), whereas males carry two homologous sex chromosomes (ZZ). The primary sex ratio can thus be determined by genetic analysis of the sex chromosome of the ovum before fertilization. Sex diagnosis is more reliable when there are more cells, i.e. sufficient DNA, for the analysis. In this study, eggs from virgin hens were incubated for 3 days and the number of cells in the germinal discs was counted. A median of 2.5 cells was counted with a range of two to 20 cells. We also counted cells in the germinal discs of unfertilized eggs of inseminated hens and recorded a median of three cells and a range of two to 40 cells. Sex diagnosis based on polymerase chain reaction (PCR) amplification of Z and W chromosomes specific fragments from the CHD1 gene in 30 incubated eggs obtained from 35-week-old virgin hens gave a ratio of 13 Z to 15 W chromosomes with two samples undetermined. The unfertilized eggs of three groups of chickens were subjected to sex diagnosis to supplement the sex ratio data of an incubation experiment (see companion paper). The high proportion of Z chromosomes diagnosed in all three groups by two independent gene products suggests a sex difference on developmental potential and/or a sex chromosome segregation biased toward males in unfertilized eggs especially at the beginning of reproduction.

Chimeric embryos—potential mechanism of avian offspring sex manipulation

“…Although analysis of developmental potential of triploids (Fechheimer 1981;Thorne et al 1991) and unfertilized blastodiscs (Klein and Grossmann 2008) suggests Z dominancy over the W in the competition environment, we argue that this does not determine the easiness of manipulating offspring sex toward one chromosome over the other. In order to predict the direction of the expected bias, one should take into account species specificity and acuteness of hormonal stimuli.…”

“…Triploids with ZWW survive only a few days of incubation, whereas individuals with ZZW and ZZZ can survive to hatching and maturity. Furthermore, unfertilized chicken eggs exhibit high incidence of Z chromosome bias (Klein and Grossmann 2008). Such an effect could be caused by sex differences in developmental potential and/or a segregation bias toward Z in unfertilized eggs (Klein and Grossmann 2008).…”