A molecular test for bird gender

    The gender of mammals and birds is determined chromosomally. However, whereas male mammals are XY (heterogametic) and females XX (homogametic), in birds it is the females that are heterogametic (WZ) and males are homogametic (ZZ) [left]. As in mammals, this makes a convenient test for gender possible.  In many species of seabirds, the sexes are not strongly differentiated morphologically. Characterization of the sex-chromosome-specific gene locus CHD (chromo - helicase - DNA binding protein) makes a definitive gender-identification test possible (Fridolfsoon & Ellegren 1999. J Avian Biol 30,116). CHD is present as a pair of duplicated gene loci on the  W and Z chromosomes (CHD-W and CHD-Z, respectively) [middle]. CHD-W and CHD-Z have conserved exons (black boxes), but differ in their intron lengths (450 vs 600 bp, respectively). A WZ female is therefore expected to have introns of two different sizes, whereas a ZZ male will have only one size class.

    In the above experiment, genomic DNA extracted from five Whiskered Auklets (Aethia pygmaea) was amplified with a pair of conserved exon PCR primers (2550F and 2718R) (Jones, Fraser, Rowe, Carr, & Taylor. 2002. Auk 109,1064). In the electrophoresis picture at right, the left-most lane is a molecular weight standard: the fourth band from the top corresponds to 603 bp. Lanes 2 & 5 shows the two-band pattern (450 bp below, 600 bp above) characteristic of a WZ female bird. Lane 3, 4, & 6 shows the single-band pattern (600bp only) characteristic of a ZZ male bird.

Figure & text material ©2008 by Steven M. Carr