Lampbrush W and Z heterochromosome characterization with a monoclonal antibody and heat-induced chromosomal markers in the newt Pleurodeles waltl: W chromosome plays a role in female sex determination.

Two subsets of lateral loops scattered on lampbrush chromosomes of the newt Pleurodeles waltl were characterized. One group was identified by labelling with a monoclonal antibody (A1). The second group was identified by the ability of the loops to be induced by heat treatment. Three loops of each subset were mapped on a short region of the two homologues of lampbrush bivalent IV. These regions appear to be heteromorphic because the six loops are always heterozygous. Five loops are found on one homologue and the sixth on the partner. The distribution of these markers in phenotypic females corresponding to the three sexual genotypes ZW, WW and ZZ shows an absolute correlation of the five loop group with the W chromosome and of the other loop with the Z chromosome. Therefore the heteromorphic regions of the homologues correspond to the differential segments of the heterochromosomes. The identification of a trisomic ZZW female suggests that the W chromosome bears female sex determinants. Furthermore the results show that heat induces loop development and that under normal conditions giant loop development is influenced by the sexual genotype.