Assessment of reproductive function in mutant restricted ovulator carrier roosters

Two splice variant forms of the chicken very low density lipoprotein receptor (VLDLR) have recently been shown to exist in both male and female chicken gonads. The longer form, containing an O-linked sugar domain, is produced in the somatic cells of the testes and ovaries, whereas the shorter form, which lacks this domain, is expressed in the male and female germ cells. The latter is essential for female reproduction, because a naturally occurring point mutation in the gene encoding the short form of the VLDLR causes its functional absence, resulting in atherosclerosis and a general failure to lay eggs. Thus, the term restricted ovulator (RO) was originally coined to describe females with the mutant gene. In contrast, the roles of the VLDLR gene products in male reproduction have yet to be defined, because only heterozygous RO roosters, which have a mutant allele of the VLDLR gene (short form) on one of their two Z (sex) chromosomes, can be obtained. However, the question of whether two normal alleles of the VLDLR gene are needed for optimal male reproductive performance can be addressed. Here, the reproductive abilities of heterozygous RO roosters were compared to their wild-type siblings. The RO roosters were found to be phenotypically normal, suggesting that only one normal allele for the VLDLR gene is needed for optimal male reproductive performance. Alternately, it is also possible that the one normal allele compensated for the presence of the mutant one in terms of VLDLR gene product production (i.e., a dosage compensation).