RESUMEN
Lipocalins are involved in the binding of small molecules like sex steroids. We show here that the previously reported tilapia male-specific protein (MSP) is a lipocalin encoded by a variety of paralogous and homologous genes in different tilapia species. Exon-intron boundaries of MSP genes were typical of the six-exon genomic structure of lipocalins, and the transcripts were capable of encoding 200 amino-acid polypeptides that consisted of a putative signal peptide and a lipocalin domain. Cysteine residues are conserved in positions analogous to those forming the three disulfide bonds characteristic of the ligand pocket. The calculated molecular mass of the secreted MSP (20.4 kDa) was less than half of that observed, suggesting that it is highly glycosylated like its homologue tributyltin-binding protein. Analysis of sequence variations revealed three types of paralogs MSPA, MSPB and MSPC. Expression of both MSPA and MSPB was detected in testis. In haploid Oreochromis niloticus embryos, each of these types consisted of two closely related paralogs, and asymmetry between MSP copy numbers on the maternal (six copies) and the paternal (three copies) chromosomes was observed. Using this polymorphism we mapped MSPA and MSPC to linkage group 12 of an F(2) mapping family derived from a cross between O. niloticus and Oreochromis aureus. Females with high MSP copy number were more frequent by more than twofold than males. Gender-MSPC combinations showed significant deviation from expected Mendelian segregation (P=0.009) suggesting elimination of males with MSPC copies. We discuss different hypotheses to explain this elimination, including possibility for allelic conflict resulted by the hybridization.