To determine whether a link exists between reproductive seasonality and the structure of the gene for melatonin receptor Mel_1a, the latter was studied in two groups of Mérinos d'Arles (MA) ewes previously chosen for their genetic value, which took into account their own out-of-season ovulatory activity adjusted by environmental parameters and that of their relatives. The genomic DNA of 36 ewes found regularly cycling in spring (group H) and that of 35 ewes never cycling in spring (group L) during the 2–3 yr before the present study was prepared, and the cDNA corresponding to almost all exon II was amplified and checked for the presence of MnlI restriction sites. The presence ( ) or absence (−) of an MnlI site at position 605 led to genotypes `` ”, `` −”, and ``−−”, whose frequencies differed significantly (P < 0.001) between the H and L groups: 52.8%, 47.2%, and 0% vs. 28.5%, 42.9%, and 28.5%, respectively. Sequencing of exon II cDNA in group L ewes with genotype −− showed the presence of only one allele − with 4 mutations, while that in ewes with genotype showed different types of alleles unrelated to the H or L groups. These alleles exhibited a combination of 1 to 7 of the 8 mutations recorded in the part of exon II studied. The genotyping of 29 ewes from the more seasonal Ile-de-France breed indicated that 38% of animals had a −− genotype and exhibited the same mutations as in the MA ewes. Finally, a comparison of ¹²⁵I-melatonin binding to membrane preparations of pars tuberalis showed a lower number of binding sites (P < 0.0005) in MA ewes with genotype than in those with genotype −− (43.2 ± 4.4 vs. 75.4 ± 8.4 fmol/mg protein in genotype and genotype −−, respectively).

In conclusion, the data show an association between genotype −− for site MnlI at position 605 and seasonal anovulatory activity in MA ewes.