The singularity of Cetacea behaviour parallels the complete inactivation of melatonin gene modules

Abstract

13Melatonin, the hormone of darkness, is a peculiar molecule found in most living organisms. 14 Emerging as a potent broad-spectrum antioxidant, melatonin was repurposed into extra 15 roles such as the modulation of circadian and seasonal rhythmicity: affecting numerous 16 aspects of physiology and behaviour, including sleep entrainment and locomotor activity.17Interestingly, the pineal gland-the melatonin synthesising organ in vertebrates-was 18 suggested to be absent or rudimentary in some mammalian lineages, including Cetacea. 19In Cetacea, pineal regression is paralleled by their unique bio-rhythmicity, as illustrated 20 by the unihemispheric sleeping behaviour and long-term vigilance. Here, we examined 21 the genes responsible for melatonin synthesis (Aanat and Asmt) and signalling (Mtnr1a 22 and Mtnr1b) in 12 toothed and baleen whale genomes. Based on an ample genomic 23 comparison, we deduce that melatonin-related gene modules are eroded in Cetacea.24 25 26 48 the existence of measurable amounts of circulating melatonin but without a clear 49 circadian rhythmicity 14 . To further address the parallelism between pineal regression and 50 melatonin synthesis and signalling, we extracted genomic information and addressed the 51 coding status of Aanat, Asmt and the melatonin receptors Mtnr1a and Mtnr1b in Cetacea 52 and in Hippopotamus amphibius (common hippopotamus), the closest extant lineage of 53 3 Cetacea. 54 55 Results 56Using the available genomic data we examined the genomic regions of melatonin 57 synthesis and signalling modules in 12 Cetacea species (Orcinus orca (orca whale), 58 Next, we examined the Asmt gene ( Supplementary Figure 4). In human, 3 distinct isoforms 92 of Asmt, resulting from the alternative splicing of exon 6 and 7, were identified 28 . Yet, only 93 isoform 1 (P46597) has been reported to display ASMT enzymatic activity 94 (Supplementary Figure 5) 28 . Thus, both human and B. taurus genes were used as 95 reference. Gene annotation revealed the presence of a conserved 1 nucleotide insertion 96 in exon 1, validated by independent SRAs in O. orca, T. truncatus, D. leucas, N. 97 asiaeorientalis, B. acutorostrata and E. robustus (Figure 2 and Supplementary Figures 6 98 and 7). In L. obliquidens, S. chinensis, P. catodon and B. mysticetus exon 1 could not be 99 retrieved using the current assemblies. Complete erosion of exon 5 was also detected in 100 O. orca, L. obliquidens, S. chinensis, N. asiaeorientalis and D. leucas, for which full genomic 101 sequences ranging from exon 4 to exon 7 were available; for P. catodon and E. robustus 102 exon 5 was identified with no deleterious mutations, while for the remaining species the 103 presence of sequencing gaps within this region impaired further assessment. The deletion 104 of exon 5 is expected to abolish enzyme activity given that this exon contains a highly 105 conserved glycine (Gly187), participating in the active site of ASMT 28 (Supplementary 106 Figures 5 and 8). Also, aside from the conserved non canonical splice site observed in...