Characterization and regulatory analysis of betaine homocysteine S-methyltransferase gene 1 (BHMT1) in mud crab: A gene responsive to salinity and feeding behavior

Betaine homocysteine S-methyltransferase gene 1 (BHMT1) encoded betaine homocysteine S-methyltransferase (BHMT) catalyzes the homocysteine-to-methionine reaction using betaine as a methyl donor. Previously, we found that BHMT was lost in the genome of the insect clade of arthropods, but was highly expressed in the mandibular organ (MO) of the mud crab Scylla paramamosain (Sp). To further explore its significance, we performed a primary regulatory analysis of BHMT in mud crabs. The open reading frame (ORF) length of Sp-BHMT1 was 1203 bp, encoding 400 amino acids. Sequence alignment revealed that BHMT1 was highly conserved in different animals, and its identity was higher in more closely related species. Sp-BHMT1 had the highest expression in the MO of both sexes, while its expression in the MO was 1.6-fold higher in males than in females; similar results were also found in the cerebral ganglion, hepatopancreas, and thoracic ganglia tissues. During larval development, Sp-BHMT1 was weakly expressed on most days but was significantly elevated on the first day of the Zoea 2nd (Z2) and Z3 stages. Sp-BHMT1 exhibited the highest expression at a salinity of 10 ‰ and the lowest at a salinity of 30 ‰. With decreasing salinity, the expression of Sp-BHMT1 increased significantly at 6 h and then returned to baseline at 8 h. After starvation treatment, the expression of Sp-BHMT1 was significantly upregulated compared to that in the feed group at all examined time points, with a peak expression at 18:00 on the third day in the starvation group. Krüppel homolog 1 (Kr-h1), a methyl farnesoate (MF) signal gene, could increase Sp-BHMT1 expression under low salinity and starvation, suggesting that a “salinity/starvation→BHMT1 → MF biosynthesis” regulatory axis might exist in crabs. This study provides valuable insights into the functional study of Sp-BHMT1 and MF biosynthetic regulation.